# 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. You will learn to interpret graphs of reaction rate, and calculate the rate of reaction at any moment during the reaction.

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, despite also involving metal + oxygen.

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. This is called completion; for the reaction in the graph, completion is at about 10 - 12 minutes. The amount of product made doesn't depend on the rate of reaction- it is fixed by the amounts of reactant put into the reaction.

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, in the graph, the first two minutes of the graph are pretty much a straight line, In that time, the reaction makes 9 cm3 gas, so

initial rate of reaction = 9 cm3 ÷ 2 min = 4.5 cm3 / min. Since 2 min = 120 s, we could also write this as

initial rate of reaction = 9 cm÷ 120 s = 0.075 cm3 / s.

As the reaction progresses, the rate of reaction slows down. The graph shows this by becoming less steep.

A better way to use the graph (and the only way that works when the graph is noticeably curved) is to draw the tangent to the graph at the moment of interest. So let's take another graph. What is the rate for this reaction at t = 40 seconds?

The red dotted line is the tangent to the graph at x = 40 seconds; it's a straight line which touches the graph at 40 seconds, without cutting it. The gradient of the graph at that moment is the same as the gradient of the tangent. But since the tangent is a straight line, we can work out its gradient using up ÷ across. Just remember that in science graphs, the up and across numbers need to relate to the numbers on the axes; you can't just count squares.

The tangent line goes from 7 cm3 at t = 0 s to 30 cm3 at t = 95 s. So the up is 30 - 7 = 23 cm3, and the across = 95 - 0 = 95 s. That means the gradient of the tangent = 23 ÷ 95 = 0.24 cm3 / s.

So the rate of reaction at 40 s = 0.24 cm3 / 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.

To speed up a reaction, we just need to make useful collisions happen more frequently. There are several ways of doing this, which you will learn about in another activity.

Use these words to complete the gaps in this paragraph:

horizontal

product

reactant

steep

vertical

Which of these is the equation to work out the rate of a chemical reaction, or a correct rearrangement of it?

rate = amount of product ÷ time

time = rate x amount of product

rate = time ÷ amount of product

amount of product = rate x time

time = amount of product ÷ rate

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 15 cm3 gas in 1.5 minutes, which of these are correct rates of reaction?

0.17 cm3 / s

1 cm3 / min

10 cm3 / min

0.1 cm3 / min

0.2 cm3 / 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 recording the volume of gas produced,

Here is a graph of some results of this experiment.

The initial reaction rate is approximately 2 cm3 / s

The initial reaction rate is approximately 0.5 cm3 / s

The total volume of gas produced at completion is 30 cm3

The reaction slows down as it progresses

Look at this graph for the progress of a chemical reaction:

What is the name given to the red dotted line on this graph?

Look at this graph for the progress of a chemical reaction:

Why is the dotted line significant?

The gradient of the line tells us the overall reaction rate.

The gradient of the line tells us the reaction rate at 20 s.

The gradient of the line tells us the reaction rate at 60 s.

The gradient of the line tells us how much product was produced in the first 30 s.

Look at this graph for the progress of a chemical reaction:

Select the best estimate of the reaction rate at 60 s.

0.02 cm3 / s

0.2 cm3 / s

0.27 cm3 / s

3.8 cm3 / s

Complete this sentence by picking a phrase from each section.

Collision theory...

0.02 cm3 / s

0.2 cm3 / s

0.27 cm3 / s

3.8 cm3 / s

Would each of these changes make a chemical reaction go faster, slower, or have no effect on the reaction rate?

Why are more energetic collisions more likely to be successful?

here is more likely to be enough energy to form bonds in the products.

There is more likely to be enough energy to break bonds in the products.

here is more likely to be enough energy to form bonds in the reactants.

There is more likely to be enough energy to break bonds in the reactants.

• Question 1

Use these words to complete the gaps in this paragraph:

horizontal

product

reactant

steep

vertical

EDDIE SAYS
Look at the graph in the introduction if you're not sure- make sure you can match the shape of the graph to this description. The end point of the reaction depends on the quantities of reactant- once one reactant has run out, the reaction cannot continue.
• Question 2

Which of these is the equation to work out the rate of a chemical reaction, or a correct rearrangement of it?

rate = amount of product ÷ time
amount of product = rate x time
time = amount of product ÷ rate
EDDIE SAYS
The equation is rate = amount of product ÷ time. You can rearrange this a bit like speed = distance ÷ time.
• Question 3

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 15 cm3 gas in 1.5 minutes, which of these are correct rates of reaction?

0.17 cm3 / s
10 cm3 / min
EDDIE SAYS
The amount of gas produced is 15 cm3, and the time taken is 1.5 min, or 90 s. So the rate = 15 ÷ 1.5 = 10 cm3 / min, or 15 ÷ 90 = 0.1667 cm3 / s, which approximates to 0.17 cm3 / s.
• Question 4

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 recording the volume of gas produced,

Here is a graph of some results of this experiment.

The initial reaction rate is approximately 0.5 cm3 / s
The reaction slows down as it progresses
EDDIE SAYS
In the first 20 seconds, 10 cm3 is produced, so the approximate initial rate is 10 ÷ 20 = 0.5 cm3 / s. When the reaction slows down, the total volume of gas produced is tending towards 25 cm3.
• Question 5

Look at this graph for the progress of a chemical reaction:

What is the name given to the red dotted line on this graph?

tangent
EDDIE SAYS
The line is called a tangent, because it touches the curve at one point, but does not cut it.
• Question 6

Look at this graph for the progress of a chemical reaction:

Why is the dotted line significant?

The gradient of the line tells us the reaction rate at 60 s.
EDDIE SAYS
The reason that tangent lines are useful is that they tell us the reaction rate at the point on the graph where the tangent touches the curved graph- in this graph, that happens at 60 s.
• Question 7

Look at this graph for the progress of a chemical reaction:

Select the best estimate of the reaction rate at 60 s.

0.27 cm3 / s
EDDIE SAYS
The tangent line goes through (0 s, 10 cm3) and (140 s, 48 cm3). That means that the gradient is (48 - 10) ÷ (140 - 0) = 0.271 cm3 / s.
• Question 8

Complete this sentence by picking a phrase from each section.

Collision theory...

EDDIE SAYS
The key point of collision theory is that the rate of a chemical reaction depends on the frequency of successful collisions between reactants.
• Question 9

Would each of these changes make a chemical reaction go faster, slower, or have no effect on the reaction rate?

EDDIE SAYS
The reaction rate depends on the frequency of successful collisions (how many per second). If the collisions are more energetic, they are more likely to be successful.
• Question 10

Why are more energetic collisions more likely to be successful?

There is more likely to be enough energy to break bonds in the reactants.
EDDIE SAYS
Before the product can be formed, we need to break apart bonds in the reactants. Energetic collisions between reactant particles help with this, because they can supply enough energy to do this.
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