# OCR AS/A Level Chemistry A

# Rates

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## Introduction

### Overview

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.

## Curriculum content

### Overview

**Content (from A Level) **

**3.2.2 Reaction rates**

(a) the effect of concentration, including the pressure of gases, on the rate of a reaction, in terms of frequency of collisions

(b) calculation of reaction rate from the gradients of graphs measuring how a physical quantity changes with time

(c) explanation of the role of a catalyst:

(i) in increasing reaction rate without being used up by the overall reaction

(ii) in allowing a reaction to proceed via a different route with lower activation energy, as shown by enthalpy profile diagrams

(d) (i) explanation of the terms homogeneous and heterogeneous catalysts

(ii) explanation that catalysts have great economic importance and benefits for increased sustainability by lowering temperatures and reducing energy demand from combustion of fossil fuels

with resulting reduction in CO_{2} emissions

(e) the techniques and procedures used to investigate reaction rates including the measurement of mass, gas volumes and time

(f) qualitative explanation of the Boltzmann distribution and its relationship with activation energy (see also 3.2.1 c)

(g) explanation, using Boltzmann distributions, of the qualitative effect on the proportion of molecules exceeding the activation energy and hence the reaction rate, for:

(i) temperature changes

(ii) catalytic behaviour (see also 3.2.2 c).

**5.1.1 How fast?**

(a) explanation and use of the terms: rate of reaction, order, overall order, rate constant, half-life, rate-determining step

(b) deduction of:

(i) orders from experimental data

(ii) a rate equation from orders of the form: rate = *k*[A]* ^{m}*[B]

*, where*

^{n}*m*and

*n*are 0, 1 or 2

(c) calculation of the rate constant, *k*, and related quantities, from a rate equation including determination of units

(d) from a concentration–time graph:

(i) deduction of the order (0 or 1) with respect to a reactant from the shape of the graph

(ii) calculation of reaction rates from the measurement of gradients (see also 3.2.2 b)

(e) from a concentration–time graph of a first order reaction, measurement of constant half-life, *t*_{1/2}

(f) for a first order reaction, determination of the rate constant, *k*, from the constant half-life, *t*_{1/2}, using the relationship: *k* = ln 2/*t*_{1/2 }

(g) from a rate–concentration graph:

(i) deduction of the order (0, 1 or 2) with respect to a reactant from the shape of the graph

(ii) determination of rate constant for a first order reaction from the gradient

(h) the techniques and procedures used to investigate reaction rates by the initial rates method and by continuous monitoring, including use of colorimetry (see also 3.2.2 e)

(i) for a multi-step reaction, prediction of,

(i) a rate equation that is consistent with the rate-determining step

(ii) possible steps in a reaction mechanism from the rate equation and the balanced equation for the overall reaction

(j) a qualitative explanation of the effect of temperature change on the rate of a reaction and hence the rate constant (see 3.2.2 f–g)

(k) the Arrhenius equation:

(i) the exponential relationship between the rate constant, *k* and temperature, *T* given by the Arrhenius equation, *k = A*e^{-Ea/RT}

(ii) determination of *E*_{a} and *A* graphically using: ln *k = –E*_{a}*/RT* + ln *A* derived from the Arrhenius equation.

## Thinking Conceptually

### Overview

**Approaches to teaching the content **

Students will be familiar with reaction rate and collision theory from GCSE but Module 3 develops the idea of activation energy and that the overwhelming majority of collisions do not lead on to reaction. Simulations are useful in showing this, although the frequency of successful collisions is necessarily greatly exaggerated. The activation energy concept can then be applied to the role of catalysts and to the effects of temperature on the Boltzmann distribution of molecular velocity.

Rate is a topic which provides plenty of opportunities for students to develop practical skills and gain greater familiarity with different ways of analysing their results, especially by using graphs to determine reaction rate, half life and activation energy.

The mathematical relationship between rate and concentration is introduced in Module 5 and applied to abstract ideas such as rate determining steps and reaction mechanisms. Plenty of examples will help students learn to apply these concepts.

**Common misconceptions or difficulties students may have **

Students will have an intuitive concept of what ‘rate of reaction’ means, probably in terms of the amount of product made in a certain time, but in Module 5 they are introduced to the more rigorous definition as ‘rate of change of concentration of a particular reactant’. They will also have difficulties with the concept of ‘zero order’, where, counter-intuitively, the concentration of one reactant has no effect on the rate of reaction.

The idea of a multi-step reaction in which one step is potentially slower than the rest is not easy to grasp but analogies may help students understand the idea of a rate determining step, as will references to some of the mechanisms they have already encountered in Organic Chemistry.

Many students will need support with the mathematical demands of this topic, which include rearranging equations, using logarithms, and calculating and using gradients. The Maths for Chemistry website may be helpful.

**Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course**

Rate and equilibrium are closely related and if students are introduced first to the effects of temperature on rate they can practice applying these to equilibrium conditions, where the rate of forward and back reactions are exactly balanced.

The concept of catalysts providing an alternative reaction pathway can be explored by reference to the role of radicals in the destruction of the ozone layer and the reactions between halogens and hydrocarbons.

Studies of rate of reaction provide evidence for the reaction mechanisms encountered in the Organic Chemistry modules. Possible rate-determining steps could be considered when these mechanisms are introduced or revised.

### Rate of reaction graphs (Royal Society of Chemistry: Assessment for Learning)

See section 35 in the link.

Students are presented with graphs for the reaction of marble chips and acid under different conditions and have to decide which graph relates to which conditions. This is a good introductory task to test recall and understanding from GCSE.

### Collision theory simulation (Simchemistry)

The rates and equilibrium model simulates collisions between particles, most of which are ‘unsuccessful’ even when the temperature is raised. Graphs are displayed showing the number of reactant and product particles against time. A quick introduction to collision theory and activation energy.

