# Understand Reaction Rates

In this worksheet, students will explore how the rate of a reaction is influenced by temperature, concentration, surface area and the use of catalysts.

### QUESTION 1 of 10

Chemicals react together when their particles collide with each other -  to collide means to bump into each other or to hit each other. Some reactions are very slow and others very fast but there are five factors that affect the rate of a reaction:​

Concentration

Temperature

Surface Area

Pressure

Catalysts

Collision theory

The collision theory states that the more collisions between particles there are, the faster the reaction. The particles must be moving very fast and have lots of kinetic energy for collisions to occur.

'Kinetic' means 'moving', so this type of energy is about the movement and collision of particles.

Let's see how the five factors above affect the number of collisions between particles.

Concentration

If the concentration of one or more of the reactants increases, the particles become more crowded. The diagram below shows the particles of two chemicals. The box on the right contains more red particles, representing one of the chemicals about to react. This means that the concentration of the 'red' chemical is now higher but the particles are still in the same space. This increases the probability of collisions and the rate of the reaction.

Temperature

When the temperature increases, the particles have more energy, as an increase in temperature means more thermal energy (heat) is present. More heat energy gives the particles more kinetic energy. More kinetic energy means more movement, so the probability of collisions increases and so does the rate of the reaction.

The graph below shows the rate of the same reaction in different temperatures. Note that the blue line, which shows the higher temperature, is much steeper than the purple line.

Surface area

When a solid chemical is broken down into smaller pieces - or even a powder - there are more particles that can react, as the diagram below shows. We say that a chemical in a powder form has a greater surface area than the same mass of the same chemical in a solid block form. An increased surface area allows for more collisions and the rate of the reaction increases.

Pressure (in gases)

An increase in pressure speeds up a reaction. It has the same effect as an increase in concentration. The way you increase pressure on a gas is by squeezing it into a smaller volume but the mass remains the same. This results in the same number of particles moving about in a smaller volume, which increases the number of collisions and the rate of the reaction.

Catalysts

A catalyst is a chemical that speeds up a reaction without being used in it. Catalysts are specific to reactions, so the catalyst for one reaction would not work for another.

Now it's time for some questions to see how much you can remember.

What is the scientific word used to describe how particles bump into each other?

Crash

Collide

What type of energy is transferred to particles and makes them move around faster?

Thermal (heat)

Chemical

Kinetic

What effect does an increase in this type of energy have on the particles?

It makes them move around faster

It decreases the number of collisions

It slows them down

It increases the number of collisions

How does an increase in concentration affect the rate of a reaction?

It speeds the reaction up

It slows the reaction down

It decreases the number of collisions

What type of energy is given to particles when the temperature increases?

Thermal (heat)

Kinetic

Sound

Which substance has an increased surface area?

Solid block of calcium carbonate

Powdered calcium carbonate

Marble chips (small blocks of calcium carbonate)

Identify a way to increase the concentration of a gas substance.

Squeeze it into a bigger volume

Squeeze it into a smaller volume

Increase the temperature

How would a decrease in the pressure of a gas affect the rate of a reaction?

It would speed it up

It would not affect it

It would slow it down

What do we call chemicals that speed up a reaction, but do not take part in it?

Enzymes

Catalysts

Triggers

Monomers

Give a reason why pressure can be increased in gases but not in solids or liquids.

Gases can be squashed

Liquids flow

Solids cannot flow

• Question 1

What is the scientific word used to describe how particles bump into each other?

Collide
EDDIE SAYS
Collide means to bump into each other and it is used in chemistry to describe the behaviour of particles. Think of them as little cars crashing into each other.
• Question 2

What type of energy is transferred to particles and makes them move around faster?

Kinetic
EDDIE SAYS
It's important to get this one sorted out because it can come up in both chemistry and physics. Any time energy that causes movement is mentioned it will be about kinetic energy. The more movement there is, the more likely it is that particles will collide with the correct speed to create a reaction.
• Question 3

What effect does an increase in this type of energy have on the particles?

It makes them move around faster
It increases the number of collisions
EDDIE SAYS
An increase in kinetic energy increases the number of collisions by making them move around faster. Remember it is all about increasing the chances that a collision will happen with the correct energy and orientation.
• Question 4

How does an increase in concentration affect the rate of a reaction?

It speeds the reaction up
EDDIE SAYS
An increase in concentration causes the particles to be closer together, which will make them collide more. This, in turn, causes the reaction rate to increase.
• Question 5

What type of energy is given to particles when the temperature increases?

Thermal (heat)
EDDIE SAYS
You might have been tempted to go for another option, but the word 'heat' in brackets was a bit of a clue! Always be on the lookout for little helpful hints like this! Thermal energy (heat) is given to particles when the temperature increases, and this gives the particles a greater kinetic energy. More kinetic energy means more energy to put into the reaction.
• Question 6

Which substance has an increased surface area?

Powdered calcium carbonate
EDDIE SAYS
Any substance in a powder form has a greater surface area. Basically, the more you open something up, the more surface area it will have. Think about a sheet on a washing line - it has a greater surface area if you spread it out than if you scrumple it up in a ball!
• Question 7

Identify a way to increase the concentration of a gas substance.

Squeeze it into a smaller volume
EDDIE SAYS
You can increase the concentration of a gas if you squeeze it into a smaller volume. This is because there will be more particles per unit area, and so more of a chance of particles colliding with the correct energy and orientation.
• Question 8

How would a decrease in the pressure of a gas affect the rate of a reaction?

It would slow it down
EDDIE SAYS
How are you getting on with these questions? Generally, a useful tip is that the rate of a reaction increases as any of the various factors increase, and so the reverse is also true - a decrease in one of the factors leads to a decrease in the rate of reaction. A decrease in pressure would slow a reaction down because there would be fewer collisions. Fewer collisions would mean less chance of the particles colliding with the correct energy and orientation. More pressure would increase the number of collisions.
• Question 9

What do we call chemicals that speed up a reaction, but do not take part in it?

Catalysts
EDDIE SAYS
Did you fall into the trap and go for enzymes? Not a bad choice but not correct in this context. Catalysts are chemicals that speed up a reaction but are not involved in the reaction itself. While an enzyme is a type of catalyst, they only work for biological reactions. Need a reaction sped up in your body, call on an enzyme - need one outside your body, use a catalyst.
• Question 10

Give a reason why pressure can be increased in gases but not in solids or liquids.

Gases can be squashed
EDDIE SAYS
Gases can be squashed because the particles are so spread out. Because of this, you can increase their pressure by squashing them. Another activity completed and hopefully, you are feeling more confident with the topic of reaction rates. Why not move on to another activity now?
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