Navigate to resources by choosing units within one of the unit groups shown below.
Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:
- Content: a clear outline of the content covered by the delivery guide;
- Thinking Conceptually: expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject;
- Thinking Contextually: a range of suggested teaching activities using a variety of themes so that different activities can be selected that best suit particular classes, learning styles or teaching approaches.
Learners should be able to demonstrate and apply their knowledge and understanding of:
EL(x) use of data from a mass spectrum to determine relative abundance of isotopes and calculate the relative atomic mass of an element
WM(i) interpretation and prediction of mass spectra:
(i) the M+ peak and the molecular mass
(ii) that other peaks are due to positive ions from fragments
(iii) the M+1 peak being caused by the presence of 13C
WM(j) the effect of specific frequencies of infrared radiation (IR) making specific bonds in organic molecules vibrate (more); interpretation and prediction of infrared spectra for organic compounds, in terms of the functional group(s) present
PL(a)(ii) techniques and procedures for paper chromatography
PL(r) the further interpretation and prediction of mass spectra:
(i) use of the high-resolution value of the M+ peak to work out a molecular formula
(ii) the mass differences between peaks indicating the loss of groups of atoms
PL(s) proton and carbon-13 nuclear magnetic resonance (NMR) spectra for the determination of molecular structure
PL(t) the combination of spectroscopic techniques (mass spectrometry, IR and NMR) to determine the structure of organic molecules
DM(n) techniques and procedures to measure concentrations of solutions using a colorimeter or visible spectrophotometer
CD(n) the general principles of gas–liquid chromatography:
(i) sample injected into inert carrier gas stream
(ii) column consisting of high boiling liquid on porous support
(iii) detection of the emerging compounds (sometimes involving mass spectrometry)
(iv) distinguishing compounds by their retention times
It is important that learners are taught to take a holistic approach to chemical analysis. The approach to analysis will depend on the context of investigation and the resources available. A mixture of chemical testing with instrumental analysis is likely to be needed to solve a problem.
Approaches to teaching the content
At GCSE, learners have become familiar with simple atomic structure and the existence of isotopes. They know how numbers of protons, neutrons and electrons affect atomic number and mass. Mass spectrometry as a device for weighing molecules will have been briefly covered in conjunction with gas chromatography.
Mass spectrometry is developed from looking at single elements to looking at more complex molecules.
In Elements of life (EL) the teaching of mass spectrometry looks at isotope abundance and the calculation of relative atomic mass. Following an explanation of what mass spectroscopy does, learners should be introduced to elemental spectra showing isotopes. They should look for and work out the percentage of each isotope in a sample, calculate the numbers of protons and neutrons and then work out the relative atomic mass. Then they should use some relative atomic masses to calculate abundances of isotopes. Extension activities might include asking learners to identify an element from the pattern of nuclides in a mass spectrum.
Once learners are confident with atoms, looking at the spectra of molecules like Cl2 can develop skills of processing data, understanding isotopes and seeing how atoms contribute to the spectra of molecules.
The workings of mass spectrometers are not assessed, although an outline of how they work will aid learners understanding, particularly when looking at fragmentation patterns.
Mass spectroscopy is revisited in What’s in a medicine (WM), where spectra of organic molecules are introduced. This is then further developed in Polymers and life (PL) in the second year of the course.
In WM mass spectroscopy can be used to work out the product of alcohol oxidation, just using the molecular ion peak to deduce whether a carbonyl or carboxylic acid has been formed. Learners need to know that a peak from 13C will contribute a small M+1 peak. There is no need to use size of the M+1 peak from 13C isotopes to count the number of carbons.
In PL more complex substances can be analysed. At this point, learners need to be able to identify identities of fragments in the spectrum. This can be done by building molecular models, breaking them and tabulating the masses of the fragments. Suitable pairs of isomers could be methoxymethane and ethanol or propanoic acid and methyl ethanoate.
Infrared (IR) spectroscopy
This technique is first introduced in WM, where the focus will be on identifying molecules encountered up to that point (hydrocarbons, alcohols, carbonyls and carboxylic acids). Initially, give learners spectra of known molecules, and focus on identifying the O–H, C–H and C=O absorptions, and the fingerprint region. At this stage, learners will need to relate spectra to given molecules, and make predictions about the peaks present in the spectrum for a given molecule.
