HSCChemistry
MODULE-8 APPLYING CHEMICAL IDEAS
ANALYSIS OF INORGANIC SUBSTANCES
Inquiry question: How are the ions present in the environment identified and measured?
– cations: barium (Ba2+), calcium (Ca2+), magnesium (Mg2+), lead(II) (Pb2+), silver ion (Ag+ ), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+)
– anions: chloride (Cl– ), bromide (Br– ), iodide (I– ), hydroxide (OH– ), acetate (CH3COO– ), carbonate (CO32– ), sulfate (SO42– ), phosphate (PO43–)
– gravimetric analysis
– precipitation titrations
– colourimetry
– ultraviolet-visible spectrophotometry
– atomic absorption spectroscopy
ANALYSIS OF ORGANIC SUBSTANCES
Inquiry question: How is information about the reactivity and structure of organic compounds obtained?
– carbon–carbon double bonds
– hydroxyl groups
– carboxylic acids (ACSCH130)
including but not limited to:
– proton and carbon-13 NMR
– mass spectroscopy (ACSCH19)
– infrared spectroscopy (ACSCH130)
CHEMICAL SYNTHESIS AND DESIGN
Inquiry question: What are the implications for society of chemical synthesis and design?
– availability of reagents
– reaction conditions (ACSCH133)
– yield and purity (ACSCH134)
– industrial uses (eg pharmaceutical, cosmetics, cleaning products, fuels) (ACSCH131) environmental, social and economic issues
ION IDENTIFICATION
OUTCOMES
analyse the need for monitoring the environment
CHEMICAL MONITORING AND MANAGEMENT
Chemical monitoring and management are needed to ensure the health and wellbeing of both plants and animals.
Monitoring also allows for more effective management.
For example, the destruction of UV-protective ozone, caused by widespread use of chlorofluorocarbons (CFCs)
- Hazardous ions in the environment such as mercury have also been identified and monitored using chemical techniques.
- Nutrients required by humans and other living organisms have also been discovered through environmental monitoring.
OUTCOMES
– cations: barium (Ba2+), calcium (Ca2+), magnesium (Mg2+), lead(II) (Pb2+), silver ion (Ag+ ), copper(II) (Cu2+),
iron(II) (Fe2+), iron(III) (Fe3+)
– anions: chloride (Cl– ), bromide (Br– ), iodide (I– ), hydroxide (OH– ), acetate (CH3COO– ), carbonate (CO32– ),
sulfate (SO42– ), phosphate (PO43–)
QUALITATIVE ION ANALYSIS
- Precipitate: Insoluble ionic salt formed when 2 ionic solutions are mixed together.
- Qualitative analysis: Identification of the constituents of components in a sample.
- Quantitative analysis: Measurement/determination of the amount (expressed in a concentration) of the constituent.
- Destructive testing: Test that irreversibly alter the composition of a sample.
PRECIPITATION REACTIONS
SOLUBILITY RULES
Precipitation reactions are useful for the rapid identification of ions.
- Solutions of specific ions are added to the unknown aqueous sample.
- The formation or absence of a precipitate can be used to determine the ion present.
- Ag+ halides precipitates decompose with exposure to light to form Ag(x). This is observed as darkening of the solid.
COMPLEXATION REACTIONS
Complexation reactions are commonly used to identify transition metal cations (due to their d orbitals)
A complexation reaction occurs when a coordination compound or complex forms.
- These consist of a central metal ion (typically a transition metal) coordinated to molecules or ions (polar & lone pair of electron molecules).
- The coordinating species are called ligands. They are typically neutrally or negatively charged. (E.g.
COMPLEXATION REACTIONS AND ION IDENTIFICATION
HYDRATED TRANSITIONAL METAL ION
When transition metal ions dissolve in water, a complexation reaction occurs:
- Water molecules (ligands) coordinate to the transition metal centre
- Charge stays the same as the original cation
- Hydrated transition metal centres often have characteristic colours that can be used to identify them.
IDENTIFICATION WITH COMPLEXATION
Complexation reactions can be also used in conjunction with precipitation reactions to identify ions.
FLAME TESTING
A flame test is used to qualitatively identify some metal ions in a sample. It can be used to distinguish between cations that undergo similar precipitation reactions. For example, Ba2+ and Ca2+ or Group 1 metals. However, not all metals produce a diagnostic flame colour.
ATOMIC EMISSION
The flame colour arises from atomic emissions of the metal.
- The heat of the flame excites electrons into higher energy levels
- Atomic emission occurs when an electron in a higher energy level returns to a lower energy level (relaxation)
- During relaxation, a wavelength of light matching the energy difference between the levels is emitted.
The colour of the flame produced in the flame test results from a combination of the most intense wavelengths emitted in the visible regions.
Metals that do not produce a characteristic flame colour typically have atomic emissions that fall outside the visible region.
CHARACTERISTIC FLAME COLOUR
LIMITATIONS OF THE FLAME TEST
- Flame tests are only useful for some metal cations.
- Destructive
- Can be ambiguous which metal ion is present
- Inconclusive results can be caused by interference created by other metal ions present in the sample. (e.g.Ba flame masks Cu, Li and Sr)
- Interference can sometimes be removed by using coloured glass filters. For example, blue cobalt glass can be used to filter out the bright yellow light produced by Na, a common impurity.
ADDITIONAL TESTS FOR ION IDENTIFICATION
SOLUTION PH
Conjugates of strong acids/bases will dissolve to produce a solution with neutral pH. Conjugate bases of weak acids/bases will produce a non-neutral pH.
ACID CARBONATE REACTION
Carbonate anions (CO32-) can be identified through the addition of an acid. Bubbles will be observed. A lime water test can be used to confirm the presence of CO2(g).
FLOWCHARTS
- To create a flowchart effectively, go from least soluble to most soluble.
- It is important to include ‘in excess’ for flowcharts with more than 1 ion in a sample.
