Rates of Reaction and Equilibrium

Why some reactions race and others crawl

Some reactions are over in a flash, while others take years. The speed of a reaction is called its rate. Reactions happen because particles bump into one another, and a bump only works if the particles hit with enough energy. So the rate depends on how often the particles collide and how hard. Anything that makes collisions more frequent or more energetic speeds the reaction up: heating the mixture, using a more concentrated solution, breaking a solid into smaller pieces, or adding a catalyst. Some reactions can also run backwards as well as forwards, and in a closed container they can settle into a balance called equilibrium. This module covers collision theory, the factors that change the rate, how a rate is measured, and what happens at equilibrium.

Rates and equilibrium words

Learn these terms before the factors are compared. They name the speed of a reaction and what changes it.

Match each factor to why it speeds a reaction

Fast reactions against slow reactions

Every rule about rate comes back to how often particles collide successfully. Comparing a fast reaction with a slow one shows the pattern.

Which change speeds the reaction?

A solid reactant is used as one large lump. Which change would make the reaction faster?

• Break it into a fine powder
• Use one even larger lump
• Cool the mixture down
• Make the solution more dilute

Think in collisions

You do not need to memorise the factors one by one. Every one of them changes the rate in the same underlying way: it changes how often particles collide, or how hard. Warm the mixture and the particles move faster, so they meet more often and hit harder. Add more particles to the same space, by raising the concentration or the pressure, and they meet more often. Spread a solid out into smaller pieces and more of it is in the path of collisions. Add a catalyst and each collision needs less energy to work, so more of them succeed. When a question describes a change, do not reach for a fact: ask whether the change makes collisions more frequent or more energetic, and the answer follows.

Pick the ways to speed a reaction

Select every change that would increase the rate of a reaction.

• Raise the temperature
• Increase the concentration
• Add a catalyst
• Cool the mixture down
• Use larger lumps of solid

Order the steps to measure a reaction rate

Put the steps of measuring a rate in a sensible order.

• Choose what to measure, such as the volume of gas given off
• Set up the apparatus and start the reaction
• Record the measurement at regular time intervals
• Plot the readings against time
• Work out the rate from how fast the amount changes

Complete the rates facts

Reactions happen when particles _____ with enough energy. Raising the _____ makes them collide more often and harder. Breaking a solid into smaller pieces increases its _____. A _____ speeds a reaction without being used up.

Work out the average rate

A reaction gives off 60 cubic centimetres of gas in 30 seconds. Work out the average rate of reaction, in cubic centimetres per second, by dividing the volume by the time.

A busy shop doorway

Picture a shop doorway. When only a few people wander past, the door is hardly ever knocked; when a crowd streams past at rush hour, it is pushed open again and again. Reactions work the same way, because particles have to bump into one another to react, so anything that makes the bumps more frequent or harder speeds things up. Warm the particles and they hurry along and meet more often; pack more of them into the same space and they cross paths more; spread a solid out so more of it is in the way of the crowd. Every rule you meet is really this one idea wearing a different coat: more successful bumps mean a quicker reaction.

Tap the factors that increase rate

Tap the TWO changes that would increase the rate of a reaction.

• raising the temperature
• cooling the mixture
• adding a catalyst
• using larger lumps

Adjust each reaction

Read each situation and choose the best answer.

• A reaction between an acid and a solid is too slow. The solid is in large lumps. What should you change to speed it up?
• You warm a reaction mixture. What happens to the rate, and why?
• A reversible reaction in a closed container reaches equilibrium. What is true at equilibrium?

Build a rates sentence

Choose the word for each gap to complete one point about rates.

Explain what controls the rate of a reaction

A friend is confused about why the same reaction can be fast one day and slow another. Using collision theory, explain what controls the rate of a reaction.

• Explain collision theory in your own words
• Explain how temperature affects the rate
• Explain how concentration and surface area affect the rate
• Explain what a catalyst does
• Explain what happens at equilibrium in a reversible reaction