Activities where learners are given a set of spectra to match to substances can stimulate thought and give controlled challenge.
IR spectroscopy is revisited in PL, where more complex molecules such as esters are examined. At A Level, learners will be expected to combine IR data with evidence from other techniques of mass spectrometry and NMR to determine the structure of an unknown organic compound. Most learners will require plenty of practice in this type of problem.
Access to the Data Sheet that will be included in the examination papers throughout teaching of this topic will be essential.
Learners will have learned about paper chromatography in KS3 and KS4, and will have covered gas chromatography in GCSE chemistry. This will be extended to the use of thin layer chromatography (TLC) as a tool for assessing the purity of synthesised compounds in WM. (See the Delivery Guide on Organic chemistry for more information about the techniques used in organic synthesis and purification.) Paper chromatography of amino acids will be used in PL as a means of identifying amino acids using Rf values.
Learners need to understand that there is an equilibrium between the sample in the mobile and stationary phases, and that solvents need to be chosen carefully so that the sample moves part of the way up the medium. They also need to understand that not all substances will be separated by a single solvent.
When learners have made organic substances (such as aspirin in WM), they should use TLC as a means of assessing the purity of their recrystallised product.
In PL, learners should use chromatography to separate and identify amino acids using known standards. They should be able to explain how to carry out an accurate practical procedure, using ideas of equilibrium between the mobile phase and the stationary phase. They should also calculate Rf values from their own data and compare them to standard versions.
Gas chromatography is studied during Colour by design (CD) as a means of identifying the oils used in oil painting. Learners should use data from analyses to identify components of mixtures.
Nuclear magnetic resonance ( NMR) spectroscopy
This can be a difficult topic, but learners are not required to have an understanding of the functional principle of this technique – the assessment is restricted to prediction and interpretation of spectra.
NMR can be introduced as a type of spectroscopy that detects specific nuclei, with chemical environments altering the chemical shift, or the position of the peaks. While carbon-13 NMR may be less familiar to some teachers than proton NMR, it is sensible to teach carbon-13 NMR first as the spectra are more straightforward. The idea of the number of different environments can then be transferred to proton NMR, where splitting patterns are added.
Learners should first be given spectra linked to known compounds, with chemical shifts linked to chemical environment and splitting patterns explained, followed by tasks where learners interpret a spectrum of an unknown substance and identify it. As with IR spectroscopy, access to the Data Sheet throughout teaching is essential.
The OCR Topic Exploration Pack on carbon-13 NMR contains three activities that can be used to introduce the concepts, covering identifying carbon environments, and prediction and interpretation of spectra.
Learners come across absorption of visible radiation in Developing metals (DM), where colorimeters are used to measure the concentration of coloured solutions. They should carry out experiments to construct a calibration curve, and use this to determine the concentration of coloured ions in a solution.
It can help to reinforce understanding of this technique by tying it in with topics covered in previous Key Stages, and earlier in the Chemistry B course. Learners can be reminded of the characteristic flame colours of different elements, and how these relate to absorption and emission spectra, as covered in EL. Similarly, some transition metal ions and complexes in solution are coloured, due to their absorption of particular wavelengths in the visible spectrum. Spectrophotometry measures the level of absorption of a particular wavelength at difference concentrations.
Common misconceptions or difficulties learners may have
Some learners with weaker mathematical skills struggle with calculating relative atomic mass and working backwards from Ar to isotope abundance.
Learners find fragmentation patterns difficult; some learners are slow to realise that fragments in a mass spectrometer are not bound by the rules that normally govern the composition of stable molecules. Breaking molecular models and tabulating the mass of the fragments helps this.
Infrared (IR) spectroscopy
Many learners try to identify too many peaks on spectra, and need to learn to leave the fingerprint zone alone. They need to understand that IR spectra show functional groups, but do not show details about the size of the molecule.
Learners will need to develop ideas about the use of non-aqueous solvents for chromatography, and the use of ultraviolet (UV) or locating agents to find the location of spots from colourless materials.
Nuclear magnetic resonance ( NMR) spectroscopy
The mechanism of NMR is not assessed in the exam, and will be difficult for all but the very brightest learners, so the emphasis should be on the atoms absorbing radiation at different frequencies depending on their chemical environment and relating the chemical shift to the type of atom using the correlation charts. Counting the hydrogen atoms on adjacent carbon atoms can be difficult. Again, only the most able learners will understand the explanation.