CATIONS
ANION
OUTCOMES
– gravimetric analysis
WHAT IS GRAVIMETRIC ANALYSIS
Gravimetric analysis uses the mass of product (precipitate) formed through a precipitate reaction to quantify a target ion. It is used in applications such as monitoring water quality, determining the mineral composition of soil and product testing.
Gravimetric analysis is typically performed with the following steps:
1) A known quantity of sample is dissolved in water
2) The target ion is precipitated from the solution using a suitable reagent
3) The precipitate is collected by filtration, then washed, dried and weighed
The reagent used in the precipitation reaction must react selectively with the target ion and form a highly insoluble precipitate in order to give accurate results. The selective reagent can be chosen using the solubility rules, while the solubility of the resulting precipitate can be found by examining the solubility product constant (Ksp).
Undesired precipitation reactions can be prevented through experimental design.
The amount of the target ion present is usually expressed as a mass percentage of the original sample.
GRAVIMETRIC ANALYSIS ASSUMPTIONS (VALIDITY)
- Only the target ion precipitates out
- All the target ion precipitates out
- All of the precipitate is collected and in a pure form
SULFATE CONTENT OF FERTERLISER
Fertilisers (a solid mixture) commonly contain ammonium sulfate (NH4)2SO4, as a sulfur source for plants. The sulfate content of fertiliser can be determined gravimetrically through the addition of Ba2+ to form a highly insoluble BaSO4.
Barium can precipitate with CO32- , Cl- , PO43- and OH- anions as well. To precipitate out:
- CO32-: Add acid in excess
- PO43 and OH-: Acidify solution (Don’t use H2SO4)
- Cl-: Add AgNO3 in excess
If other anions precipitate with Ba, the mass of the residue collected will increase. Therefore, a higher w/w % will be calculated than actually present.
EXAMPLE QUESTION 1
The following results were obtained for the gravimetric analysis of sulfate ions in several fertiliser samples
There are two methods:
PROS AND CONS OF GRAVIMETRIC ANALYSIS
ADVANTAGES
- It is very accurate, if the gravimetric analysis is carefully designed (assumptions) and conducted.
- It does not require speciality equipment to perform.
- It does not require the construction of a calibration curve, which can be time consuming
DISADVANTAGES
- The target ion of interest must be able to form a sufficiently insoluble precipitate for accurate results (A limitation of the technique)
- The precipitate must be highly pure
- It is extremely time consuming compared to modern analytical techniques
- Concentration of the target ion must be high enough to produce a precipitate that can be accurately weighed.
OUTCOMES
– gravimetric analysis
– precipitation titrations
TITRATION
Precipitation titration are commonly used in the quantification of halides (Cl-, Br-, I- ). In these procedures, silver ions are usually the titrant and consequently they are known as argentometric titrations.
The main three titrations used are:
MOHR’S METHOD (DIRECTION TITRATION)
Mohr’s method quantifies chloride, bromide and cyanide ions using direct titration with a standard silver nitration solution.
Yellow potassium chromate (K2Cr4) is used as the indictor:
- Silver preferentially precipitates with halide ions
- When excess silver is added, it will react with chromate ions to produce a reddish-brown precipitate
LIMITATIONS OF MOHR’S METHOD
The presence of multiple anions in a sample can lead to invalid results. Carbonate and phosphate ions will precipitate with silver.
The conical flask must be thoroughly stirred during the titration to ensure the Ag+ ions are evenly distributed, so AgCrO4 will not precipitate early.
The titration must be conducted at a pH 6.5-9.0
- At pH < 6.5, the chromate ion (a weak base) accepts H+ to become HCrO4- , so there will not be enough chromate in solution to produce the end point colour change. HCrO4- is also bright orange, which obscures the endpoint.
- At pH > 9.0, the concentration of OH- is too high. It will form a precipitate with Ag to produce AgOH which is a brown precipitate. It also obscures the endpoint and increases the silver needed to reach it.
Mohr’s method is suitable for the analysis of chloride and bromide ions, but not iodide or thiocyanate ions. This is because the chromate ions are strongly adsorbed to the surface of AgI and AgSCN precipitates, making the endpoint indistinct.
In this precipitation titration, the concentration of the indicator used is quite low as adding too much chromate indicator will lead to an intense yellow colour which masks the end point. Hence the excess titrant required to cause a colour change is relatively large.
This excess silver required to reach the end point leads to a systematic error, which can be corrected using a blank titration.
- The same quantity of chromate indictor is to be used in the titration is added to a suspension of calcium carbonate (which imitates the white AgX precipitate).
- AgNO3 is added to the solution until a colour change occurs.
- The volume of AgNO3 required to cause the colour change in the blank is the excess volume required. This volume is subtracted from the titration volumes.
VOLHARD’S METHOD (BACK TITRATION)
Volhard’s method quantifies anions used in back titration procedure.
LIMITATIONS OF VOLHARD’S METHOD
The Volhard titration must be performed in low pH to prevent the precipitation of Fe3+ indictor as Fe(OH)3. At low pH, there is a relatively low [OH- ], therefore the precipitate Fe(OH)3 will not form.
The Volhard method is less valid if the first precipitate formed is more soluble than AgSCN.
- For example, AgCl (??? = 1.77 × 10−10) is more soluble than the AgSCN (??? = 1.03 × 10−12). This means that AgCl can dissolve during the titration leading to a greater titration volume required.
To avoid this problem, the procedure can be modified.
- The first precipitate can be removed via filtration before titration. However, this can lead to inaccuracies and is less convent.
- A dense organic liquid like nitrobenzene or chloroform can be added to the conical flask to act as a barrier between the precipitate and aqueous layer.
FAJAN’S METHOD (DIRECT TITRATION AND ADSORPTION INDICATOR)
In the analysis of chloride, an anionic dye is added to the sample.
The most suitable indicator depends on the analyte.
