 # Collision theory

In this activity, you will learn what chemists mean by collision theory, and how it explains whether a chemical reaction will happen quickly or slowly. Key stage:  KS 4

Curriculum topic:   Chemistry: Rate and Extent of Chemical Change

Curriculum subtopic:   Factors that Influence Reaction

Difficulty level:   ### QUESTION 1 of 10

The speed of a chemical reaction makes a big difference to its effects. Fireworks involve metals reacting with oxygen in a fraction of a second... ... but rusting happens over years. In this activity, and the next one, you will learn how we can describe this difference, and use an aspect of particle theory called collision theory to explain why some reactions are fast and some are slow. That helps people make good reactions (like pretty fireworks) go faster, and make bad reactions (like rusting) go more slowly.

Rates of reaction

Suppose we start a chemical reaction and record how much product is made over time. Unless something very strange happens, the graph will look like this; The first part of the graph is a straight line. This is also the time when the reaction is fastest. As time passes, the reaction starts to go more slowly, then it stops. The reaction rate tells us how quickly the reaction happens. To work out the rate of reaction, we calculate

rate of reaction = amount of reaction ÷ time taken

For example, if a reaction makes 15 g of product in 50 seconds, we would do this calculation,

rate of reaction = 15 g ÷ 50 s = 0.3 g / s.

Collision theory

There are several factors which change the rate of reaction. One big idea links them all, called collision theory.

For two particles to react, they have to collide, in the right orientation and with enough energy to break the atomic bonds in the particles.

The more frequently these collisions happen, the faster the rate of reaction will be.

Once we understand collision theory, we can explain why some reactions are fast and some are slow. We can also speed up or slow down reactions if that is what we need to do.

Think about these chemical reactions. Do they happen quickly or slowly?

If you plot a graph of the progress of a chemical reaction, what goes on the horizontal (x) axis?

Amount of product

Time

If you plot a graph of the progress of a chemical reaction, what goes on the vertical (y) axis?

Amount of product

Time

Which of these words describe the progress of a chemical reaction at the beginning of the reaction?

fastest

slowest

straight line

curved

Which of these is the right equation to work out the rate of a chemical reaction?

rate = amount of product x time

rate = amount of product ÷ time

rate = time ÷ amount of product

If we react magnesium with hydrochloric acid, the equation is

Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g).

We track this reaction by collecting the volume of gas produced,

If we collect 28 cm3 gas in 35 s, what is the rate of reaction?

0.8 cm3 / s

1.0 cm3 / s

1.25 cm3 / s

If we react calcium carbonate with hydrochloric acid, the equation is

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

We track this reaction by recording the loss of mass as carbon dioxide escapes.

If the loss of mass 1.5 g in 50 s, what is the rate of reaction?

3 g / s

0.03 g / s

30 g / s

If we react aluminium with hydrochloric acid, the equation is

2 Al (s) + 6 HCl (aq) →2 AlCl3 (aq) + 3 H2 (g).

We track this reaction by collecting the volume of gas produced,

If we collect 35 cm3 gas in 70 s, what is the rate of reaction in cm3 / s.

3 g / s

0.03 g / s

30 g / s

For two particles to react, what has to take place? Tick all the statements which are true.

the particles have to have sufficient energy

the particles have to collide

the particles have to move slowly enough

the orientation of the particles has to be correct

the particles have to want to react

Why is collision theory important?

Atoms are always colliding in solutions.

The more frequently successful collisions happen, the slower the rate of reaction.

The reactants need to collide with the products.

The more frequently successful collisions happen, the faster the rate of reaction.

• Question 1

Think about these chemical reactions. Do they happen quickly or slowly?

EDDIE SAYS
Explosions and combustion are fast reactions; they will be complete in a few seconds. Rusting metal takes several years, and weathering of rocks can take thousands of years.
• Question 2

If you plot a graph of the progress of a chemical reaction, what goes on the horizontal (x) axis?

Time
EDDIE SAYS
Time is the independent variable in this experiment, so it goes on the x-axis.
• Question 3

If you plot a graph of the progress of a chemical reaction, what goes on the vertical (y) axis?

Amount of product
EDDIE SAYS
The amount of product is the dependent variable in this experiment, so it goes on the y-axis.
• Question 4

Which of these words describe the progress of a chemical reaction at the beginning of the reaction?

fastest
straight line
EDDIE SAYS
The first part of the graph is a straight line- it curves later on as the reaction nears completion. The first part of the graph is steeped, showing the reaction is fastest.
• Question 5

Which of these is the right equation to work out the rate of a chemical reaction?

rate = amount of product ÷ time
EDDIE SAYS
The equation is rate = amount of product ÷ time. The more product made, the faster the rate is. The less time taken, the faster the rate.
• Question 6

If we react magnesium with hydrochloric acid, the equation is

Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g).

We track this reaction by collecting the volume of gas produced,

If we collect 28 cm3 gas in 35 s, what is the rate of reaction?

0.8 cm3 / s
EDDIE SAYS
The amount of gas produced is 28 cm3, and the time taken is 35 s. So the rate = 28 ÷ 35 = 0.8 cm3 / s.
• Question 7

If we react calcium carbonate with hydrochloric acid, the equation is

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

We track this reaction by recording the loss of mass as carbon dioxide escapes.

If the loss of mass 1.5 g in 50 s, what is the rate of reaction?

0.03 g / s
EDDIE SAYS
The mass change is 1.5 g, and the time taken is 50 s. So the rate = 1.5 ÷ 50 = 0.03 g / s.
• Question 8

If we react aluminium with hydrochloric acid, the equation is

2 Al (s) + 6 HCl (aq) →2 AlCl3 (aq) + 3 H2 (g).

We track this reaction by collecting the volume of gas produced,

If we collect 35 cm3 gas in 70 s, what is the rate of reaction in cm3 / s.

EDDIE SAYS
The amount of gas produced is 35 cm3, and the time taken is 70 s. So the rate = 35 ÷ 70 = 0.5 cm3 / s.
• Question 9

For two particles to react, what has to take place? Tick all the statements which are true.

the particles have to have sufficient energy
the particles have to collide
the orientation of the particles has to be correct
EDDIE SAYS
The basis of collision theory is that, for particles to react they have to collide, in the right orientation and with enough energy to break the atomic bonds in the particles.
• Question 10

Why is collision theory important?

The more frequently successful collisions happen, the faster the rate of reaction.
EDDIE SAYS
The rate of reaction depends on how frequently successful collisions happen. Successful collisions are ones where reactants collide in the right orientation with enough energy to break bonds in the reactants, so the products can form.
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