# Understand 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, linking these to collision theory and applying them to real phenomena.

### QUESTION 1 of 10

Suppose we have a chemical reaction that we want to happen faster. For example, if we can speed up the reactions in fireworks, the bang will be louder, the sparks will move faster and be brighter. It will be great.

Imagine running a factory that makes chemicals for medicines. If we can make the chemicals we need more quickly, we can sell them faster and more profitably (time is money). On the other hand, some chemical reactions are harmful- like the chemical weathering of buildings and statues. Ideally, we want harmful chemical reactions to happen as slowly as possible. Either way, it is useful to control the rate of a chemical reaction.

There are five ways that we can speed up (or slow down) a chemical reaction, all linked by the Big Idea of 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.

From this, we get five factors which affect the rate of a reaction.

• temperature
• concentration
• pressure
• surface area
• catalysts

All of these factors affect the reaction rate by changing the frequency of collisions between reactant particles, or changing the likelihood that a collision will be successful (right orientation and enough energy). As well as learning the five factors, think about the connections to collision theory.

Temperature

When the temperature increases, the particles have more energy. More energy means more movement, so the frequency of collisions increases. Also, the collisions that happen have more energy, so are more likely to cause bond breaking. Both these factors increase the rate of reaction.

The graph below shows the rate of the same reaction in different temperatures. The blue line shows the progress of the reaction at a higher temperature than the purple line. The blue line is steeper, showing that the reaction happens faster. Notice that the final amount of product made is the same for the two temperatures. That's because the amount of product made depends on the amount of reactant used, not the rate of reaction.

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 so the rate of the reaction.

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.

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. One example of this is that custard powder is very explosive, because the powder has very small grains.

Catalysts

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. In biology, catalysts are called enzymes- it's a different name for the same idea.

Catalysts don't increase the frequency of collisions, but they do make collisions more likely to be successful. In the diagram, the catalyst is the brown shape at the bottom of the picture. It holds the reactants in specific positions and orientations, which are the correct ones for a reaction to happen. The catalyst (or enzyme) can also make chemical bonds to the reactants which weaken some of the bonds which need to be broken for a reaction to take place. If less energy is needed to turn a collision into a reaction, there is a higher chance that a collision will cause a reaction. In terms of energy level diagrams, we say that the catalyst has reduced the activation energy

If we want a reaction to go more quickly, we can try these things:

• increase temperature
• increase the concentration of the reactants in solution
• increase the pressure of the reactants, if they are a gas
• increase the surface area, by grinding the solid into smaller pieces
• using a catalyst (called an enzyme in biology)

If we want a reaction to go more slowly, we can try the opposite of these things.

Match these half-sentences about the effect of changing temperature on reaction rates.

## Column B

Increasing temperature
reduces the rate of reaction.
Particles with more kinetic energy
collide more often and with more energy.
Therefore reducing temperature
increases the kinetic energy of the particles.

Glow sticks work because of a chemical reaction. When you bend a glow stick, you start to mix and react the chemicals inside. Energy is transferred out of the glow stick as light.

Complete the gaps in this paragraph about the chemical reaction in glowsticks. Each missing word is either increases or decreases.

## Column B

Increasing temperature
reduces the rate of reaction.
Particles with more kinetic energy
collide more often and with more energy.
Therefore reducing temperature
increases the kinetic energy of the particles.

Why do we pour water onto acid burns? You need a phrase from each column to make a complete answer.

## Column B

Increasing temperature
reduces the rate of reaction.
Particles with more kinetic energy
collide more often and with more energy.
Therefore reducing temperature
increases the kinetic energy of the particles.

Complete the gaps in this paragraph about reaction rates. Each missing word is either increases or decreases.

## Column B

Increasing temperature
reduces the rate of reaction.
Particles with more kinetic energy
collide more often and with more energy.
Therefore reducing temperature
increases the kinetic energy of the particles.

A group of students are investigating the rate of the chemical reaction between acid and marble rock (calcium carbonate). The reaction is taking too long to happen. Pick a way of making the reaction go faster, and an explanation for why it would help.

## Column B

Increasing temperature
reduces the rate of reaction.
Particles with more kinetic energy
collide more often and with more energy.
Therefore reducing temperature
increases the kinetic energy of the particles.

This question is about surface area and volume.

Imagine you had 27 little cubes, each 1 cm x 1 cm x 1 cm.

Here are three ways you could arrange them:

A 27 totally separate cubes

B 3 separate square blocks, each 3 cm x 3 cm x 1 cm

C 1 big cube, 3 cm x 3 cm x 3 cm

Work out the surface area of each arrangement.

## Column B

Increasing temperature
reduces the rate of reaction.
Particles with more kinetic energy
collide more often and with more energy.
Therefore reducing temperature
increases the kinetic energy of the particles.

Washing depends on chemical reactions to break up particles of dirt. Which of these sentences helps explain why enzymes are often added to washing powders?

Enzymes are a type of catalyst.

Catalysts are not used up in chemical reactions.

Enzymes are found in nature.

Catalysts make chemical processes happen faster.