PROS AND CONS OF FAJAN’S METHOD
The Fanjan’s method relies on a large precipitate surface area, to allow sufficient dye to adsorb so the colour change is easily visible.
- Dextrin (starch) can be added to prevent silver halide precipitates from aggregating
- A high concentration of spectator ions can also caused aggregation, so this method cannot be used with all samples.
- The analyte must also be concentrated enough, as the colour will not be seen if there is too little precipitate.
The pH of the titration reaction must be carefully controlled as anionic dye indicators are conjugate bases of weak acids. This means it will react with free H+ and will not adsorb onto the surface of the precipitate.
OUTCOMES
– atomic absorption spectroscopy
ATOMIC ABSORPTION
The Bohr model of the atom where electrons are contained in shells around the nucleus of the atom:
- The further away from the nucleus, the higher the energy level of the shell
- These shells have discrete or quantised energy levels
- The lowest energy electronic configuration of an atom is called the ground state
When electrons in the ground state (gas) absorb light energy (photons) corresponding to a difference between levels, they are excited to a higher energy level.
The light that passes through the sample without being absorbed can be collected as an absorption spectrum.
- The absorbed wavelengths of light can be seen as black lines on a bright background.
- The energies of the absorbed wavelengths match energy gaps between two shells.
As each element has a unique set of energy levels, the wavelengths of light absorbed by each element is characteristic.
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
Atomic absorption spectroscopy is a sensitive and highly selective technique that can be used to measure small
concentrations of metal ions. It has an important role in the detection of toxic heavy metals.
In AAS, the concentration of the analyte is calculated from the amount of wavelength of light absorbed by the sample.
- The wavelength used in the analysis is based on atomic absorptions of the analyte.
- The amount of light absorbed is called the absorbance. It is determined from the relative intensities of light before and after the sample
Where:
- A: Absorbance which has no units as its dimensionless
- I0: The intensity of light before the sample
- I: The intensity of light after the sample
The measured absorbance is directly proportional to the concentration of analyte present in the sample.
HOW DOES IT WORK
The hollow cathode lamp contains the element which being tested. It emits a certain spectral pattern – unique to each metal.
CALIBRATION USING STANDARD SOLUTIONS
The calibration curve is used to establish the relationship between measured absorbance and the analyte concentration for a particular instrument. It is produced by measuring the absorbance of standard solutions
Absorbance is proportional to concentration for dilute samplesin AAS.
APPLICATIONS OF AAS
- It is quick, easy, accurate and highly sensitive and is commonly used to determine the concentrations of over 65 elements.
- AAS provides a means of investigating phenomena which could not be studied before, such as trace nutrients and heavy metal pollution.
ADVANTAGES AND DISADVANTAGES OF AAS
Advantages
- Simple procedure
- Very sensitive technique (can measure ppb)
- Extremely accurate
- Specific (can analyse a single ion in a mixture)
- Although destructive, uses a very smallsample size
- Very fast analysis (1 minute per element)
- Little sample preparation required
Disadvantages
- High initial costs
- Can only test for one atom or ion at a time with most instruments
- Only metals and a few non-metals can be tested
- Cannot distinguish between different oxidation states of the metal (e.g. Fe2+ and Fe3+)
- Destructive analysis
- Only test aqueous solutions
CLASS NOTES FOR AAS
Some elements produce very intense spectral lines that serve as a unique characteristic marker for their presence.
This can be used as a qualitative indicator. When a flame is looked at through a spectrometer it can be further analysed to determine the concentration of the substance.
AAS is a technique used to identify the presence and concentration of substances by analysing the spectrum produced when a sample is vaporised and absorbs certain frequencies of light. It is primarily used to determine the concentrations of cations.
AAS measures the amount of light absorbed by the sample
OUTCOMES
– colourimetry
COLOURIMETRY
Colourimetry is a technique for determining the analyte concentration based on the absorbance of a coloured solution.
- The amount of light that is absorbed by a sample is compared to the amount absorbed by a blank or reference
- The blank contains all components of the measured sample except the analyte
A coloured solution mostly absorbs its complementary colour, which is directly opposite in the colour wheel.
The wavelength of light used in colourimetry corresponds to the strongest absorption of the analyte of interest, known as the absorption maximum (????)
- This wavelength is used as sensitivity increases with the intensity of absorption, allowing for more accurate data to be collected
- Requires an intensely coloured analyte
The main coloured species analysed by colourimetry are transitional metal complexes. Transition metal ions are often weakly coloured in solution, so they are commonly converted to intensely coloured transition metal complexes to allow for quantitative analysis. The colour of complex ions depends on:
- Central metal ion
- Ligands
HOW COLOURIMETRY WORKS
The calculation of absorbance is similar to the method used in atomic absorption spectroscopy (AAS).
- However, the amount of light absorbed by the sample is calculated relative to the amount of light absorbed by a blank sample.
- This is to remove the influence of the solvent and other substances present in the sample on the measured absorbance
- A: Absorbance which has no units as its dimensionless. Typically, between 0.3 and 2.5 (AU)
- I0: The intensity of light passing through the blank sample
- I: The intensity of light passing through the analyte sample
- E: Molar absorptivity (also known as extinction coefficient) (: L cm−1 mol−1)
- l: Path length of the sample in cm
- c: Concentration of the substance in the sample in mol/L
Many factors can influence the absorbance readings therefore, it is always more accurate to construct a calibration curve by measuring the absorbance of standard solutions under the sample experimental conditions. This helps to reduce systematic error.
Similar to AAS, absorbance is proportional to concentration for samples with moderate absorbances (0.3 – 2.5). Samples should be concentrated or diluted so that measured absorbances are in the range where the relationship is linear.
DETERMINATION OF IRON CONTENT USING COLOURIMETRY
Due to the pale green colour of Fe3+ , it cannot be directly measured using colourimetry, and requires conversion to the intensely coloured [Fe(SCN)]2+ complex.
The complex ion strongly absorbs blue light at a wavelength of 447 nm.
