# Understand Reaction Rates

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

### 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.

There are five ways that we can speed up 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

Temperature

When the temperature increases, the particles have more energy. More energy means more movement, so the frequency of collisions increases and the collisions that happen are harder- they have more energy. 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.

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.

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

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

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.

Imagine you are reacting magnesium metal with hydrochloric acid. If we reduce the concentration of the acid by adding water, how will the reaction change? Choose a change and a reason.

Suppose we have 2 g calcium carbonate. Which of these forms has the highest 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, are not used up in the reaction?

Catalysts

Monomers

Triggers

If we increase the temperature of a substance, how does that change the way the particles in the substance move?

They move faster

They move slower

They move the same way

If the particles in a substance are moving faster, how will that affect collisions between the particles?

Collisions will be more frequent

Collisions will be less frequent

Collisions will be more energetic

Collisions will be less energetic

Fill in the gaps in this paragraph. Each missing word is either enzymes or catalysts.

Collisions will be more frequent

Collisions will be less frequent

Collisions will be more energetic

Collisions will be less energetic

Which of these methods would make a chemical reaction happen faster?

increase temperature

increase pressure

reduce concentration

increase surface area

removing a catalyst

Which of these methods would make a chemical reaction happen more slowly?

increase temperature

reduce pressure

reduce concentration

grinding a reactant into a power

removing a catalyst

• Question 1

Imagine you are reacting magnesium metal with hydrochloric acid. If we reduce the concentration of the acid by adding water, how will the reaction change? Choose a change and a reason.

EDDIE SAYS
If we dilute the acid, the collisions between metal and acid particles will be less frequent, because the acid particles will be more spread out. That will make the reaction go more slowly.
• Question 2

Suppose we have 2 g calcium carbonate. Which of these forms has the highest surface area?

Powdered calcium carbonate
EDDIE SAYS
Imagine starting with a big block and cutting it into smaller pieces. Each cut adds more surface area. Any substance in a powder form has more surface area than a single block of the same substance.
• Question 3

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 4

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 collisions would be less frequent. Fewer collisions mean less chance of the particles colliding with the correct energy and orientation. More pressure will increase the number of collisions.
• Question 5

What do we call chemicals that speed up a reaction, are not used up in the reaction?

Catalysts
EDDIE SAYS
Catalysts are chemicals that speed up a reaction but are not involved in the reaction itself.
• Question 6

If we increase the temperature of a substance, how does that change the way the particles in the substance move?

They move faster
EDDIE SAYS
If you heat something up, you put more energy into the something. Looking at the particles, the increased energy makes them move round faster.
• Question 7

If the particles in a substance are moving faster, how will that affect collisions between the particles?

Collisions will be more frequent
Collisions will be more energetic
EDDIE SAYS
Imagine a hall with some people in. If they walk, they won\'t collide very often, and the collisions will be gentle. If they run, there will collide more frequently, and the collisions will hurt more. People walking / running round a room are quite a good model for particles in a gas.
• Question 8

Fill in the gaps in this paragraph. Each missing word is either enzymes or catalysts.

EDDIE SAYS
Enzymes are important for living things- life depends on lots of chemical reactions happening quickly. You will learn lots about enzymes in biology; remember that (in general terms) enzymes and catalysts have similar effects.
• Question 9

Which of these methods would make a chemical reaction happen faster?

increase temperature
increase pressure
increase surface area
EDDIE SAYS
Take these ideas back to collision theory; all the ways of increasing reaction rate work by making successful collisions between particles more frequent.
• Question 10

Which of these methods would make a chemical reaction happen more slowly?

reduce pressure
reduce concentration
EDDIE SAYS
Take these ideas back to collision theory; all the ways of reducing reaction rate work by making successful collisions between particles less frequent. Grinding reactants into powders increases surface area, so that will make the reaction faster.
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