Energy and matter
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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(v) the electromagnetic spectrum in order of increasing frequency and energy and decreasing wavelength: infrared, visible, ultraviolet
EL(w) transitions between electronic energy levels in atoms:
(i) the occurrence of absorption and emission atomic spectra in terms of transition of electrons between electronic energy levels
(ii) the features of these spectra, similarities and differences
(iii) the relationship between the energy emitted or absorbed and the frequency of the line produced in the spectra, \(\displaystyle \Delta E = h\nu\)
(iv) the relationship between frequency, wavelength and the speed of electromagnetic radiation, \(\displaystyle c = v\lambda\)
(v) flame colours of Li+, Na+, K+, Ca2+, Ba2+, Cu2+
OZ(r) the formation and destruction of ozone in the stratosphere and troposphere; the effects of ozone in the atmosphere, including:
(i) ozone’s action as a sunscreen in the stratosphere by absorbing high-energy UV (and the effects of such UV, including on human skin)
(ii) the polluting effects of ozone in the troposphere, causing problems including photochemical smog
OZ(s) the principal radiations of the Earth and the Sun in terms of the following regions of the electromagnetic spectrum: infrared, visible, ultraviolet
OZ(t) the effect of UV and visible radiation promoting electrons to higher energy levels, sometimes causing bond breaking
OZ(u) calculation of values for frequency, wavelength and energy of electromagnetic radiation from given data
WM(j) the effect of specific frequencies of infrared radiation making specific bonds in organic molecules vibrate (more)
O(n) the ‘greenhouse effect’, in terms of:
(i) solar energy reaching Earth mainly as visible and UV
(ii) Earth absorbing some of this energy, heating up and radiating IR
(iii) greenhouse gases (e.g. carbon dioxide and methane) in the troposphere absorbing some of this IR, in the ‘IR window’
(iv) absorption of IR by greenhouse gas molecules increases the vibrational energy of their bonds, the energy is transferred to other molecules by collisions, thus increasing their kinetic energy and raising the temperature
(v) greenhouse gas molecules also re-emitting some of the absorbed IR in all directions, some of which heats up the Earth
(vi) increased concentrations of greenhouse gases leading to an enhanced greenhouse effect
DM(m)(ii) the origins of colour in transition metal complexes in terms of the splitting of the d orbitals by the ligands and transitions between the resulting electronic energy levels
CD(b) the structure of a dye molecule in terms of the chromophore and:
(i) functional groups that modify the chromophore
CD(m) the origins of colour (and UV absorption) in organic molecules
Approaches to teaching the content
Energy is not the same as matter, although the two are closely related and associated. Energy has the ability to cause a change in matter. This may be a physical change, e.g. a solid melting, or a chemical change producing new substances with new properties – a chemical reaction. All chemical reactions involve the transfer of energy.
An important theme running throughout several of the modules in the specification is the idea of electromagnetic waves as an energy source and the effect they can have on matter. In Elements of life (EL), the relationship between the frequency of a particular source of radiation, and its wavelength and energy is discussed, as well as the link between electronic energy and the absorption and emission of light. In The ozone story (OZ), the idea of UV absorption is extended into the role of UV energy in atmospheric chemical reactions. In the module What’s in a medicine (WM), the effect of IR radiation on molecules is dealt with.
The A level module Oceans (O), discusses the link between the absorption of IR radiation and possible ‘global warming’. Finally, in the A level modules Developing metals (DM) and Colour by design (CD), the interaction between visible light and d-block compounds and organic dye molecules, respectively, is explored.
Electronic energy changes
Having established the electromagnetic spectrum as an energy source, the nature of the interaction between the different regions of the spectrum and matter is gradually considered. Building on GCSE ideas, the origin of flame colour is developed in EL in terms of excited electrons re-emitting energy, in the form of light, by dropping back down quantised electronic energy levels.
A useful starting point is for learners to use their mobile phones to video the flame colours produced by various metal salts. This practical work can then be extended using direct vision spectroscopes. Although tricky to use, learners should be able to see different coloured lines ‘light up’, often against a background ‘rainbow’ spectrum (due to stray white light). Groups can then put together a presentation, using their videos and freely available online images of emission spectra, to further develop the concept of quantised electronic energy levels. The similarities and differences between emission and absorption spectra should also be discussed, as well as the idea of atomic line spectra being unique to the individual elements.