DETERMINING STOICHIOMETRY USING COLOURMETRY
Colourimetry can be used to determine the stoichiometric ration of reactants in the formation of an ionic species such as a coordination complex. The absorbances of a series of solutions containing different ratios of reactants are measured at a wavelength absorbed by the product. The correct stoichiometry of reactants is given at the maximum absorbance.
The measured absorbances of the solutions are plotted against the volume of a reactant used, and the maximum absorbance is found using lines of best fit.
The ratio of the reactants in the product corresponds to the moles combined to achieve the maximum absorbance.
OUTCOMES
– ultraviolet-visible spectrophotometry
SPECTROSCOPY OF ORGANIC COMPOUNDS
The wavelengths absorbed by an organic molecule can be matched to its structural fragments. A structural fragment that absorbs a characteristic wavelength is called a chromophore.
The overall spectrum can also be used as a signature or fingerprint of the whole molecule. By comparing the collected spectrum with a database of standards, the compound can be identified.
UV-VISIBLE SPECTROPHOTOMETRY
UV-visible spectrophotometers can also be used to measure the absorbance of a sample over a range of wavelengths.
- The prism is rotated so that different wavelengths (190 – 700 nm) pass through the monochromator to the sample then to the detector.
- The absorbance at each wavelength is recorded.
- The UV-visible spectrum plots absorbance against wavelength
A UV-visible spectrum is often used to find the absorption maximum of a sample.
ULTRAVIOLET-VISIBLE SPECTROSCOPY OF ORGANIC COMPOUNDS
It is primarily used in detecting the presence of conjugation within organic molecules.
Conjugation is where molecules have alternating double or triple and single bonds.
The UV-Vis spectra of organic compounds arise as a result of electronic transitions, just as they do for metals and transition metal complexes.
Organic compounds can absorb UV and visible radiation as the wavelength in these regions correspond to the energy gaps between molecular orbitals.
The energy gap, and consequently the wavelength required for an electronic transition, depends on the type of bonding.
- For C-C and C-H single bonds, the energy gaps are very large and require shorter wavelengths in the far UV region (<150 nm)
- For double bonds or triple bonds (C=C, C=O, C=N) or bonds with heteroatoms that have lone electron pairs (C-Cl, C-O), the energy gaps are smaller therefore light of longer wavelengths is absorbed (150-200 nm)
When conjugation exists in a molecule, the energy gaps are even smaller, resulting in longer wavelengths being absorbed (>200 nm)
- If the conjugation is extended further, the energy gap will be small enough for the molecule to absorb wavelengths in the visible region (400-800 nm), resulting in a highly coloured compound.
In practice, the range from 190 to 700 nm is detectable by a typical commercial UV-Vis spectrometer.
- UV-Vis spectroscopy of organic compounds is limited in the most part to conjugated systems
- For measurements below 190nm, a vacuum spectrometer and a suitable UV light source are required.
INTERPRETING UV-VIS SPECTRA
A UV-Vis spectrum is a plot of absorbance versus absorbed wavelengths. The diagram below is the UV-Vis spectrum for buta-1,3-diene.
The interpretation of the UV-Vis spectrum of organic compounds is straightforward. By comparing Amax with a database of standards, the type of bonding in a compound can be identified
The exact wavelengths absorbed depends on the rest of the molecule, hence compounds have distinctive UV-Vis spectra.
The presence of strong absorption bands in the spectrum above 200nm can generally be used to indicate the presence of conjugation in a molecule. The greater the extent of conjugation, the longer the wavelength absorbed.
OUTCOMES
– carbon–carbon double bonds
– hydroxyl groups
– carboxylic acids (ACSCH130)
CARBON-CARBON DOUBLE BONDS
All halogens are coloured (bromide, chloride, fluoride and iodide). The addition reaction, halogenation, can be used as an indicator for alkenes and alkynes through the bromine test.
Once bromide is added to a substance, it is an alkene or alkyne when there is a rapid spontaneous decolourisation of Br(l). This should be done in the absence of UV light.
HYDROXYL GROUPS (ALCOHOLS)
Alcohols have hydroxyl (-OH) functional groups. The presence of the hydroxyl functional group can be detected by reacting a dry sample of the alcohol with a small piece of active metal such as sodium.
The evolution of a gas indicates a reaction has occurred. The gas evolved can be collected and tested with a lit splint.
If a ‘pop’ sound occurs, H2 has evolved and a hydroxyl group is present.
Alternatively, the pH of the alkoxide salt solution formed can be tested with an indicator. Alcohols are neutral. The alkoxide ion produces a strong base.?
The reduction of permanganate or dichromate in acidic solutions results in distinctive colour changes, which can be used to distinguish between the types of alcohol
CARBOXYL GROUP
Carboxylic (alkanoic) acids contain the carboxyl (-COOH) functional group. They are weak acids as the hydrogen atom bonded to the oxygen atom will partially dissociate in water.
The presence of a carboxyl group can be determined by an indictor test.
- An aqueous solution of the carboxylic acid can be tested with blue litmus paper. The presence of the carboxyl group will turn the litmus paper red.
Alternatively, a carbonate test can be performed. Carboxylic acids react with metal carbonates and metal hydrogen carbonates to produce a salt, carbon dioxide and water. For example:
Effervescence (bubbles) indicates that a reaction has occurred. The gas evolved can be collected and confirmed as a carbon dioxide by bubbling it through limewater (calcium hydroxide).
ANALYSING A SAMPLE
INFRARED SPECTROSCOPY
Infrared spectroscopy analyses the interaction of molecules with IR light. It primarily gives information about the bonds or functional groups present in a molecule and can be used for fingerprinting purposes.
The absorption of IR radiation by an organic compound is associated with a change in the vibrational energy levels of the molecule. IR induces molecular vibrations.
A molecule can be thought of as a group of spherical masses (atoms) connected by strings (covalent bonds).
