Identifying unknowns AS
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3.1.4 Qualitative analysis
(a) qualitative analysis of ions on a test-tube scale; processes and techniques needed to identify the following ions in an unknown compound:
- CO32–, by reaction with H+(aq) forming CO2(g) (see 2.1.4 c)
- SO42–, by precipitation with Ba2+(aq)
- Cl–, Br–, I– (see 3.1.3 g)
(ii) cations: NH4+, by reaction with warm NaOH(aq) forming NH3.
4.2.4 Analytical techniques
(a) infrared (IR) radiation causes covalent bonds to vibrate more and absorb energy
(b) absorption of infrared radiation by atmospheric gases containing C=O, O–H and C–H bonds (e.g. H2O, CO2 and CH4), the suspected link to global warming and resulting changes to energy usage
(c) use of an infrared spectrum of an organic compound to identify:
(i) an alcohol from an absorption peak of the O–H bond
(ii) an aldehyde or ketone from an absorption peak of the C=O bond
(iii) a carboxylic acid from an absorption peak of the C=O bond and a broad absorption peak of the O–H bond
(d) interpretations and predictions of an infrared spectrum of familiar or unfamiliar substances using supplied data
(e) use of infrared spectroscopy to monitor gases causing air pollution (e.g. CO and NO from car emissions) and in modern breathalysers to measure ethanol in the breath
(f) use of a mass spectrum of an organic compound to identify the molecular ion peak and hence to determine molecular mass
(g) analysis of fragmentation peaks in a mass spectrum to identify parts of structures
Approaches to teaching the content
Qualitative analysis is essentially a practical skill and can be introduced by simple test tube experiments. For example, an unlabelled sample could be tested alongside the chloride, bromide and iodide when students investigate the reactions between halides and silver nitrate (specification section 3.1.3g).
Spectroscopy will probably involve looking at spectra of simple compounds with emphasis on pattern recognition. There is no need to explain the practical basis of these techniques (especially as the modern manifestations of these techniques bear little resemblance to textbook descriptions).
Common misconceptions or difficulties students may have
Analytical techniques involve moving from the particular to the general and concentrating on relevant observations, whether in test tubes or copies of spectra. Many students take time to adjust to ideas of pattern recognition.
Students often struggle with the precise description of changes of state, for example mistaking a suspension/ precipitate for a ‘cloudy solution’ or suggesting that ‘the solution goes white’. They also have difficulty moving from these macroscale observations to generalisations at the particle level.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
Qualitative inorganic analysis provides students with opportunities to develop their skills of observation and interpretation. They have to distinguish changes of state, describe them precisely and deduce the likely identity of products. The topic provides opportunities to practise writing balanced chemical and ionic equations and to revise the reactions of acids and alkalis in terms of H+ and OH- ions.
The interpretation of IR and mass spectra will be set in the context of Organic Chemistry. Students will have to draw displayed formulae, showing all bonds, in order to suggest sensible identities for fragments formed during mass spectrometry, while the interpretation of IR spectra generates familiarity with different functional groups.
The learner resource provides practice at identifying which ions are involved in each reaction and at eliminating ‘spectator ions’ from the ionic equation. It includes precipitation reactions and the reaction of a carbonate with an acid.
This resource is intended for students aged 14-16 but parts would be useful at AS Level. The first two pages of the student worksheet use simple models to explain the processes going on when a precipitate forms. The final page, the production of lead iodide, is a good way of testing whether students are able to apply these ideas.
These activities can be projected onto a whiteboard and used as a quick check of understanding. Starters 10.1.3 and 10.2 are on MS and IR spectroscopy respectively. The answers and a suggested mark scheme are provided so students can check their own work.
Inorganic analysis is essentially a practical activity. It will form part of the practical assessment at A Level so students must practice making the correct observations as well as being able to interpret their results.
Spectroscopy is likely to be taught by reference to printed spectra but these can be placed in various contexts throughout the Organic Chemistry course. For example, when studying the conversion of an alcohol to a halogenoalkane students could compare the spectra of the reactant and product. They could look at the spectra of both possible products when discussing the oxidation of primary alcohols. When a new compound is encountered, students could use its mass spectrum and percentage composition to calculate the molecular formula. (Relevant spectra can be downloaded from the internet. See below for links to some online sources).
Students may be able to experience practical IR spectroscopy by visiting local HE institutions or through the Royal Society of Chemistry’s 'Spectroscopy in a Suitcase' scheme.
(The resource includes a test for nitrate ions which is not on the specification)! Simple test-tube reactions allow students to discover the expected observations when testing for anions. To fulfil the requirements of the specification, students will have to be taught to test for carbonates first and only proceed to test for halide or sulphate ions when carbonate has been eliminated.
This is an illustration of bond vibration at characteristic frequencies.
Hang some 100g masses from a spring. The spring will oscillate at a steady frequency which changes when masses are added or removed. This is an analogy for the difference in natural frequency of vibration of covalent bonds when atoms of different masses are present, for example C-H and O-H bonds. Adding a second spring in parallel to the first also changes the frequency, analogous to the difference between C-O and C=O bonds.
The topic can be introduced by issuing students with spectra for some simple molecules. They work together to draw displayed formulae for the compounds and try to identify how the differences in structure are related to the presence of additional peaks on the spectrum. Students often mis-identify C-H peaks as O-H groups. This activity will help them to realise that almost all organic spectra contain a C-H stretch so they will have to look for a further peak in this part of the spectrum to identify an O-H. PowerPoint slides for this sort of activity can be found by clicking on the link.
This booklet contains a wealth of internet links and information for teachers.
Student worksheet CA1 is a good introduction to the science of the greenhouse effect. It deals with the black-body emission spectra of the sun and the earth and absorption of IR radiation by atmospheric gases such as CO2 and water vapour. Details about changes in dipole moment associated with bond vibrations are not on the specification but help to link IR spectroscopy with aspects of covalent bonding.
This resource concentrates on the steady-state between incoming and outgoing radiation. It is mainly aimed at 14-16 year olds but parts can be used at AS Level, for example the modelling spreadsheet MCC_EnergyBalance. Students can download and use this to model the effect on global temperatures of changes in the amount of energy absorbed by the atmosphere.
This is an introductory task intended for group work. Students work through example spectra to identify the molecular and (M+1) peaks and suggest identities for some fragments. Although the principles of mass spectrometry are not on the specification students have to realise that only charged particles will be separated and detected.
Schools which are members of CLEAPSS can download booklet L202, which provides some background information about spectroscopy together with IR, mass and NMR spectra for 40 common compounds. A smaller range of spectra can be downloaded from the second link.
The National Institute for Science and Technology (U.S.) and the National Institute of Advanced Industrial Science and Technology (Japan) are searchable databases which can provide spectra for a wide variety of compounds.
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