# 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. The student will then link these factors to the idea of collision theory.

### 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.

Why does increasing temperature make the rate of chemical reactions increase? Tick all the correct answers.

Particles collide less frequently

Collisions have more energy

Particles collide more frequently

Collisions have less energy

Blocks of rock break into smaller pieces

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.

Imagine taking two identical glow sticks, and putting one in a hot place, and the other in a cold place. How would the two glow sticks be different?

Particles collide less frequently

Collisions have more energy

Particles collide more frequently

Collisions have less energy

Blocks of rock break into smaller pieces

Pick two phrases to complete this sentences explaining how concentration affects rate of reaction.

"If the concentration of a reactant solution increases...

Particles collide less frequently

Collisions have more energy

Particles collide more frequently

Collisions have less energy

Blocks of rock break into smaller pieces

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

Particles collide less frequently

Collisions have more energy

Particles collide more frequently

Collisions have less energy

Blocks of rock break into smaller pieces

A group of students are investigating the rate of the chemical reaction between acid and marble rock (calcium carbonate). They find that the reaction is happening too quickly to study. How could they slow down the reaction, so they could make better observations?

Grind the rock into finer pieces

Use larger pieces of rock

Use thin slices of rock

The next two questions are about the surface area of differently-sized pieces of a solid.

Let's start with a cube, 2 cm x 2 cm x 2 cm.

What are its total surface area and volume?

Grind the rock into finer pieces

Use larger pieces of rock

Use thin slices of rock

The next two questions are about the surface area of differently-sized pieces of a solid.

Let's start with a cube, 2 cm x 2 cm x 2 cm.

In the last question, you worked out that the total surface area of this cube is 24 cm2.

Now cut it into 8 pieces, each of them a 1 cm x 1 cm x 1 cm cube.

What is the total surface area of all the cubes now? What is the total volume of all the cubes now?

Grind the rock into finer pieces

Use larger pieces of rock

Use thin slices of rock

Which of these sentences about catalysts are true? Tick the correct ones.

Catalysts cause chemical reactions to happen more quickly.

Catalysts have to be replaced, because they get used up.

Catalysts make particles collide more frequently

Catalysts make it more likely that a collision will be successful

We can use the same catalysts for all reactions.

What is the name for catalysts in biological systems?

How do catalysts affect the activation energy of chemical reactions?

Activation energy increases

Activation energy is unchanged

Activation energy decreases

• Question 1

Why does increasing temperature make the rate of chemical reactions increase? Tick all the correct answers.

Collisions have more energy
Particles collide more frequently
EDDIE SAYS
Reaction rates increase with the frequency of successful collisions (correct orientation, enough energy). Increasing temperature helps with both factors.
• 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.

Imagine taking two identical glow sticks, and putting one in a hot place, and the other in a cold place. How would the two glow sticks be different?

EDDIE SAYS
The reaction will be faster in the hot glow stick. That means that energy is released more quickly, so the hot stick will be brighter than the cold one. Because the reaction is faster, the reaction will reach completion sooner, so the hot stick will glow for less time than the hot one. Try it and see!
• Question 3

Pick two phrases to complete this sentences explaining how concentration affects rate of reaction.

"If the concentration of a reactant solution increases...

EDDIE SAYS
If there are more reactant particles in a fixed volume of solution, collisions will happen more frequently, increasing the reaction rate. This is why concentrated acids and alkalis are more dangerous than dilute ones.
• 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). They find that the reaction is happening too quickly to study. How could they slow down the reaction, so they could make better observations?

Use larger pieces of rock
EDDIE SAYS
If we change the size and shape of the rock pieces, we will need to reduce the total surface area. We can do this by having larger pieces. The next questions will take you through the maths of this.
• Question 6

The next two questions are about the surface area of differently-sized pieces of a solid.

Let's start with a cube, 2 cm x 2 cm x 2 cm.

What are its total surface area and volume?

EDDIE SAYS
There are 6 faces, each of them 4 cm2, so the total surface area is 24 cm2. The volume is 2 x 2 x 2 = 8 cm3.
• Question 7

The next two questions are about the surface area of differently-sized pieces of a solid.

Let's start with a cube, 2 cm x 2 cm x 2 cm.

In the last question, you worked out that the total surface area of this cube is 24 cm2.

Now cut it into 8 pieces, each of them a 1 cm x 1 cm x 1 cm cube.

What is the total surface area of all the cubes now? What is the total volume of all the cubes now?

EDDIE SAYS
Each cube has 6 faces, each of them 1 cm2. There are 8 cubes, so the total surface area is 8 x 6 x 1 = 48 cm2. Cutting the block into smaller pieces won't change the volume, so that is still 8 cm3.
• Question 8

Which of these sentences about catalysts are true? Tick the correct ones.

Catalysts cause chemical reactions to happen more quickly.
Catalysts make it more likely that a collision will be successful
EDDIE SAYS
Catalysts do speed up reactions- by lining up reactants correctly, and weakening the bonds which need to be broken. This doesn't change the frequency of collisions, but does increase the chance that they will be successful. Because catalysts hold particles in specific ways, each reaction needs its own catalyst.
• Question 9

What is the name for catalysts in biological systems?

enzyme
enzymes
EDDIE SAYS
Enzymes are catalysts made and used by living organisms, to make biochemical reactions (like digestion, or photosynthesis) happen faster. The idea is the same as inorganic catalysts, but they have different properties, because they are made of proteins.
• Question 10

How do catalysts affect the activation energy of chemical reactions?

Activation energy decreases
EDDIE SAYS
Catalysts reduce the activation energy for the reaction, because of the way the reactants bond to the catalyst. That means that any collisions are more likely to have enough energy to make a reaction happen.
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