Carbon-13 NMR has not previously been included in this specification, so older texts are unlikely to cover this technique, and teaching and learning resources may be fewer (or higher level). However, existing resources for the 2008 OCR Chemistry A specification will include material on carbon-13 NMR, as will any new materials created specially for the 2015 Chemistry B (Salters) specification.
Learners may struggle to remember that the colour of absorption, and therefore the wavelength that should be used in colorimetry, is complementary to the colour of the substance being tested.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
While the various analytical techniques are taught at different points of the course, it is helpful to give learners a holistic view of analysis, and the different information provided by different types of analysis. For example, while qualitative tests can confirm the presence of a particular functional group, IR spectroscopy and mass spectrometry can provide more detailed structural information. Chromatography shows up the number of components in a mixture, and can be used to confirm the identity of a compound if reference information is available.
Energy and matter
The idea that matter and radiation interact underlies many of the analytical techniques covered: IR spectroscopy, NMR spectroscopy and colorimetry. The introduction of these techniques can provide opportunities to revisit and build on those core ideas. In turn, the absorption of IR radiation by molecules first introduced in the context of IR spectroscopy in WM will be revisited in the second year of the course in Oceans (O), where learners discover how absorption of IR by gases in the atmosphere contributes to the ‘greenhouse effect’.
See the Delivery Guide on Energy and matter for more information about the interactions between matter and radiation.
Mass spectrometry of isotopes reinforces teaching of nuclear structure. In both IR and NMR spectroscopy, the idea of isotopes can be revisited. Up to this point, the existence of isotopes is often seen as simply a means to an end when it comes to relative atomic mass calculations, or as something that only matters in nuclear equations. The application of isotopes in carbon-13 NMR, along with the differences in bond strength caused by substitution of a hydrogen atom for a deuterium atom, will help to demonstrate to learners the important physical variations between atoms with the same electronic configuration.
Use and consolidation of the terms ‘mobile phase’ and ‘stationary phase’ while discussing paper and thin layer chromatography will set learners up for understanding gas chromatography later in the course.
Mass spectrometry is first applied in Elements of life (EL), to the relative abundance of isotopes. A context for this is provided by the fact that relative isotopic abundance is not the same in every sample: it may depend on the location the sample was taken from, and it can change over time (due to radioactive decay). Thus, measurement of isotopic abundance is used to determine the source or age of samples and artefacts.
In What’s in a medicine (WM), learners must apply mass spectrometry to the identification of organic compounds. Many contexts are possible here; the storyline itself introduces the technique in the context of medicines, and so learners can be given problems centred around analysing samples of medicines (whether naturally occurring or synthetic), or the analysis of medical samples such as blood or urine, for the presence of drugs or organic compounds. This can be further extended into the realms of forensic analysis.
IR spectroscopy is similarly introduced in the context of medicines in WM, and similar contexts can be used in teaching; a well-known example is the detection of alcohol using breathalysers. A common approach is to combine teaching of this technique with discussion of the oxidation of alcohols, and the use of O–H and C=O absorptions to determine whether a carboxylic acid, aldehyde or ketone has been formed (or even to see if the alcohol has failed to oxidise).
IR spectroscopy is also used in astronomical research, and has led to the discovery of complex organic molecules in space. This forms an alternative, potentially very engaging, context for IR problems.
Chromatographical techniques are applied throughout the course to organic compounds, which again gives a very wide scope for potential applications. While TLC and paper chromatography are mainly used to check samples for purity or analyse mixtures with a small number of components, gas–liquid chromatography is often applied to much more complex samples. Potential contexts include the analysis of water samples for pollutants.
Colorimetery/spectrophotometry is introduced in Developing metals (DM), as a technique for determining concentrations of coloured solutions. It is introduced in the context of transition metal complexes, but the technique can be applied to any coloured solution, and so could be revisited in Colour by design (CD). It can enhance learning if learners are able to use a spectrometer or colorimeter to look at the absorbance of light at different frequencies by dyes, and to observe that absorption is at the complementary colour to that observed in reflection. They should then compare the spectra of apparently similar pigments – this can be by using the context of art restoration or forgery.
OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. We update our resources on a regular basis, so please check the OCR website to ensure you have the most up to date version.
© OCR 2015 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.