- At room temperature, the atoms within the molecules are constantly in motion, vibrating back and forth.
- The fundamental vibrational motions include stretching and bending.
However, unlike masses on a spring, the energy of molecular vibration is quantised. This means every vibration can only occur at specific frequency that is characteristic to the type of motion, the atoms attached and the bond strength.
When a molecule is irradiated with energy that matches the energy gap for one of its vibrational modes, it will absorb the energy and be promoted to a higher vibrational energy level. This results in increased amplitude for the vibration.
- The energy difference between the vibrational levels corresponds to the frequencies in the infrared region
Since different types of bonds give rise to different vibrational frequencies, each functional group will absorb in a different IR range. This absorption frequency can be used as a diagnostic marker for the presence of functional groups.
An important aspect of IR spectroscopy is that not all vibrations can be detected.
- IR active vibrations are those that cause a change in the dipole moments of a molecule.
- IR inactive vibrations do not cause changes in the dipole moment of the molecule. For example, vibrations of symmetrical non-polar molecules like H2 do not result in the absorption of IR energy. Thus, no peak will be observed for these vibrations.
- The reason for this requirement involves the mechanism by which the radiation transfer its energy to the molecule.
INFRARED SPECTROMETER
- The IR radiation source is an inert solid that is heated electronically to cause thermal emission of radiation in the infrared region.
- This incident light is split into two equivalent beams that are passed through a reference and the sample,
where some energy is absorbed, and some is transmitted.
- The beam chopper alternatively passes the beams to the monochromator which filters out a single wavelength of light.
- The detector calculates the ratio between the intensities of the reference and sample beams to determine the absorbance.
INTERPRETATION OF INFRARED SPECTRUM
An infrared spectrum is constructed by plotting transmittance vs wavenumber.
- The transmittance is plotted, hence absorbed wavelengths are shown by troughs pointing downwards.
- Infrared absorption wavelength is reported in wavenumbers which is a type of frequency measurement. It is used because the numbers are more convenient.
- The x-axis is not a linear scale. It changes at 2000 cm-1
- 3600 − 2300 ??−1: stretching of single bonds to hydrogen
- 2300 − 1900 ??−1: stretching of triple and consecutive double bonds
- 1900 − 1500 ??−1: stretching of carbonyls and double bonds
- 1500 − 666 ??−1: fingerprint region. Many overlapping signals are present
in this region, leading to a unique spectrum for different molecules. Peaks in the region are not normally interpreted in detail but comparison of this region with a database of standards allows for the identification of specific compounds.
The exact frequencies absorbed depends on the rest of the molecule, hence compounds have unique infrared absorption spectra. Infrared absorption beaks are described qualitatively as strong (s), medium (m), or weak (w). Additionally, broad (br) troughs may be observed.
ADVANTAGES AND DISADVANTAGES OF IR SPECTROSCOPY
ADVANTAGES
- Infrared spectroscopy requires milligram-sized samples, but these can usually be recovered, depending on the technique used.
- Infrared spectra are very easy and quick to run, and are relatively inexpensive
- Infrared spectra can be used for fingerprinting purposes
DISADVANTAGES
- Functional groups can be identified from the spectrum, but other structural features cannot be determined.
- It can be difficult to resolve spectra from complex mixtures. The presence of multiple organic substances can result in masking and distortion of characteristic absorptions.
- The sample must be very dry. Water present in samples will make it appear as if the compound contains O-H bonds.
OUTCOMES
– mass spectroscopy (ACSCH19)
COMBUSTIVE ANALYSIS
Elemental analysis of organic compounds is the determination of mass fractions or percentages of carbon, hydrogen and heteroatoms in a sample. This information can be used to determine the empirical formulae of unknown compounds.
- Empirical formula: simplest whole number mole ratio
- Molecular formula: moles of atoms in the molecule
The mass of each element in a compound can be obtained experimentally by means of combustion analysis.
- This process involves burning a weighed sample of the organic compound in excess oxygen, then collecting and weighing the combustion products
- From these mass measurements, the percentage composition of the sample can be determined
DETERMINING EMPIRCAL FORMULA
1) Assume that you have 100g of the compound.
2) Calculate the moles of each element
3) Find the simplest whole number ratio
DETERMINING MOLECULAR FORMULA
A molecular formula can always be determined by multiplying the subscripts in the empirical formula by an integer.
To determine the molecular formula, the number of empirical units would need to be calculated. Therefore, you need to know the approximate molar mass.
MASS SPECTROMETRY
Mass spectrometry is an analytical technique for studying the chemicals present in samples, and for probing the molecular structure of compounds. A mass spectrometer measures the masses of molecules and atoms by volatising then ionising them.
The most common type of mass spectrometer is the electron ionisation magnetic sector instrument
- A very small (picogram to nanogram) sample is injected into the mass spectrometer and vaporised into its gaseous state.
- The sample is then ionised by bombardment by a high energy stream of electrons, which knock valence electrons from the molecules in the sample, creating cations.
− Many of these cations are unstable and will decompose into fragment (daughter) ions and radicals.
- The cations are then accelerated into a magnetic field which causes the ions to travel on a curved path. Uncharged radicals cannot be accelerated
- The curvature depends on the mass-to-charge (m/z) ratio of the particles. The larger the ion, and the lower the charge, the lower the deflection.
- By varying the strength of the magnetic field, ions of differing mass can be brought to focus on the detector
- The detector is composed of a conductive metal. Like an anode, it is a source of electrons. When the positive ions collide with it, they gain an electron and become neutralised. A small current is produced, amplified and recorded as a spectrum by a computer.
1) Vaporisation
2) Ionisation
3) Acceleration
4) Deflection
5) Detection
INTERPRETING MASS SPECTRA
The mass spectrum records the abundances of the fragments of different m/z ratio relative to the most intense peak (the base peak).
- Each peak represents an ion with a specific m/z ratio. Since the charge of these fragments are usually +1, the value of m/z is usually the same as mass.
