 # 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, 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.

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.

Which of these words or phrases is linked to the amount of product produced in a chemical reaction?

stops increasing eventually

increases slowest at first

is zero at the beginning

increases fastest at first

carries on increasing forever

Use a phrase from each column to make the equation to work out the rate of a chemical reaction.

stops increasing eventually

increases slowest at first

is zero at the beginning

increases fastest at first

carries on increasing forever

If we react zinc with hydrochloric acid, the equation is

Zn (s) + 2 HCl (aq) → ZnCl2 (aq) + H2 (g).

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

If we collect 14 cm3 gas in 35 s, what is the rate of reaction?

stops increasing eventually

increases slowest at first

is zero at the beginning

increases fastest at first

carries on increasing forever

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

MgCO3 (s) + 2 HCl (aq) → MgCl2 (aq) + H2O (l) + CO2 (g)

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

If the loss of mass 3.0 g in 2 min, what is the rate of reaction in cm3 / s?

stops increasing eventually

increases slowest at first

is zero at the beginning

increases fastest at first

carries on increasing forever

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. Estimate the total volume of gas which would be produced in this experiment if it ran to completion (so all the possible gas were produced).

20 cm3

25 cm3

30 cm3

35 cm3

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.

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. What is the initial reaction rate for this experiment?

0.2 cm3 / s

0.6 cm3 / s

0.8 cm3 / s

1.0 cm3 / s

If we need to make a chemical reaction run more slowly, which of these strategies will work?

reduce the frequency of collisions between reactants

increase the frequency of collisions between reactants

make the collisions stronger

make the collisions more gentle

Someone says "If we need to make more product in a chemical reaction, we need to increase the reaction rate."

This idea always works

This idea sometimes works

This idea never works

• Question 1

Which of these words or phrases is linked to the amount of product produced in a chemical reaction?

stops increasing eventually
is zero at the beginning
increases fastest at first
EDDIE SAYS
Look at the graph in the introduction if you're not sure. When the reaction starts, no product has been made, but the rate of production is high (the graph is quite steep). As time passes, the graph gets less steep (the rate of reaction decreases), until the reaction finishes.
• Question 2

Use a phrase from each column to make the equation to work out the rate of a chemical reaction.

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 3

If we react zinc with hydrochloric acid, the equation is

Zn (s) + 2 HCl (aq) → ZnCl2 (aq) + H2 (g).

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

If we collect 14 cm3 gas in 35 s, what is the rate of reaction?

EDDIE SAYS
The amount of gas produced is 14 cm3, and the time taken is 35 s. So the rate = 14 ÷ 35 = 0.4 cm3 / s.
• Question 4

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

MgCO3 (s) + 2 HCl (aq) → MgCl2 (aq) + H2O (l) + CO2 (g)

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

If the loss of mass 3.0 g in 2 min, what is the rate of reaction in cm3 / s?

EDDIE SAYS
2 min is 120 s, so the rate = 3.0 ÷ 120 = 0.025 g / s.
• Question 5

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. Estimate the total volume of gas which would be produced in this experiment if it ran to completion (so all the possible gas were produced).

25 cm3
EDDIE SAYS
By the end of the graph, the gradient of the line is already almost flat, which shows the reaction is nearly complete. 25 cm3 is a reasonable limit; it's hard to imagine the graph reaching 30 cm3.
• Question 6

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 7

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.
• 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 recording the volume of gas produced,

Here is a graph of some results of this experiment. What is the initial reaction rate for this experiment?

0.6 cm3 / s
EDDIE SAYS
The graph is fairly straight for the first 20 s, and in this time about 11 cm3 gas is produced. That means the initial rate is 11 cm3 ÷ 20 s = 0.55 cm3 / s, which is 0.6 to 1 decimal place.
• Question 9

If we need to make a chemical reaction run more slowly, which of these strategies will work?

reduce the frequency of collisions between reactants
make the collisions more gentle
EDDIE SAYS
This is collision theory again, but in reverse. The less frequently collisions between reactants with the right energy and orientation to break bonds happen, the slower the reaction will be.
• Question 10

Someone says "If we need to make more product in a chemical reaction, we need to increase the reaction rate."

This idea never works
EDDIE SAYS
The amount of product produced depends on the amount of reactants you have. If you make the reaction rate faster, you can produce product in less time, but you can\'t change the final total amount of product made.
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