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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 which best suit particular classes, learning styles or teaching approaches.
Content (from A Level)
Learners should be able to demonstrate and apply their knowledge and understanding of:
DF(h) the terms catalyst, catalysis, catalyst poison, heterogeneous
DF(i) a simple model to explain the function of a heterogeneous catalyst
OZ(e) the term activation enthalpy; enthalpy profiles
OZ(f) the effect of concentration and pressure on the rate of a reaction, explained in terms of the collision theory; use of the concept of activation enthalpy and the Boltzmann distribution to explain the qualitative effect of temperature changes and catalysts on rate of reaction; techniques and procedures for qualitative experiments in reaction kinetics including plotting graphs to follow the course of a reaction
OZ(g) the role of catalysts in providing alternative routes of lower activation enthalpy
OZ(h) the term homogeneous catalysis and the formation of intermediates
CI(a) the terms:
(i) rate of reaction
(ii) rate constant, including units
(iii) order of reaction (both overall and with respect to a given reagent), use of '\(\displaystyle \propto\)'
(iv) rate equations of the form: rate = k[A]m[B]n where m and n are integers; calculations based on the rate equation; the rate constant k increasing with increasing temperature
CI(b) the use of given data to calculate half-lives for a reaction
CI(c) techniques and procedures for experiments in reaction kinetics; use of experimental data [graphical methods (including rates from tangents of curves), half-lives or initial rates when varying concentrations are used] to find the rate of reaction, order of a reaction (zero-, first- or second-order), rate constant and construction of a rate equation for the reaction
CI(d) the Arrhenius equation and the determination of Ea and A for a reaction, given data on the rate constants at different temperatures
CI(e) the term rate-determining step; relation between rate-determining step and the orders and possible mechanism for a reaction
CI(g) the determination of the most economical operating conditions for an industrial process using principles of equilibrium and rates of reaction
PL(f) the shape of the rate versus substrate concentration curve for an enzyme-catalysed reaction; techniques and procedures for experiments involving enzymes
PL(g) the characteristics of enzyme catalysis, including: specificity, temperature sensitivity, pH sensitivity, competitive inhibition; explanation of these characteristics of enzyme catalysis in terms of a three-dimensional active site (part of the tertiary structure)
APPROACHES TO TEACHING THE CONTENT
Chemical kinetics is a key topic at AS and A Level and ideas such as catalysis, rate of reaction, experimental techniques and procedures in kinetics and application of kinetics are developed through the course.
Learners will have already met the idea of catalysis at Key Stage 4 and are reminded of the meaning of catalyst and catalysis in Developing fuels (DF). The dramatic demonstration ‘Elephant’s toothpaste’ – the decomposition of hydrogen peroxide coloured by food dyes and catalysed by metal oxides – can be used to introduce this topic. The emphasis at this stage is on heterogeneous catalysis; for many learners it will be the first time a simple model is introduced to explain how such a catalyst works. This model also helps learners understand how the function of a catalyst can be impaired by a catalyst poison.
Kinetic ideas are met again in The ozone story (OZ) when the term activation enthalpy is introduced in the context of enthalpy profiles. These ideas also build upon the concept of energy level diagrams that have been met during the study of energetics in DF. Enthalpy profiles can be drawn for catalysed and uncatalysed reactions on the same diagram to bring out the key point that catalysts work by providing an alternative reaction route that has a lower activation enthalpy than the uncatalysed route. This means that more particles in the reacting system will have sufficient energy to be involved in a collision leading to a reaction so that the rate of reaction is higher (this ties in with discussion of the Boltzmann distribution – see ‘Rates of reaction’ below). Here is an opportunity to emphasise that the activation enthalpy of the uncatalysed reaction has not been lowered. It remains the same. The key point is that the catalyst provides an alternative route with a lower activation enthalpy.
It is helpful to use enthalpy profiles to explain the model of how a heterogeneous catalyst works, met in DF, since learners often find making such links between different modules difficult. Adsorption of reactant molecules on the surface of a catalyst in a reaction between gases, for example, provides an alternative route to collisions of molecules in the gas phase. The reaction on the surface of the catalyst involves a lower activation enthalpy since bonds are weakened by the process of adsorption.
