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C7.3 Reaction Profiles

GCSE Combined Science · AQA · Chapter C7: Energy Changes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: collisions and activation energy, drawing and reading reaction profiles, catalysts, and bond breaking and making. 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

  • Explain that reactions happen only when particles collide with enough energy (the activation energy)
  • Draw and label reaction profiles for exothermic and endothermic reactions
  • Read the activation energy and overall energy change from a reaction profile
  • Show the effect of a catalyst on a reaction profile
  • Explain that bond breaking is endothermic and bond making is exothermic (Higher tier)

Key words

  • Reaction profile: A graph of energy against the progress of a reaction, showing the reactants, the products and the energy barrier between them.
  • Activation energy: The minimum amount of energy that particles must have to react when they collide.
  • Overall energy change: The difference in energy between the products and the reactants.
  • Catalyst: A substance that speeds up a reaction without being used up. It provides a pathway with a lower activation energy.

Why reactions need activation energy

Chemical reactions can only happen when reacting particles collide with each other with sufficient energy. The minimum energy needed is the activation energy. This is why a gas cooker needs a spark to light, even though burning methane is very exothermic.

Key fact: Once an exothermic reaction starts, the energy it releases supplies the activation energy for more particles, so the reaction keeps going.

Drawing reaction profiles

A reaction profile shows energy (vertical axis) against the progress of the reaction (horizontal axis). The curve rises from the reactants to a peak and then falls to the products.

  • Products compared with reactants; Exothermic lower in energy; Endothermic higher in energy
  • Overall energy change arrow; Exothermic points down (energy given out); Endothermic points up (energy taken in)
  • Activation energy; Exothermic from the reactants up to the peak; Endothermic from the reactants up to the peak

Exam tip: Label the axes (energy; progress of reaction), the reactants and products (you can write their formulae), the activation energy arrow from the reactants to the top of the curve, and the overall energy change arrow from reactants to products.

Reading values from a profile

Worked example: activation energy and energy change

Reactants = 120 kJ/mol, peak = 290 kJ/mol, products = 40 kJ/mol.
Activation energy = peak − reactants = 290 − 120 = 170 kJ/mol.
Overall energy change = products − reactants = 40 − 120 = −80 kJ/mol.
The products are lower than the reactants, so the reaction is exothermic.

Answer: activation energy 170 kJ/mol; energy change −80 kJ/mol (exothermic)

Catalysts and reaction profiles

A catalyst provides a different pathway for the reaction with a lower activation energy. On a profile, the catalysed curve has a lower peak, but the reactants and products stay at the same levels, so the overall energy change does not change. With a lower activation energy, more collisions have enough energy to react, so the rate increases (see C8).

Key fact: A catalyst is not used up, so it does not appear in the overall equation. Enzymes are biological catalysts.

Bond breaking and bond making (Higher tier)

In a reaction, bonds in the reactants are broken and new bonds are formed in the products.

  • Breaking bonds needs energy, so it is endothermic.
  • Making bonds releases energy, so it is exothermic.
  • If more energy is released making new bonds than is taken in breaking old bonds, the reaction is exothermic. If less, it is endothermic.

Link: The rising part of a reaction profile is where bonds start to break. You will calculate energy changes from bond energies in C7.4.

Common mistakes

  • Drawing the activation energy arrow from the products, or from zero. It starts at the reactants.
  • Drawing the overall energy change arrow from the peak. It goes from the reactants level to the products level.
  • Saying a catalyst changes the overall energy change. Only the activation energy is lowered.
  • Labelling the x-axis “time”. Use “progress of reaction”.
  • (Higher tier) Saying that breaking bonds releases energy. Breaking bonds always takes in energy.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the activation energy? (Choose from: The minimum energy that particles must have to react, The energy given out by a reaction, The energy stored in the products, The energy needed to melt the reactants.)

Answer: The minimum energy that particles must have to react

Working: Particles must collide with at least the activation energy for a reaction to happen.

Question 2. True or false? Adding a catalyst changes the overall energy change of a reaction.

Answer: False

Working: False. A catalyst lowers the activation energy only. The reactants and products are at the same energy levels, so the overall energy change is the same.

Question 3. How does a catalyst change a reaction profile? (Choose from: The peak is lower, The products are lower, The reactants are higher, The peak is higher.)

Answer: The peak is lower

Working: The catalyst provides a pathway with a lower activation energy, so the peak of the curve is lower.

Related lessons

Continue in the full course

This lesson is section C7.3 (AGCSC2403) 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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