### Rate simulation (University of Colorado PhET project)

Students can use this simulation to experiment with the effects of temperature and concentration on reaction rate. There is a useful graphic which superimposes the average energy of the system on an enthalpy profile and counters to display how the numbers of reactant and product particles change. The simulation can also be used to investigate equilibria.

### Advanced rate graphs (TES)

Students are presented with unlabelled reaction graphs and asked to identify the axes and suggest a title. This is an excellent check to see whether they can distinguish between plots of concentration v time and rate v concentration.

### Rate determining step (Video: Richard Thornley)

This is one of a series of videos to support IB Chemistry but most are relevant to the A Level course. The videos are concise and very well put together. This one provides a simple visual analogy for a multi-stage reaction with a rate-determining step. Others explain the rate constant and its units, the graphical determination of activation energy and other topics which many students find difficult.

The whole IB Chemistry series is available by clicking on the second link. This is potentially a very useful site as an idea bank for teachers and for use by students.

### Assessment for learning (Royal Society of Chemistry: Learn Chemistry)

Section 7 deals with rate of reaction graphs and the Maxwell-Boltzmann distribution.

Section 1 has worksheets on the rate determining step, rate equations and the Arrhenius equation.

### Arrhenius presentation

### Reaction rates, focussing on Arrhenius

## Thinking Contextually

### Overview

Rate and order of reaction are empirical concepts so it is important that students have the chance to carry out experiments to measure rate and see how it is affected by concentration and temperature. Several different methods can be used for continuous monitoring of reactions, such as measuring volumes of gas produced, removing samples for titration and colorimetry (the latter lends itself to datalogging). Initial rates methods can also be used, such as the iodine-clock, where the concentration of iodide ions remains constant and the ‘end-point’ occurs when a certain amount of iodine has been produced.

When carrying out practical work students should be aware of relative amounts of each reactant so that they can decide which concentrations are changing significantly. For example, in the reaction between marble chips and HC*l* the reaction rate slows down as the acid is used up, whereas in the similar reaction between magnesium ribbon and HC*l* the acid is almost certain to be in excess.

In addition to the practical components this topic also contains mathematical and other theoretical concepts, such as reaction mechanism, activation energy and the Boltzmann distribution. These provide opportunities to use videos and ICT simulations. Students may also find ICT useful in graph plotting and for analysing the results of their experiments.

### Rate of decomposition of hydrogen peroxide

### The rate of reaction of calcium carbonate and hydrochloric acid

### The rate of reaction of magnesium and hydrochloric acid

### Rates - Iodine Clock

### Rates - Thiosulfate and acid

### Rates - Activation energy

### Investigating the reaction between potassium manganate(VII) and ethanedioic acid

### Reaction rates

A pack containing three activities to review and extend ideas about reaction rates from Module 3 and introduce the Arrhenius equation.

Reaction rates, focussing on Arrhenius - Topic exploration - Teacher-pack (PDF, 1MB)

### A visible activated complex (Royal Society of Chemistry: Classic Demonstrations )

This colourful demonstration of the role of cobalt ions in catalysis provides a perfect answer to any student who believes “a catalyst is not involved in a reaction” and can lead on to a discussion of their role in providing an alternative reaction pathway with lower activation energy.

### Reaction kinetics: Calcium carbonate + hydrochloric acid (Nuffield Foundation)

This resource comprises a detailed lesson plan and associated material for explaining the concept of rate equations and half life. It includes a student worksheet for determining the order of the reaction between marble chips and HC*l* and explains how the concentration of HC*l* at any time can be deduced from the amount of CO_{2} produced.

### Reaction kinetics: Magnesium + hydrochloric acid (Nuffield Foundation)

The worksheet describes how to measure the rate of reaction by collecting hydrogen gas in a measuring cylinder. A Level students could use a burette to improve precision. The reaction can be repeated with different concentrations of HC*l*. In this case the HC*l* is in considerable excess so initial rates should be compared rather than attempting a concentration v time graph.

### Reaction kinetics: Iodine and propanone

The acid-catalysed reaction between iodine and propanone is a good example of a reaction which is zero order with respect to one reactant (iodine). The worksheet gives practical details for determining the rate of change of concentration of iodine by removing samples and titrating them with sodium thiosulfate. It includes data which students can use to determine the overall rate equation and suggest a possible series of steps.

### A colorimetric kinetics experiment (Scottish Schools Education Research Centre)

This paper provides experimental details for using a colorimeter to follow the reaction between blue food dye and household bleach. Sample results are included. The paper uses the integrated forms of rate equations (which are not on the specification) but students can plot absorbence or concentration against time and either measure the rate at different concentrations or find the half life to determine the order with respect to dye concentration. They can also compare the initial rates with different concentrations of bleach.

### Hydrolysis of bromobutane

Data and questions about the rate equation for the hydrolysis of two bromoalkanes, leading on to questions about rate determining steps and reaction mechanisms. This provides some of the evidence for the nucleophilic substitution mechanism encountered in Module 4.

### Determining activation energy (Royal Society of Chemistry: Learn Chemistry)

The worksheet provides a method for following the reaction between sodium thiosulfate and hydrochloric acid. The initial rate of the reaction will be proportional to the rate constant. So the gradient of a graph of:

In \(\displaystyle = (\frac{1}{\text{time}})\) V \(\displaystyle = \frac{1}{\text{temperature}} \) (K) will be \(\displaystyle = \frac{Ea}{R}\)

The same approach can be used for ‘clock reactions’, such as the reaction between potassium iodide and potassium iodate.

## Acknowledgements

### Overview

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