The idea of a homogeneous catalyst is introduced in OZ. This will be a new idea to most learners and carefully worded questions can be used to bring out the similarities and differences between homogeneous and heterogeneous catalysis. The general idea that all catalysts work by providing an alternative route with a lower activation enthalpy still applies but the model used for a heterogeneous catalyst is not applicable. The function of intermediate formation during homogeneous catalysis is introduced as equivalent to the solid surface during heterogeneous catalysis. Drawing an enthalpy profile with a slight dip in the curve of the catalysed reaction to represent the formation of an intermediate is an effective way of emphasising the importance of this part of the alternative route for the reaction.
Teachers using the reaction between peroxodisulfate ions and iodide ions to explore rates of reaction may wish to use this opportunity to explain why this reaction is catalysed by iron(II) ions in terms an alternative route involving the formation of iron(III) ions as an intermediate. Alternatively, this explanation could be delayed until the oxidation states of transition metals are discussed in module DM.
The effect of a homogeneous catalyst can be effectively illustrated by some visually impressive demonstrations. The reaction between sodium thiosulfate and hydrogen peroxide is catalysed by a small amount of ammonium molybdate(VI). If a few drops of alkaline universal indicator are added to the reaction mixture a colour change enables the progress of the catalysed and uncatalysed reaction to be compared.
The role of heterogeneous catalysts is further developed in the context of enzymes in Polymers and life (PL). The function of the enzyme as a catalyst is approached through an explanation of the shape of a reaction rate against substrate concentration graph. Here is an opportunity to develop the model of heterogeneous catalysis introduced in DF to include a description of the active site of an enzyme. This concept uses ideas about ionic and covalent bonds introduced in EL and intermolecular bonds introduced in OZ to explain substrate interactions and helps learners understand why the enzyme catalyst provides an alternative reaction route with a lower activation enthalpy. Learners can be given data to plot their own graph of reaction rate against substrate concentration and asked to interpret its shape in terms of substrate–enzyme interactions. The hydrolysis of urea, catalysed by the enzyme urease, is a suitable example.
A secure understanding of the three dimensional structure of the active site of enzymes and the bonds that hold the tertiary structure of the enzyme in place allows learners to make sense of the characteristics of enzyme catalysis. These include specificity, sensitivity to temperature and pH, and competitive inhibition. Carefully worded questions can be used to ask learners to explain these characteristics in terms of the shape of the active site and the bonds involved in maintaining this shape. Changing the temperature and/or pH of a solution of hydrogen peroxide to which the enzyme catalase is added in the form of a potato extract can be used to illustrate some of the enzyme catalysis characteristics.
Rates of reaction
Learners should be aware of the effect of concentration of a reactant and the pressure in a gaseous mixture on the rate of reaction from Key Stage 4, usually with an explanation in terms of collision theory. These ideas are revisited in OZ.
The qualitative effect of temperature on reaction rate is also introduced in OZ. To explain this effect the Boltzmann distribution of the energy of reacting particles at different temperatures is needed together with the concept of activation enthalpy. Understanding of the way in which temperature affects the rate of reaction is deepened in the second year of the course when the Arrhenius equation is introduced in The chemical industry (CI). This allows learners to use their own experimental or given data to determine the activation enthalpy for a reaction and to calculate the value of A in the Arrhenius equation.
Several simple examples can be used to illustrate the qualitative effect of concentration and temperature on reaction rate, of which the reaction between magnesium and dilute acids to produce hydrogen is a very straightforward and reliable example. The ‘iodine clock’ reaction between peroxydisulfate and iodide ions can be carried out at different temperatures to enable an ‘Arrhenius’ graph to be plotted from which Ea and A can be determined.
Learners are moved on from a qualitative view of kinetics to a quantitative approach in CI. Rate equations are developed which requires understanding of rate of reaction, rate constant and order of reaction. A good way of helping learners develop their understanding of these topics is to provide them with data showing the relative rate of reaction when the concentration of reactants is changed. It is important that they realise that they can only be sure about the effect of the concentration of a given reactant if the concentrations of other reactants remain constant.
A small but sometimes overlooked point that can be covered at this time is that the rate constant increases with temperature. This is why the rate of reaction depends upon temperature.
