Introducing Rates of Reaction
Introduction

You know that some reactions happen very quickly (for example, potassium reacting with water). On the other hand, some reactions are very slow (for example, the rusting of iron or the setting of concrete). Many chemists in industry spend their careers trying to speed up or slow down chemical reactions. In this unit we will see some ways to measure how quickly a reaction takes place.

Measuring rates
The only way we can find out about the rate of a particular reaction is to carry out an experiment. The
balanced equation
A balanced equation is a chemical reaction represented by the formulae of reactants and products, showing the same number of each type of atom before and after the reaction.
e.g. 2Mg(s)  +  O2(g)    2MgO(s)
balanced equation
for the reaction tells us nothing about its rate. So we need to have some way of measuring either:

  • the rate at which reactants are used up in a reaction, or

  • the rate at which products are formed in a reaction.

Figure 1.   Modelling rates of reaction.
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In real experiments, we find some property of the reacting mixture that changes as the reaction takes place and we measure that. That's easy in reactions that produce a gas as one of the products – for example, marble chips (containing calcium carbonate) reacting with hydrochloric
acid
An acid is a substance that forms a solution with a pH value of less than 7. Acidic solutions contain an excess of hydrogen ions, H+(aq).
acid
:


CaCO3(s)  + 2 HCl(aq)     CaCl2(aq)  +  H2O(l)  +  CO2(g)

Here we can follow the rate at which carbon dioxide gas is given off, using one of the methods shown in Figs.2–4 below.

Figure 2.   Measuring the volume of gas given off (1).
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Figure 3.   Measuring the volume of gas given off (2).
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Figure 4.   Measuring the mass of gas given off.
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If the reaction involves a
precipitate
A precipitate is an insoluble solid formed when two solutions react together.
precipitate
forming quite slowly, then you can time how long it takes for the solution to become opaque, as shown in Fig.5:

Figure 5.   Measuring the time it takes for a certain amount of precipitate to be produced.
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There are other ways of measuring the rate of a chemical reaction, but we will only be using the above methods in this chapter.

You are investigating the reaction between magnesium ribbon and dilute sulfuric acid. Which of the methods below could you use to follow the rate of the reaction?
  • Measure volume of gas given off at regular time intervals
    Measure the time taken for a mark on a piece of paper under the reaction vessel to disappear when viewed from above
    Measure the loss in mass of the apparatus at regular time intervals as the reaction proceeds
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Graphs and rates
We often display the results of experiments to follow the rate of a reaction on a graph. This helps us to 'see' what was happening over the course of the reaction. Look at the graph in Fig.6 below:

Figure 6.   Graph showing the changing rate of a reaction.
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The line has the classic shape of a rate of reaction graph. It starts off steep, becoming shallower until it levels off. You can tell the rate of reaction at any particular time by the slope (gradient) of the line.

The steeper the slope of the graph, the faster the reaction at that point.
Choose the correct option to complete this statement: In the reaction between marble chips and dilute hydrochloric acid, …
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Collision theory
It is not difficult to imagine that, in order for a chemical reaction to take place, the reacting particles must collide. Look at the model below:
Figure 7.   Particles must collide before a reaction can take place.
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However, not all collisions in a reacting mixture result in a reaction. The particles (molecules or ions) in the mixture will have a whole range of different energies. Some have lots of energy and move about quickly; others have a low energy and move more slowly.

In order for the collision to produce a reaction, the particles must have enough energy to allow the reaction to take place. This minimum amount of energy is called the activation energy of the reaction.

Look at the model in Fig.8 below:

Figure 8.   Particles must have sufficient energy to react together.
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As you can see above, the particles with insufficient energy just collide with each other without reacting.

Decide whether the following statements about the particles in a reacting mixture are true or false.
  • All the particles have the same energy.
    The particles have a range of energies.
    All collisions between particles result in a reaction.
    Only collisions with energy greater than the activation energy result in a reaction.
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Summary


We can measure the rate of reaction by following the change in some property of the reacting mixture over time.

We can show the changing rate of a reaction by plotting a graph of amount of reactant remaining or amount of product formed against time..

At any moment during the reaction: the steeper the slope of the graph, the faster the reaction at that point.

The reaction finishes where the line levels off.

The collision theory states that particles must collide before they can react, and that only collisions with sufficient energy (greater than the activation energy) will result in a reaction.

Exercises
Figure 9.  
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1. Look at the graph in Fig.9. Match the labels on the graph to the description of the reaction:
  • A
    B
    C
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2. Look at the graph in Fig.10 below.

  • How much reactant did we start with? g
    How long did it take for the reaction to finish? s
    How much reactant was left after 10 seconds? g
    How much reactant had been used up after 20 seconds? g

  • Click here to mark the question
Figure 10.  
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3. Between which range of times was the reaction in Fig.10 at its fastest?
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4. In a reacting mixture, some particles collide but do not react. Why is this?
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5. What do we call the minimum amount of energy needed for particles to react?
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