The mass spectrum can be used to determine the molecular formula of a substance.
- The ions with the greatest mass will correspond to a molecule that has only lost a single electron (parent peak), giving relative molecular mass of the intact molecule
- This can be combined with data from elemental analysis to calculate molecular formula
The masses of the detected fragments can be used to deduce structural information about the molecule. Working backwards, the identified fragments can be reassembled to generate the original molecule.
The fragment ions that are observed and the abundance of each fragment (height of each peak) is dependent on the stability of the ion
- The more stable the fragment ion, the higher its abundance
- The base peak represents the fragment ion with the highest stability
Different types of fragments have different stabilities, which result in unique fragmentation patterns. The overall fragmentation pattern can be used for fingerprint purposes to identify the molecule by comparison with the spectra of known compounds from a library.
ISOTOPES IN MASS SPECTRA
Some elements have more than one isotope in high abundance. In particular, chlorine and bromine have isotopes that differ by a mass of 2.
The presence of these isotopes can be observed in the mass spectra of chlorinated or brominated compounds.
USES OF MASS SPECTROMETRY
Due to the sensitivity of mass spectrometers, the mass spectra of compounds can be used as fingerprints for identifying compounds.
- Mass spectrometry can be used to unambiguously identify a substance as no two substance produce the same fragmentation pattern
- Experimentally obtained spectra can be compared with databases of known substances
Mass spectrometry is often used in conjugation with chromatographic separation and is routinely used to analyse a wide range of industrial, environmental and forensic samples. It is also used for:
- Detect and identify the use of steroids in athletes
- Identify compounds in illicit drug samples
- Monitor the breath of patients by anaesthetists during surgery
- Determined whether honey is adulterated with corn syrup
- Locate oil deposits by measuring petroleum precursors in rock
- Monitor fermentation processes for the biotechnology industry
- Detect dioxins in contaminated fish
- Establish the elemental composition of semiconductor materials
ADVANTAGES OF MASS SPECTROMETRY
- Can be used to determine a substances molecular formula
- The masses of fragment ions can be used to identify structural fragments in the molecule
- Isotopes can be determined, and used to match a sample to a particular source
- It is quick, highly sensitive technique that can be performed on very small samples
DISADVANTAGES OF MASS SPECTROMETRY
Mass spectrometry is less useful in the analysis of very complex mixtures
- When hundreds of compounds are present in a sample, there may be so many peaks present that it becomes impossible to determine what fragments are present
- In these cases, mass spectrometry can be used to determine molecular masses and molecular formula, but cannot be used to determine the structure of components
- Overshowed by the large quantity of structural information available in NMR spectroscopy.
– proton and carbon-13 NMR
NUCLEAR SPIN
Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful techniques for the analysis of organic compounds.
If a nucleus contains an odd number of protons and/or odd number of neutrons, it can exhibit spin (similar toe electron spin). This means that the nucleus will behave like a magnet in a magnetic field, and can be detected by NMR (NMR-active or spin-active).
The spins of the nuclei in a sample are not oriented, and are initially degenerate (same energy). However, when placed in an external magnetic field, the spin-active nuclei will adopt one of two states:
1) A higher energy spin state aligned against the magnetic field (antiparallel)
2) A lower energy spins state aligned with the magnetic field (parallel)
The difference in energy between the two spin states is Δ?, and this amount of energy lies within the radiofrequency region of the electromagnetic spectrum.
One way of producing an NMR spectrum:
- The sample is placed in a magnetic field
- Radio waves of different frequencies are sent through the sample
- When the energy of the wave matches Δ? for a particular nucleus, it can absorb the energy and transition from the lower energy spin state to the higher energy spin state. The nucleus can release the energy again to return to the lower spin state. Switching between these two states is called resonance.
- The frequency at which resonance occurs is recorded as a signal in the NMR spectrum
CARBON-13 NMR SPECTRA
Most carbon atoms in a sample will usually be carbon-12, which cannot be analysed using NMR. However,
approximately 1% of a typical sample of carbon will consist of the carbon-13 isotope, which is spin-active and can be
detected in NMR spectroscopy.
A carbon-13 NMR spectrum gives information about the number and type of carbon environments in a molecule.
The frequencies that are absorbed in an NMR experiment are proportional to the size of ΔE. Electrons are moving charged particles, so they will create a small magnetic field around the nucleus which will change the size of ΔE, and hence the absorbed frequencies. This is known as electron shielding.
Electron shielding creates unique chemical environments in a molecule.
- Electron shielding changes depending on the atoms and bonds around the nucleus (indicated by the position of the signal)
- The number of unique chemical environments for a particular type of nucleus will determine the number of unique ΔE, which will match the number of signals in an NMR spectrum.
CARBON ENVIRONMENTS
If the carbon nuclei can be interchanged via bond rotation or rotation in space, they will be in the same chemical environment and produce the same signal.
CHEMICAL SHIFT
The resonance frequencies in an NMR spectrum are converted to values known as chemical shift, measured in parts per million (ppm).
- Converting to chemical shift removes the impact of magnetic field strength, which increases ΔE
- This allows the comparison of spectra collected in spectrometers of different strengths
The chemical shift (position of the signal) that a nucleus produces depends on electron shielding (what is bonded to the carbon).
- A nucleus surrounded by low electron density is less shielded, so it experiences more of the magnetic field and resonates at higher frequencies (higher ppm).
- A nucleus surrounded by high electron density is more shielded and will resonate at a lower frequency (lower ppm).
Tetramethylsilane (TMS) is usually added to samples as a standard.
Different types of carbon environments have characteristic chemical shifts, generally between 0 – 200 ppm.
PROTON NMR SPECTRA
The number of signals in the spectrum matches with the number of unique hydrogen environments.
