R3 What are the mechanisms of chemical change?
This final section of the Reactivity strand looks at how electrons rearrange in different ways during chemical reactions. The first part covers Brønsted-Lowry acids where a pair of electrons split heterolytically to donate a proton and form a conjugate base, whereas Brønsted-Lowry bases accept a proton. The next parts cover firstly redox reactions caused by electron transfer and finally electron and electron-pair sharing reactions which cover free radical substitution, and reactions involving nucleophiles and electrophiles
R3 What are the mechanisms of chemical change?
Introduction
This is the largest of the six main topics with 24 teaching hours recommended for Standard Level and 45 hours teaching time recommended for Higher Level. It is divided into three main strands which in the past have essentially gone under the headings of Acids and bases (R3.1), Oxidation and reduction (R3.2) and Organic chemistry (R3.3 & R3.4).
Topic R3.1 - Proton transfer follows on neatly from Topic R2.3 - How far? as much of the chemistry of acids and bases concerns equilibrium in aqueous solution. Since students will already have come across some classic acid-base reactions I usually start with these and do some introductory practical work before discussing the Brønsted-Lowry theory involving proton transfer. Clearly the topic lends itself to much practical work including titrations, test-tube reactions, the use of data loggers and pH probes etc. and simulations. Because measurements are easily made many of the practicals provide good scaffolding in preparation for the internally assessed individual scientific investigation.
The core part of topic R3.2 - Electron transfer reactions (which essentially translates as redox reactions) involves the definitions of oxidation and reduction and the concepts of oxidation states and oxidation numbers. It includes simple half-equations and oxidizing and reducing agents together with some idea about an activity series. Simple examples are given of spontaneous reactions to produce electricity (voltaic cells) and non-spontaneous reactions where electricity is used to bring about reactions (electrolysis involving molten electrolytes). A more quantitative approach is included in the Additional Higher Level material where prominence is given to the electrochemical series, more complex redox reactions and the electrolysis of aqueous solutions (including the factors affecting the discharge of products). The importance of spontaneous and non-spontaneous as applied to redox reactions is stressed and the relationship between the Gibbs energy and the electromotive force of the cell, ΔG⦵ = − nFE⦵ is covered.
The whole topic lends itself to good practical work - both simple test-tube redox reactions and quantitative redox reactions.
The third strand is actually subdivided into R3.3 Electron sharing reactions and R3.4 Electron pair sharing reactions which essentially cover organic reactions as the nomenclature and classification of organic compounds etc. has already been covered in S3.2 Functional groups - classification of organic compounds and for Higher Level students the stereochemistry and identification of organic compounds using spectroscopy in S3.2 (AHL) Functional groups - stereoisomerism and S3.2 (AHL) Mass spectrometry, IR & 1H NMR spectroscopy. The oxidation of alcohols and the reduction reactions leading to the formation of alcohols have also already been covered under electron transfer reactions in R3.2.
R3.3 is concerned with free radical substitution reactions and R3.4 covers nucleophilic substitution and electrophilic addition reactions and includes the use of 'curly arrows'. The AHL material for electron pair sharing reactions includes Lewis acid and base theory and also electrophilic substitution reactions as well as looking in greater depth at the mechanisms of nucleophilic substitution and electrophilic addition.
Some points to consider
Students should be able to recall the characteristic properties of acids and bases and you should give them plenty of practice at writing equations for the reactions of acids with hydroxides, oxides, carbonates, hydrogencarbonates and other bases such as ammonia as well as reactive metals. The distinction between strong and weak acids is covered and this emphasises the importance of language as students should be able to distinguish between the words “strong”, “concentrated” and “corrosive” when applied to an acid. The concept of the pH scale is included and at both Standard and Higher Level calculations involving the formula pH = − log10[H+(aq)] are required. At both levels students also need to understand the use of indicators and pH meters in titrations. At Higher Level more types of calculations are required including the manipulation of logarithms for pH, pOH, Ka, Kb, pKa and pKb calculations and buffer calculations. For this reason I usually leave teaching this topic until the second year of the course when students should be more able to cope with the mathematics. However, as with Equilibrium calculations, they are not expected to solve quadratic equations but use approximations instead.
For R3.2 stress that much of the energy involved in life on Earth from photosynthesis, respiration, the burning of fossil fuels and even the energy to power students’ smart phones is all the result of redox reactions.
Both the person that holds it and the smart phone itself are powered by redox reactions.
Clearly this is an important topic for students to understand but it is also a topic which relates intimately to the whole of the rest of the chemistry programme. Rather than teach it in isolation links can (and should) be continuously made to all of the other topics.
Just some examples you could use are:
- The use of half-equations and quantitative calculations from redox equations.
- Electron arrangement to show oxidation is loss of electrons and reduction is gain of electrons.
- The trend from reducing agents to oxidizing agents across the elements of Period 3 and the variable oxidation states of transition metals.
- The redox reaction between sodium and chlorine to exemplify ionic bonding.
- Enthalpies of combustion as fossils fuels are oxidized by oxygen.
- The use of catalysts in redox reactions such as the decomposition of hydrogen peroxide or the formation of ammonia in the Haber process.
- The voltaic cell formed from two half-cells will have zero voltage once the redox reaction between the two half-cells has reached equilibrium.
- One of the characteristic reactions of an acid is the redox reaction between the acid and a reactive metal to produce hydrogen.
- The free radical substitution reaction between chlorine and alkanes in ultraviolet light is also a redox reaction.
- Uncertainties when calculating results from redox titrations and when using apparatus such as a voltmeter.
There are also many excellent links to both the Nature of Science (NOS) and Theory of Knowledge (TOK) that can be made. These include the ‘phlogiston theory’, the use of language when naming compounds, the validity of assumptions when devising the rules of oxidation states and the questioning of definitions of redox in terms of electron transfer.
The organic reactions covered in R3.3 and R3.4 provides a useful tool to reinforce many of the concepts that students have already met. How many teachers stress that many organic reactions are in fact redox reactions as there is a change in the oxidation state of carbon during the course of the reaction? At Higher Level the syllabus also now clearly links the movement of electron pairs (shown by the use of curly arrows) as examples of Lewis acid-base theory.
Each of the accompanying pages give you thorough coverage of each of the R3 sub-topics including further insight, teaching tips, slide galleries and further resources together with ten multiple choice quiz questions and short answer questions all with worked answers.
R3.1 Brønsted-Lowry acids and basesOnce you have finished teaching the whole topic you can give the 20 question multiple choice tests on S3 (together with worked answers):
R3 Proton transfer reactions