Rates of Reaction_Chemistry AQA GCSE_Additional Science

Rates of reaction

The rate of a reaction can be measured by the rate at which a reactant is used up, or the rate at which a product is formed.
The temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts, are all factors which affect the rate of a reaction.
Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.

Measuring rates

Different reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. For example, the chemical weathering of rocks is a very slow reaction: it has a low rate of reaction. Explosions are very fast reactions: they have a high rate of reaction.

Reactants and products

There are two ways to measure the rate of a reaction:

1. Measure the rate at which a reactant is used up
2. Measure the rate at which a product is formed

The method chosen depends on the reaction being studied. Sometimes it is easier to measure the change in the amount of a reactant that has been used up; sometimes it is easier to measure the change in the amount of product that has been produced.

Things to measure

The measurement itself depends on the nature of the reactant or product:

• The mass of a substance - solid, liquid or gas - is measured with a balance
• The volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette

It is usual to record the mass or total volume at regular intervals and plot a graph. The readings go on the vertical axis, and the time goes on the horizontal axis.

For example, if 24 cm³ of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12 cm³ hydrogen / min.

Factors affecting the rate

You will be expected to remember the factors that affect the rate of reactions, and to plot or interpret graphs from rate experiments.

How to increase the rate of a reaction

The rate of a reaction increases if:

• The temperature is increased
• The concentration of a dissolved reactant is increased
• The pressure of a reacting gas is increased
• Solid reactants are broken into smaller pieces
• A catalyst is used

Rate of reaction and changing conditions

The graph above summarises the differences in the rate of reaction at different temperatures, concentrations and size of pieces. The steeper the line, the greater the rate of reaction. Reactions are usually fastest at the beginning, when the concentration of reactants is greatest. When the line becomes horizontal, the reaction has stopped.

Collisions and reactions

You will be expected to explain, in terms of particles and their collisions, why changing the conditions of a reaction changes its rate.

Collisions

For a chemical reaction to occur, the reactant particles must collide. Collisions with too little energy do not produce a reaction.

The collision must have enough energy for the particles to react. The minimum energy needed for particles to react is called the activation energy.

Changing concentration or pressure

If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:

• There are more reactant particles in the same volume
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing particle size

If a solid reactant is broken into small pieces or ground into a powder:

• Its surface area is increased
• More particles are exposed to the other reactant
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing the temperature

If the temperature is increased:

• The reactant particles move more quickly
• More particles have the activation energy or greater
• The particles collide more often, and more of the collisions result in a reaction
• The rate of reaction increases

Using a catalyst

Catalysts increase the rate of reaction without being used up. They do this by lowering the activation energy needed. With a catalyst, more collisions result in a reaction, so the rate of reaction increases. Different reactions need different catalysts.
Catalysts are important in industry because they reduce costs.

Rates of reaction

Test:

1. Using data from the table, when did the reaction stop? 

60 s
80 s
100 s

2. Using data from the table, what was the mean rate of reaction? 

 

2 cm³/s
0.4 cm³/s
0.5 cm³/s

3. Why does reaction rate increase as the temperature increases? 

 

Particles begin to collide at higher temperatures
At higher temperatures particles move faster and collide more often
There are more particles at higher temperatures, so they collide more

4. Which line represents the fastest reaction?

A
B
C

5. Which curve represents the lowest concentration of acid for a certain reaction? 

 

A
B
C

6. Which curve represents the highest pressure of two reacting gases? 

 

A
B
C

7. Why does reaction rate increase as the concentration increases? 

 

The particles have more energy so there are more collisions
There are more particles so there are more collisions
The surface area is increased so there are more collisions

8. Which of the following statements about collisions is correct? 

 

All colliding particles have the same amount of energy
Only fast-moving particles collide with each other
Reactions can happen if the colliding particles have enough energy

9. A successful collision is one in which: 

 

The particles hit each other
Enough energy is transferred for particles to react
Enough energy is transferred for particles to bounce off each other

10. Which curve represents the 'uncatalysed reaction' but with a catalyst added? 

A
B
C

To check your answers visit:
http://www.bbc.co.uk/apps/ifl/schools/gcsebitesize/science/quizengine?quiz=add_aqa_rates_reactiontest&templateStyle=science

Resource: http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/ (15.12.2012)