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7.3 Reaction Profiles

GCSE Chemistry · AQA · Chapter 7: Energy Changes

A free GCSE Chemistry lesson for AQA (8462) from OBA School: drawing and reading reaction profiles, activation energy and overall energy change. 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 chemical reactions happen only when particles collide with enough energy
  • Define 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
  • (Higher tier) explain that bond breaking is endothermic and bond making is exothermic

Key words

  • Reaction profile: A graph of energy against the progress of a reaction, showing reactants, 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 (written ΔH at A-level).
  • Catalyst: A substance that speeds up a reaction without being used up. It provides a route 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. Bonds in the reactants must start to break before new bonds can form. The minimum energy needed is the activation energy. This is why many fuels need a spark or flame to start burning, even though burning is exothermic.

Key fact: Once an exothermic reaction starts, the energy it releases can supply the activation energy for more particles, so it 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 released); Endothermic points up (energy taken in)
  • Activation energy; Exothermic measured from the reactants up to the peak; Endothermic measured 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 = 150 kJ/mol, peak = 330 kJ/mol, products = 60 kJ/mol.
Activation energy = peak − reactants = 330 − 150 = 180 kJ/mol.
Overall energy change = products − reactants = 60 − 150 = −90 kJ/mol.
The products are lower than the reactants, so the reaction is exothermic.

Answer: Activation energy 180 kJ/mol; energy change −90 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 are at the same levels. So a catalyst does not change the overall energy change. With a lower activation energy, more collisions have enough energy to react, so the rate increases (more on rates in Chapter 8).

Bond breaking and bond making (Higher tier)

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

  • Breaking bonds needs energy (atoms are pulled apart against the attraction holding them together), so it is endothermic.
  • Making bonds releases energy, so it is exothermic.
  • If more energy is released making bonds than is taken in breaking bonds, the reaction is exothermic overall. If less, it is endothermic.

Link: The early part of a reaction profile (rising to the peak) is where bonds start to break. This is why every reaction, even a very exothermic one, has an activation energy. You will calculate energy changes from bond energies in section 7.4.

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.
  • Forgetting to label the axes, or 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 activation energy? (Choose from: The minimum energy particles need to react when they collide, The energy released by a reaction, The energy of the products, The energy needed to melt the reactants.)

Answer: The minimum energy particles need to react when they collide

Working: Activation energy is the minimum energy colliding particles must have to react.

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; reactant and product levels stay the same.

Question 3. Which describes the reaction profile of an exothermic reaction? (Choose from: The curve rises to a peak, then falls to products below the reactants, The curve rises to a peak, then falls to products above the reactants, The curve falls steadily with no peak, The products and reactants are at the same level.)

Answer: The curve rises to a peak, then falls to products below the reactants

Working: There is always a peak (activation energy). For exothermic reactions the products end up lower.

Related lessons

Continue in the full course

This lesson is section 7.3 (AGCCH0703) of our free GCSE Chemistry 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 Chemistry course.

 

 

 

 

 

 

 

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