By using enzymes, we can wash clothes at lower temperatures than without enzymes.

Would you expect a single enzyme to be able to clean all stains? Select an answer and a reason.

Enzymes are a type of catalyst.

Catalysts are not used up in chemical reactions.

Enzymes are found in nature.

Catalysts make chemical processes happen faster.

By using enzymes, we can wash clothes at lower temperatures than without enzymes.

How does reducing the activation energy affect the rate of a chemical reaction?

Catalysts increase the activation energy

Catalysts don't affect the activation energy

Catalysts reduce the activation energy

Which of these ideas would work? Tick all the correct answers.

Diluting acid in water to make it react more quickly

Grinding up charcoal to make it burn faster

Compressing a gas to make it react faster

Warming an acid to make metal react faster

Using the catalytic converter from a car to make plastics more quickly

• Question 1

Match these half-sentences about the effect of changing temperature on reaction rates.

## Column B

Increasing temperature
increases the kinetic energy of t...
Particles with more kinetic energ...
collide more often and with more ...
Therefore reducing temperature
reduces the rate of reaction.
EDDIE SAYS
Temperature affects reaction rates a lot. Remember that both the frequency and the energy of the collisions increases with increasing temperature.
• Question 2

Glow sticks work because of a chemical reaction. When you bend a glow stick, you start to mix and react the chemicals inside. Energy is transferred out of the glow stick as light.

Complete the gaps in this paragraph about the chemical reaction in glowsticks. Each missing word is either increases or decreases.

EDDIE SAYS
Increasing temperature causes the chemical reaction to be faster, so more energy is released per second, making the glow stick brighter. Because the reaction is faster, the reaction will reach completion sooner, so a hot stick will glow for less time than a cold one. Try it and see!
• Question 3

Why do we pour water onto acid burns? You need a phrase from each column to make a complete answer.

EDDIE SAYS
If we add more water to an acid splash, the concentration will be reduced. This means that hydrogen ions will interact with the skin less.
• Question 4

Complete the gaps in this paragraph about reaction rates. Each missing word is either increases or decreases.

EDDIE SAYS
This example shows the big idea really well; more frequent collisions lead to a faster rate of reaction. We can\\\'t change the pressure of liquids or solids like this, because the particles are always touching in those states.
• Question 5

A group of students are investigating the rate of the chemical reaction between acid and marble rock (calcium carbonate). The reaction is taking too long to happen. Pick a way of making the reaction go faster, and an explanation for why it would help.

EDDIE SAYS
Grinding the rock into smaller pieces will make the surface area smaller, which increases the reaction rate. The next question will help you think about this some more.
• Question 6

This question is about surface area and volume.

Imagine you had 27 little cubes, each 1 cm x 1 cm x 1 cm.

Here are three ways you could arrange them:

A 27 totally separate cubes

B 3 separate square blocks, each 3 cm x 3 cm x 1 cm

C 1 big cube, 3 cm x 3 cm x 3 cm

Work out the surface area of each arrangement.

EDDIE SAYS
Each little cube has 6 faces, each of them 1 cm2. There are 27 cubes, so the total surface area for A is 27 x 6 x 1 = 162 cm2. For B, each slice has a total surface area of 30 cm2, and there are three of them. For C, there is one cube, each face has area 9 cm2, and there are six of them. If you stick blocks together, you lose surface where the blocks join. Conversely, if you break them apart into smaller pieces, it makes more surface area. The idea of surface area comes up a lot in biology and chemistry- think about the shape of leaves.
• Question 7

Washing depends on chemical reactions to break up particles of dirt. Which of these sentences helps explain why enzymes are often added to washing powders?

Enzymes are a type of catalyst.
Catalysts make chemical processes happen faster.
By using enzymes, we can wash clothes at lower temperatures than without enzymes.
EDDIE SAYS
Chemical reactions happen much faster if the right catalyst is present. Increasing temperature (by using hot water) is another way to speed up the relevant reactions, but this can damage clothes and requires large amounts of energy.
• Question 8

Would you expect a single enzyme to be able to clean all stains? Select an answer and a reason.

EDDIE SAYS
Most biological washing powders contain at least two types of enzyme- lipase breaks down oily stains, protease works on proteins, and carbohydrase breaks down starch.
• Question 9

How does reducing the activation energy affect the rate of a chemical reaction?

Catalysts reduce the activation energy
EDDIE SAYS
Activation energy is how much energy we need to make a reaction begin to happen (like the energy from a match making a Bunsen burner start to burn gas). The lower it is, the more likely any one collision is to have enough energy to overcome the activation energy.
• Question 10

Which of these ideas would work? Tick all the correct answers.

Grinding up charcoal to make it burn faster
Compressing a gas to make it react faster
Warming an acid to make metal react faster
EDDIE SAYS
Most of these are examples of the ways of speeding up a reaction that you have learned. Diluting an acid will slow down the reaction. The catalytic converter in a car works on converting combustion products (like carbon monoxide) into safer ones (like carbon dioxide); it doesn't help with other reactions.
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