The Effect of Temperature
Introduction

We all use low temperatures to help stop foods going off, by using fridges and freezers. In this unit we will investigate the effect of temperature on rate of reaction and explain our observations using the collision theory (see the unit Introducing Rates of Reaction).

Changing temperature
We can look at the reaction between sodium thiosulfate and 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
in Fig.1 to see how changing the temperature affects the rate of a reaction. In this reaction, a fine
precipitate
A precipitate is an insoluble solid formed when two solutions react together.
precipitate
of sulfur is formed. We can use the formation of the precipitate to judge the rate of the reaction at different temperatures.

Figure 1.   The effect of temperature on rate of reaction.
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Which of the following factors should be kept the same in the experiment shown in Fig.1 above?
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Which solution reacts faster?
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This gives us evidence that:

As we raise the temperature, the rate of reaction increases.


Temperature has a large effect on the rate of reaction. For many reactions, an increase in temperature of just 10 °C will roughly double the rate of reaction.

You can also monitor the rate of reaction between the sodium thiosulfate and hydrochloric acid by using a light sensor. The solution gets more cloudy as the reaction carries on, so you can record the rate at which light is blocked by the precipitate of sulfur. We can then convert these readings into amounts of sulfur formed to get a graph like the one in Fig.2 below:

Figure 2.   The reaction at 30 °C and 40 °C.
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How can you tell from the graph that the reaction at 40 °C is about twice as fast the reaction at 30 °C?
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How can you tell from the graph that the same amount of products are produced at both temperatures?
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Explaining the effect of temperature
So why does temperature have such a big effect on the rate of reactions? There are two factors at work here:
  • As the temperature is raised, the particles gain energy and move around more quickly. This means that there will be more chance of reactant particles colliding. There will be more collisions between particles in any given time, and more frequent collisions result in an increased rate of reaction.

  • The increased energy of the particles means that a greater proportion of the collisions in the reacting mixture actually produce a reaction. More collisions happen with enough energy to exceed the activation energy of the reaction. (Remember that the activation energy of a reaction is the minimum amount of energy needed before a reaction can take place.)



The second factor accounts for most of the effect that temperature has on the rate of reaction.

Figure 3.   Effect of temperature on the rate of reaction.
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Which two of these statements are valid reasons why temperature increases the rate of reaction?
  • Increasing the temperature lowers the activation energy of a reaction.
    Increasing the temperature results in a higher rate of collision between particles.
    Increasing temperature produces more effective collisions with enough energy for a reaction to occur.
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Summary


As we increase the temperature, we increase the rate of reaction.

The main reason for this is that a higher proportion of the collisions at the higher temperature will have sufficient energy to result in a reaction. In other words, more particles collide with an energy greater than the activation energy for the reaction.

In addition, as we increase the temperature, the particles gain more energy and move around faster. This increases the chance of collisions between reactant particles, producing more frequent collisions.

Exercises
1. What two substances are produced when magnesium ribbon reacts with dilute sulfuric acid?
  • Hydrogen
    Magnesium hydroxide
    Magnesium sulfate
    Sulfur dioxide
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2. The reaction in question 1 was used to judge the effect of the temperature on the rate of reaction. List two factors that must be kept constant in the tests.
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Figure 4.  
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3. The results of measuring the rate of
product
A product is a substance formed in a chemical reaction.
product
formed at different temperatures are shown in Fig.4. Match the curves to the temperatures.
  • A
    B
    C
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4. Explain why increasing the temperature increases the rate of reaction.
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