Finding the kinetic half-life of a reaction is particularly useful for confirming that a reaction is first order because the half-life in this case is constant. The catalytic decomposition of hydrogen peroxide lends itself to this kind of study.
Learners need to be able to explain why the order of a reaction may be different for different reactants. To do so they need to understand the idea of rate-determining step and to appreciate that this is linked to possible mechanisms for the reaction. Many learners will find this aspect of kinetics quite difficult and will need particular help in suggesting reaction mechanisms from orders of reaction.
Techniques and procedures
Reaction kinetics is very much an experimental area of study since this is the only way that the order of a reaction, for example, can be determined. In OZ learners are expected to know about and make use of techniques and procedures for qualitative experiments in kinetics, including plotting graphs to follow the course of reaction. Good experiments to use are measuring the volume of hydrogen produced over time in the reaction of magnesium with acids, and the volume of oxygen produced during the catalytic decomposition of hydrogen peroxide. These can be used for PAG 9 of the Practical Endorsement.
The practical techniques and procedures are developed further in CI, including the use of experiments to generate data that can be used to find the rate of reaction at different times by drawing tangents to progress curves or by finding the initial rate of reaction when varying concentrations of reactants. Kinetic half-lives can also be calculated from progress curves. This kind of information should be used by learners to determine order of reaction, the rate constant and rate equation for the reaction. Measurement of rate of reaction by an initial rate method can be used for PAG 10 of the Practical Endorsement.
Learners can use a range of experiments to explore the effect on reaction rate of changing the concentration of reactants, and so determine the order of reaction with respect to specific reactants and for the overall reaction. These include ‘clock reactions’ such as the those between peroxydisulfate and iodide ions and between bromide and bromate ions in acid solution, as well as reactions where change is monitored continuously such as the reaction of magnesium with acids or the reaction of propanone with iodine in acid solution.
Common misconceptions or difficulties students may have
One of the misconceptions that some learners have is that all chemical reactions are fast. The problem with this idea is that changes such as rusting that take place in a time frame much longer than a typical chemistry lesson may not be regarded as chemical reactions at all. One way of addressing this view is to discuss a range of changes such as the reaction of sodium with water, letting milk go sour and dissolving rocks to highlight the variation in rates of reaction.
Another confusion that becomes apparent when looking at energy changes during reactions is the idea that fast reactions are always exothermic. This may arise because exothermic reactions such as the reaction between magnesium and acids are often used to introduce rates of reaction. Learners with this view can find it hard to understand why some exothermic reactions are actually very slow because they have not appreciated that the rate depends in part on the activation enthalpy. They may be further confused when they find that some very vigorous reactions such as that between sodium hydrogencarbonate and acids are endothermic.
Catalysts are another source of misconceptions. It is quite common for learners to write that a catalyst ‘lowers’ the activation enthalpy. This view suggests that the learner has not sufficiently understood that the activation enthalpy of the reaction remains unaltered. The catalyst provides an alternative route for the reaction which involves a lower activation enthalpy than the uncatalysed reaction route.
The function of enzymes as catalysts may also be problematic. Learners who also study biology may describe the behaviour of enzyme and substrate in terms of a ‘lock and key’ model. Unfortunately they may view this idea as an ‘explanation’ of the function of an enzyme catalyst instead of realising that it is a helpful analogy. This can get in the way of a chemical understanding in terms of bonds holding the tertiary structure of a protein in a particular 3-D orientation and bonds between substrate molecule and parts of the active site.
Learners have been taught from an early stage in their study of chemistry that particles have a greater kinetic energy or move about more rapidly at higher temperatures. They use this idea to explain why the reaction rate increases with temperature arguing that particles will collide more frequently. This however is only a minor reason for the increase in reaction rate. The dominant factor is the increased number of successful collisions between particles because there are more collisions with energy greater than the activation enthalpy. Learners need to develop their thinking as they revisit the effect of temperature on reaction rate.
Another area in which learners have to modify and build upon earlier ideas is the effect of concentration on reaction rate. This topic is usually introduced in Key stage 4 using reactions that are first order with respect to the reagent investigated. This can cause learners to generalise and to think that the rate of reaction is always directly proportional to the concentration of all reactants. This can make understanding reactions that are second or zero order very difficult to accommodate within the existing conceptual framework. It may well be necessary to explicitly point out that initial ideas about the effect of concentration have to be modified to take account of more information. Introducing the concept of rate-determining step is a useful way of helping learners appreciate why reactants may be second or zero order.
