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C8.2 Collision Theory and Surface Area

GCSE Combined Science · AQA · Chapter C8: Rates and Equilibrium

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: why particles must collide with enough energy to react, what activation energy is, and why smaller pieces react faster. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • State collision theory and define activation energy
  • Draw and label a reaction profile showing the activation energy
  • Calculate surface area to volume ratios for cubes and use them to compare lumps and powders
  • Explain, using collision theory, why increasing surface area increases the rate of reaction
  • Interpret graphs comparing powders and lumps

Key words

  • Collision theory: Chemical reactions can only happen when reacting particles collide with sufficient energy.
  • Activation energy: The minimum amount of energy that particles must have to react when they collide.
  • Successful collision: A collision with energy equal to or greater than the activation energy, so a reaction happens.
  • Frequency of collisions: How many collisions happen each second.
  • Surface area to volume ratio: Surface area ÷ volume. Cutting a solid into smaller pieces increases it.
  • Reaction profile: A diagram showing the energy of reactants and products and the activation energy.

Collision theory

Particles in liquids, gases and solutions are always moving. For a reaction to happen, reacting particles must collide, and the collision must have at least the activation energy. Most collisions do not have enough energy: the particles just bounce apart.

Key fact: The rate of reaction depends on the frequency of successful collisions: more successful collisions per second means a faster reaction. So anything that makes collisions more frequent, or gives a greater proportion of them enough energy, speeds up the reaction.

Activation energy and reaction profiles

A reaction profile shows energy on the vertical axis. The activation energy is the difference between the energy of the reactants and the top of the “hill”. Think of it as an energy barrier that the colliding particles must get over.

Exam tip: Measure activation energy from the reactants line to the peak, not from zero and not from the products.

What affects the rate?

  • Surface area of a solid; What changes smaller pieces / powder; Effect on collisions more frequent collisions (this lesson)
  • Temperature; What changes higher temperature; Effect on collisions more frequent and more energetic collisions (C8.3)
  • Concentration or pressure; What changes more particles in the same volume; Effect on collisions more frequent collisions (C8.4)
  • Catalyst; What changes added catalyst; Effect on collisions lower activation energy, so more collisions are successful (C8.5)

Surface area and surface area to volume ratio

A solid can only react at its surface, where particles of the other reactant can reach it. If you break a solid into smaller pieces, more of its particles are on the surface. The total surface area goes up, even though the mass stays the same.

Worked example: surface area to volume ratio

A cube has sides of 3 cm. Each face = 3 × 3 = 9 cm², and a cube has 6 faces.
Surface area = 6 × 9 = 54 cm². Volume = 3 × 3 × 3 = 27 cm³. SA : V = 54 : 27 = 2 : 1.
Cut into 1 cm cubes: 27 cubes, each with SA = 6 cm². Total SA = 27 × 6 = 162 cm². SA : V = 162 : 27 = 6 : 1.

Answer: the ratio increases from 2 : 1 to 6 : 1

Maths skill: For any cube, SA : V = 6 ÷ side length. So if the side is made 10 times smaller, the surface area to volume ratio becomes 10 times bigger. Always state the unit of length: a ratio of 6 per cm is not the same as 6 per m.

Worked example: comparing two sizes of chip

A large marble chip is roughly a 2 cm cube; a small chip is roughly a 0.5 cm cube.
Large: SA : V = 6 ÷ 2 = 3 per cm. Small: SA : V = 6 ÷ 0.5 = 12 per cm.
For the same total mass of marble, the small chips expose 12 ÷ 3 = 4 times as much surface to the acid.

Answer: about 4 times the surface area, so collisions with acid particles are much more frequent

Explaining the effect of surface area

Smaller pieces → larger surface area to volume ratio → more particles exposed → more frequent collisions → faster rate.

The energy of the collisions does not change, so the proportion of collisions that are successful stays the same. There are simply more collisions each second.

Here 0.20 g of calcium carbonate reacted with excess hydrochloric acid, once as powder and once as large chips.

The final volume is the same because the same mass of calcium carbonate (the limiting reactant) was used. Surface area changes how fast the gas is made, not how much.

Key fact: Fine powders, such as flour, custard powder or coal dust, have such a huge surface area that a cloud of them in air can ignite and explode. Factories control dust for this reason.

Common mistakes

  • Saying “the particles collide more”. Say more frequent collisions (more collisions per second).
  • Saying a bigger surface area gives the particles more energy. Surface area only changes the frequency of collisions.
  • Saying powder makes more product. With the same mass of solid, the final amount of product is the same.
  • Thinking every collision causes a reaction. Only collisions with energy at least equal to the activation energy are successful.
  • Measuring activation energy from the products or from zero on a reaction profile.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the activation energy of a reaction? (Choose from: The minimum energy that particles must have to react when they collide, The energy given out when a reaction finishes, The energy needed to melt the reactants, The total energy of the products.)

Answer: The minimum energy that particles must have to react when they collide

Working: Activation energy is the minimum energy particles must have to react when they collide.

Question 2. The diagram shows the reaction profile for a reaction. Which arrow, A, B or C, shows the activation energy? (Choose from: A, B, C.)

Answer: B

Working: B goes from the reactants up to the peak, so it is the activation energy. C is the overall energy change (reactants to products) and A runs from the products to the peak.

Question 3. Fill in the missing word. For a reaction to happen, particles must collide with at least the ______ energy.

Answer: activation

Working: Collisions must have at least the activation energy to be successful.

Related lessons

Continue in the full course

This lesson is section C8.2 (AGCSC2502) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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