Different types of hydrogen environments have characteristic chemical shifts
- Decreased electron shielding leads to downfield shifts (higher ppm)
- TMS is also used as a standard and has a chemical shift of 0.0ppm
The chemical shifts can be divided into general regions:
- 0-1.5 ppm: protons on saturated carbon centres, or on carbons attached to saturated centres
- 1.5-2.5 ppm: protons on carbons attached to unsaturated centres (alkenes, alkynes)
- 2.5-4.5 ppm: protons on carbons attached to electronegative atoms
- 4.5-6.5 ppm: protons on alkene carbons
- 6.5-8.0 ppm: protons on aromatic rings (benzene)
Additional information can be obtained from higher resolution H-NMR spectra: from signal splitting and the area under each integral.
HYDROGEN ENVIRONMENTS
INTEGRALS AND SPIN-SPIN SPLITTING IN H-NMR
INTEGRALS (AREA UNDER CURVE)
The area under the signal corresponds to the number of hydrogen nuclei in that environment. Use a ruler to measure the ratio.
SPLITTING (MULTIPLICITY)
In H-NMR each signal can be split into multiple peaks, called a multiplet. Splitting arises from the interaction of the spins of nearby nuclei, called coupling.
Splitting follows the n + 1 rule: protons that have n protons in a different chemical environment on immediately adjacent carbons show n + 1 peaks in their signal.
The heights of the peaks in multiplet matches Pascal’s triangle, with taller peaks in the middle and a symmetrical structure.
ADVANTAGES AND DISADVANATGES OF NMR SPECTROSCOPY
NMR spectroscopy by far is the most powerful technique for determining and confirming the structures of organic compounds, due to the large amount of structural information given in a spectrum.
NMR is less sensitive than other techniques, so the sample sizes required are larger than for other techniques.
However, NMR spectroscopy is non-destructive for samples can be recovered.
Highly complex, large molecules will produce a spectrum that are difficult to interpret.
Compounds must be dissolved in a suitable solvent to produce a clear NMR spectrum
NMR spectrometers maintenance requires technical support.
OUTCOMES
– availability of reagents
– reaction conditions (ACSCH133)
– yield and purity (ACSCH134)
– industrial uses (eg pharmaceutical, cosmetics, cleaning products, fuels) (ACSCH131) environmental, social and economic issues
CHEMICAL SYNTHESIS
Chemical synthesis refers to the purposeful use of chemical reactions to obtain a desired product. A synthesis reaction is when smaller compounds undergo a chemical reaction to produce a larger compound (product).
The development of chemical syntheses for desirable product compounds has been of fundamental importance to our standard of living and has addressed issues such as:
- Replenishment of depleting resources
- Large-scale production of compounds that are scarce in nature
- Development of unique chemicals to address specific needs
DESIGNING A CHEMICAL SYNTHESIS PROCESS
CONSIDERATIONS FOR REACTION PATHWAY DESIGN
Desired products typically cannot be produced in a single step from the available starting materials. Instead, a multi
step chemical synthesis is required.
A reaction pathway is commonly designed by working backwards from the desired product and identifying the
intermediate products and reaction conditions that would result in the formation.
The analysis of a compound to devise a suitable reaction pathway is called a retrosynthetic analysis.
- A retrosynthetic analysis can often identify a number of alternative reactions pathway to produce a desired product. This can be seen for the synthesis of butanone.
A number of factors are considered when selecting the most desirable reaction pathway:
- Availability of reactants and reagents
- Yield and purity of the product
- Reaction conditions
- Economic factors such as industrial uses of the products and by-products, the cost of reagents and the efficiency of the process
- Environmental impacts of reagents, products and waste generated by the process itself
- Social impacts of the synthesis such as the health hazards associated with reagents, products and waste,
AVAILABILITY OF REAGENTS
The availability of reagents is a significant factor in the selection of a reaction pathway.
- Hard to obtain
- ↓Supply → ↑Price → ↑Cost of production
YIELD AND PURITY
The yield of a reaction is the amount of product obtained compared to the theoretical maximum expressed as a percentage. The theoretical maximum amount of product that can be formed from a reaction is based on the amount of the limiting reagent.
The amount of product that is obtained from a reaction (actual yield) is commonly expressed as a percentage of the theoretical maximum (theoretical yield).
An additional consideration in the selection of a reaction pathway is the number of steps required to convert a given starting material to the desired product compound.
- The overall yield of a reaction pathway typically decreases with the number of steps in the reaction sequence
- A reaction pathway with a large number of single steps reactions will result in a low yield.
The overall yield for a multi-step reaction pathway can be calculated from the yield for each of the individual steps 
LINEAR AND CONVERGENT SYNTHESIS
A linear synthesis is where each intermediate product was the reactant for the next subsequent step.
One strategy to improve the overall product yield of a multi-step reaction pathways is convergent synthesis.
- A convergent synthesis reduces the number of linear steps in a sequence by using individual reaction pathways to synthesis the intermediate products
- The intermediate products are then combined in a single step to generate the final desired product.
A convergent reaction pathway typically has a higher overall yield compared to the same reaction pathway performed as a linear sequence.
SELECTIVITY
An additional consideration in the selection of a reaction pathway is the selectivity of each of the individual steps to form the desired product. TLDR: basically, consider the major and minor products.
PURITY
Purity refers to the amount of desired product in a sample, expressed as a percentage.
- Product purity is commonly used as a measure of quality of a product.
- Typically, higher purity chemicals are more labour intensive to produce than lower purity chemicals due to the need for additional purification processes.
Product purity is continually monitored throughout chemical manufacture to ensure that the product meets specifications. This process is commonly referred to as quality control.
REACTION CONDITIONS
Reaction conditions must be considered when designing syntheses.
- Conditions are adjusted to ensure the desired product can be obtained in high yield and at a reasonable reaction rate.
- This is particularly important in the chemical industry, where small gains in yield and rate can dramatically improve profitability.