An aspect of chemical kinetics that some learners find difficult is representing changes graphically and interpreting kinetics graphs. This is maybe because it is something they do not do very often. An effective way of addressing this issue is to ask learners to match a set of unlabelled graphs with a set of possible titles such as ‘concentration against rate for a zero order reaction’.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course.
The way that heterogeneous catalysts function is covered in DF. This paves the way for the use of catalysts in the laboratory and in industry, particularly in the context of organic chemistry.
Heterogeneous catalysts are used in cracking processes, for example the cracking of a hydrocarbon over a heated catalyst in DF. Also in DF, the reactions of alkenes are studied including the reaction with hydrogen to give an alkene using a nickel or platinum catalyst, and the reaction with water in the presence of a catalyst to give an alcohol. Later on, in WM, aluminium oxide is used as a heterogeneous catalyst in the reverse of this reaction, the dehydration of alcohols to form alkenes. Another aluminium compound, anhydrous aluminium chloride, is used as a catalyst in Friedel–Crafts alkylation and acylation reactions in Colour by design (CD).
Homogeneous catalysts are introduced in OZ. In DM, both homogeneous and heterogeneous catalysts are revisited in the context of the catalytic activity of transition metals and their compounds. Homogeneous catalysts are explained in terms of the variable oxidation states of transition metals whilst heterogeneous catalysts are explained in terms of the ability of transition metals to use 3d and 4s electrons of the atoms on the catalyst surface to form weak bonds to reactants. This topic provides an excellent opportunity to summarise the key points about catalysis that have been met throughout the course.
The term catalyst is defined in DF as a substance that is not used up during the reaction. This is a key idea to remind students about when discussing the chemical basis of the depletion of ozone in the stratosphere due to haloalkanes in OZ. It is important because the chlorine atoms become involved in catalytic cycles in which ozone molecules are converted to oxygen molecules and chlorine atoms are regenerated to take part in further cycles.
Catalysts are mentioned in CI when factors affecting equilibria are considered to point out that in contrast to changes in concentration, pressure and temperature they have no effect on the position of equilibrium or the magnitude of the equilibrium constant. This is because they increase the rate of both forward and reverse reactions equally. However, this increase in rate at which equilibrium is established is very important in industrial processes and must be taken into account when deciding on the most economical conditions for an industrial process. It is important that learners use principles of both equilibrium and rate of reaction to decide on optimum conditions. Asking learners to suggest optimum conditions for industrial processes, including the use of an appropriate catalyst, is an effective way of reinforcing these ideas.
A variety of short starter activities to help embed learning in the important skill areas of advanced chemistry. Covers:
Rate determining step
Calculating reaction rate
Measuring reaction rate in the lab
Determining the rate equation
Arrhenius and rate
The qualitative effects of concentration and temperature on the rate of a reaction are introduced initially in The ozone story (OZ), while looking at the rate of formation and breakdown of ozone in the atmosphere.
These ideas are developed in The chemical industry (CI), where a quantitative approach is added to introduce rate equations, rate constant and order of reaction in the context of making and using phosphate and sulfate fertilisers.
Rates of reaction are an important aspect of industrial processes as the manufacture of ammonia, sulfuric acid and nitric acid illustrate very clearly.
Catalysts are introduced in Developing fuels (DF) in the context of the use of catalytic converters in cars to reduce the amount of pollutants in exhaust emissions. This context is revisited in Developing metals (DM) where applications of transition metals are explored.
The role of catalysts is developed further in OZ, in considering the destruction of ozone in the stratosphere in catalytic cycles involving chlorine atoms from haloalkanes.
Finally, enzymes as catalysts are explored in Polymers and life (PL) during a study of proteins.
Module CI is a study of how chemists use industrial processes to benefit mankind. This module considers by way of case studies the manufacture of nitric acid via the production of ammonia, and the manufacture of sulfuric acid and ethanoic acid. The role of catalysts as part of the most economical operating conditions is a vital part of all of these processes.
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