The concept of wave–particle duality can be touched on here, with the alternative idea of the different frequencies of light being regarded as carriers of specific packets of energy (quanta) or photons.
Linking energy to the frequency of a light source can be effectively shown using photosensitive paper and there are several practical possibilities (see Activities).
It is important that learners see, and clearly understand, simplified diagrams illustrating changes between electronic energy levels and the resultant spectra. Choosing the correct direction and relative size of arrows in such diagrams, and relating to the type and pattern of spectra, requires practice.
Changes in electronic energy due to the absorption of frequencies of light in the visible are revisited in later units (see ‘Conceptual links to other areas of the specification’).
In O, the theme of UV and visible light promoting electrons to higher energy levels is continued, with the consequence that, if the frequency (and therefore energy) of the light form is high enough, the interaction with some molecules can cause homolytic bond fission, resulting in radical formation.
The excellent Harvard lecture demonstration of the reaction between hydrogen and chlorine, found on YouTube, shows how light of the correct frequency can cause bond fission with a very fast chain reaction resulting. Teachers can then relate this relatively straightforward example of initiation, propagation and termination to radical reactions in the ozone layer. The light-activated bromination of hexane is a slightly calmer practical way of reinforcing this.
Quantitative ideas are also introduced in this unit with calculations involving the relationship between frequency, energy and bond enthalpy being explored. In such calculations, learners need to have a sound basis of the units used. For example, calculating energy using the relationship \(\displaystyle \Delta E = h\nu\) will give an answer in joules; if this energy is to be related to bond breaking then learners must be clear that a multiplication by the Avogadro constant is required, as well as the conversion, to or from, kJ (bond enthalpies measured in kJ mol−1).
Vibrational energy changes
In the module WM, the effect of IR radiation on covalent molecules is examined. Here the lower energy of IR radiation causes an increase in vibrational energy in bonds rather than in electronic energy. Because different bonds have different bond strengths (bond enthalpies), different frequencies of IR are needed to cause an increase in vibrational energy for the different bonds, and this leads us to IR spectroscopy as an analytical technique.
A very useful explanation of the fundamental ideas of IR spectroscopy comes from the Royal Society of Chemistry’s Learn Chemistry resources. This resource allows teachers (or learners) to interrogate IR spectra of relatively simple molecules, and links the IR absorptions with animations of the different bond vibrations. This visual approach helps learners grapple not only with how IR works and how it can be used, but also with the consequences of bond vibration, such as potential global warming.
These effects of infrared radiation are picked up again in the second year, when the mechanism of the ‘greenhouse effect’ is discussed in O. Here, the strength of absorption for specific bonds in the IR spectrum can be correlated with the greenhouse factor of a gas containing those bonds. Calculations involving Planck’s constant can also be reintroduced at this stage as a means for learners to decide which bonds in molecules are responsible for absorption of infrared radiation.
Nuclear magnetic resonance (NMR) spectroscopy
The introduction of NMR spectroscopy in Polymers of life (PL) does not require an understanding of how the technique works in terms of low-energy radiowave quanta causing changes to the energy of certain atomic nuclei. However, you may wish to give a brief introduction. A simple, practical demonstration using plotting compasses (with one having the glass top removed) in the presence of a magnetic field, such as produced by a large bar magnet, can be used to illustrate ideas.
- Energy can be given to the free magnet by twisting the pointer against the applied field. On letting go, the pointer will spin back, releasing energy. This is analogous to how atomic nuclei are manipulated in NMR.
- Changing the strength of the applied field makes it harder/easier to twist the needle against the field.
- The same effect would be found if the strength of the free magnet were changed for a fixed applied field (changing the local field, i.e. proton environment).
- In ‘real’ NMR, the energy source is radiofrequency.
Learners often find NMR difficult; while simplistic, any practical analogy will help their confidence.
It might be, at this stage, an appropriate time to draw together the effect of the interaction of the different areas of the electromagnetic spectrum on matter, including changes in rotational energy brought about by absorption in the microwave region. The link between the frequency of the light form and the size of quanta absorbed should be stressed.