To increase the viability of an industrial process:
- Maximise yield
- Maximise rate
- Minimise waste
o Use the by-product of the reaction for something else
o Recycle energy
- Safety
Reaction conditions that are commonly optimised to improve the yield and/or rate of a chemical process include:
- Concentration of reactants and products
- Temperature
- Pressure
- Catalyst
However, there are other considerations in addition to product yield and reaction rate when selecting optimal temperature and pressure conditions:
- High temperatures can be unsafe (increase in costs)
- Specialised equipment and greater energy is required to create and maintain high pressures
- Side reactions are more likely at higher temperatures
- Reactants, products and catalysts often decompose at high temperatures
- Catalysts are often very expensive relative to the other reagents in the process.
ENVIRONMENTAL ISSUES
Most industrial processes consume large amounts of energy. For example, the production of ammonia via the Haber process accounts for around 1.2% of the global energy consumption each year. Issues of high energy consumption may include:
- Co2(g) + H2O(I) (Greenhouse gases)
- Co(g) (Toxic gas)
- NxOy(g) (Associated with photochemical smogs)
- So2(g) (Associated with the formation of acid rain)
Whenever possible, energy should be recycled to minimise environmental impact and reduce costs
THERMAL POLLUTION
Many industrial processes use high temperatures processes, which means cooling the products is often necessary. Water is commonly used as a coolant as there are huge water bodies and due to its high heat capacity.
Discharging warm water into the environment may cause thermal pollution. The temperature of water body may increase, causing the concentration of oxygen to decrease. This potentially kills off aquatic life.
Holding ponds can be used to allow water to cool before it is discharged. Releasing warm water into the ocean is also a feasible solution, as the ocean is large enough to dissipate heat quickly.
TOXIC EMISSIONS
Chemical processes should be designed to use substances that minimise hazards to users and the environment. For example, the production of sulfuric acid is associated with fugitive emission of SO2(g). It is a toxic gas and forms an acidic solution in water.
Toxic emissions should be carefully monitored to reduce environmental impact.
WASTE DISPOSAL
Depending on the chemical process, different types of waste are generated which must be appropriately handled.
- Acidic wastes are generated by many industrial processes.
- Solid wastes, if inert, are frequently discharged into the ocean.
- Toxic waste disposal is often expensive, and there are usually government regulations that dictate how they must be handled.
- Non-toxic ionic wastes are often discharged into the ocean.
CASE STUDY: PRODUCTION OF SULFURIC ACID
Sulfuric acid is manufactured using the contact process (as the reaction occurs on the surface of the catalyst).
The key step in the contact process is the conversion of SO2 to SO3. It is the yield determining step as it is a reversible process.
The industrial yield is typically 90-95%.
- 30-50% excess oxygen is present in the mixture. (Oxygen enriched air)
- The reaction mixture is passed over beds of vanadium oxide catalyst pellets (V2O5)
- 400 − 550℃ moderate temperature. The reaction mixture is progressively cooled as the reaction is
exothermic.
- SO3 is removed from the reaction mixture before it is passed over a final bed of vanadium oxide.
- A low pressure of 1-2 atm is used
Contact Process - Reactions
H2So4 is used in many other chemical processes such as the production of fertilisers, synthetic detergents and HCl.
CONTACT PROCESS CONSIDERATIONS
So2(g) and So3(g) forms acidic rain in the atmosphere. They are both toxic gases as well.
CASE STUDY: HABER PROCESS
The Haber process is the main source of ammonia
- Ammonia is an important raw material for nitrogen-based fertilisers
- Modern agriculture is dependent on replenishing nitrogen in the soil with fertilisers, as nitrogen is required for protein production and leaf growth.
TEMPERATURE
A moderate temperature of 400 − 500℃ is used in industry.
- ↑ Temperature → Favours the endothermic reaction (LHS) → ↓ Yield %
- ↓ Temperature → Favours the exothermic reaction (RHS) → ↑ Yield%
A moderate temperature is considered a compromise between rate a yield as it achieves an economically viable rate and yield.
PRESSURE
A pressure of around 200 atm is used in industry
- ↑ Pressure → Favours the side with the least gas mols (RHS) → ↑ Yield%
- A high pressure also benefits rate of reaction as there are significantly more collisions and potential effective collisions resulting in reactions.
However, high pressures are expensive as energy is required to create high pressure, and equipment must be reinforced to contain increased pressure. High pressure can also pose a safety risk
CATALYST
A magnetite catalyst containing iron (II/III) oxide (Fe3o4) is used. The catalyst provides an alternative energy path that is lower than the original. It increases the rate of reaction and does not affect yield.
REMOVAL OF PRODUCT
The ammonia can be separated from the unreacted reactants through liquifying it by cooling the system to −33℃. The ammonia can be removed, and the unreacted gases are recycled back into the reaction chamber to reduce waste.
YIELD
Yield is typically 10-20% in the Haber Process.
OTHER CONDITIONS/CONSIDERATIONS
- Oxygen must be excluded to avoid explosions
- Heat produced by the exothermic reaction is recycled to pre-warm the input gases
- Carbon monoxide and sulfur contaminants in the reactants must be monitored as they will poison thecatalyst and decrease efficiency.
ENVIRONMENTAL IMPACTS
The production of ammonia via the Haber process accounts for around 1.2% of the global energy consumption per year. The source of energy is from the combustion of fossil fuels/
- CO2(g) + H2O(I) are both greenhouse gases
- Co(g) is a toxic gas
- NxOy is associated with photochemical smog
- So2(g) is associated with the formation of acid rain
Since the Haber process is an exothermic reaction, the heat produced from the reaction can be recycled. When warm water is discharged into the environment it can be problematic as oxygen has a lower solubility at high temperatures.
PRODUCTION OF ETHENE
Crude oil → Cracked to smaller fragments
ATOM ECONOMY
The principle of atom economy seeks to design chemical reactions such that the greatest number of atoms from reagents are incorporated into the desired product.
The atom economy of a process is calculated as a percentage based on the molecular mass of the desired product and the sum of the molecular masses of the reactants
