Reaction Rates

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

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

You're just chilling by yourself one day - minding your own business and browsing insta when suddenly a friend come up behind you and gives you a hug. That would be weird right? Until you worked out who they were that is. But what if you really wanted to hug people all of the time, and then when you hug them stay attached to them forever unless someone who hugs better comes along. Okay, I might be stretching the analogy a bit now, but it is the same with chemicals. When they react, they hug each other, but they only do it in a particular way that is not weird. They need to be travelling at the right speed (they can't do a running hug - or a slow-motion hug) and they need to be facing the right direction (no weird hugs from behind or the side). Thats what we will be looking at in this activity - how particles find a hug, or how they react with each other.

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:

• temperature
• concentration
• pressure
• surface area
• 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'; it is easy to remember because it is about particles moving and colliding.

Let's now see how the five factors above affect the number of collisions between particles. You will notice it will be all about the number of collisions.

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 showing the total amount of product as the reaction at a higher temperature progresses 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 more 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.

What is the scientific verb used to describe how particles bump onto 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? Tick two boxes.

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 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 verb used to describe how particles bump onto 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
Any time energy that causes movement is mentioned, it will be about kinetic energy. The more movement there is the more likely that 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? Tick two boxes.

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 chance 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 and, therefore, 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
Thermal energy (heat) is given to particles when the temperature increases and this results in the increase of kinetic energy of the particles. More kinetic energy means more energy that it can 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 more surface area. Basically - the smaller you make something the more surface area it will have.
• 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 reaction?

It would slow it down
EDDIE SAYS
A decrease in pressure would slow a reaction down because there would be fewer collisions. Fewer collisions mean less chance of the particles colliding with the correct energy and orientation. More pressure will 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
Catalysts are chemicals that speed up a reaction but are not involved in the reaction itself. Whilst 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 of 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.
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