Common misconceptions or difficulties learners may have
The origin of colour
Probably the most common misconception some learners take with them into examinations is in associating the origin of colour in pigments or dyes to emission. It is important that learners realise that pigments and dyes look coloured because our eyes only see those wavelengths of light that have not been absorbed by the coloured material. Excited electrons do lose their absorbed energy, but not as photons in the visible range, hence the colour seen is not due to emission of such photons. The excess energy in pigments and dyes is lost as kinetic energy via vibrational, rotational and translational changes.
Calculations using the links between energy and frequency or wavelength and bond enthalpy often cause learners some difficulty, despite the fact that most simply involve a reorganisation of the salient expressions. The powers of 10 involved in these calculations often put learners off and careful practice is necessary.
The ‘greenhouse effect’
When describing the sequence of events that leads to the creation of the greenhouse effect, learners often confuse the process of absorption and re-emission of radiation by the Earth as reflection – perhaps seeing the layer of greenhouse gases as some sort of ‘one-way mirror’ that allows heat through in one direction but not the other. There is a good online simulation by PhET which uses different colours to represent photons of visible or infra-red light and also demonstrates what is happening to molecules when radiation is absorbed.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
The colours of d-block metal compounds
Changes in electronic energy brought about by the absorption of frequencies in the visible are important in understanding why d-block compounds are usually coloured. Thus, in DM, the connection between electronic structure and the splitting of d-orbital energies should be discussed. Key in this dialogue should be a consideration of those factors that control the relative size of the energy gap and hence, ultimately, the colour of a particular ion.
The role of absorption of light in the analytical technique of colorimetry is also introduced in DM. Here, emphasis should be based on the link between the concentration of the absorbing species and the value of the absorption as indicated by the colorimeter reading. The ideas here allow the use of colorimetry, not only as a technique to determine the concentration of a coloured compound, but also in kinetics experiments involving coloured species.
The colours of organic molecules
In the module CD, the origin of colour in organic dye molecules is explored. Here, yet again, the idea of an electronic energy gap is key in any explanation. In this case, we also have to introduce, or perhaps re-introduce, the idea of delocalised electrons and their associated energy levels, the extent of the delocalisation being responsible for the particular frequencies absorbed. Thus, benzene only absorbs in the UV region and hence is colourless, but an azo dye, with more extended delocalisation, absorbs in the visible region (and is, therefore, coloured). This is because the gap between upper, excited, delocalised electronic energy levels and lower levels gets smaller with increasing delocalisation, allowing lower energy photons (i.e. in the visible region) to cause promotion of electrons from lower to upper levels.
This topic enjoys a wide range of interesting and colourful contexts, which can be used to reinforce learners’ understanding.
Elements from the sea (ES) provides simple examples of photochemical reactions with the darkening of silver halides and their consequent use in photographic film.
This introduces learners to the idea that light is another way of providing the necessary energy to overcome activation energy barriers for many reactions. This concept is quickly further developed in OZ, which looks at the effect of UV light on the skin and the dangers of bond fission damaging the delicate molecules in the skin. This context can be followed up by a consideration of sunscreens (including the ozone layer itself).
The module WM allows learners to examine the IR spectra of medicinal molecules and the information it provides, and this is taken further in PL where IR and NMR spectra are used to determine the molecular structure in a wide range of organic molecules. (See also the Delivery Guide on Analytical techniques for more information about spectroscopic techniques and the interpretation of spectra.)
The role of IR radiation is considered in a global context in O where learners consider the ‘greenhouse effect’ and the idea of global warming.
DM looks at d-orbital splitting in the context of pigments used in art. The key ideas here involve factors that affect the magnitude of the d-orbital split and consequently the photons of light necessary to promote d electrons into higher energy orbitals. Learners often find this topic very interesting and it is possible, for more able learners, to edge into the idea of interactions between orbitals being important in determining how d-orbitals split and the magnitude of the splitting.
Finally, CD explores dye molecules and the role of molecular structure in the absorption of visible light. Experimental work on the synthesis of different azo dyes can be used to show the effect of increasing the conjugated system/delocalisation on the frequency of light absorbed. Great care must be taken with this work to prevent learners becoming confused with colour due to emission.
A sample of glass is made by heating a mixture of lead oxide, zinc oxide and boric acid strongly until it melts. The glass formed is coloured by adding traces of various transition metal oxides.
This unusual context illustrates the different colours produced when the magnitude of the d-orbital splitting changes.
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