5.3 Reaction profiles and activation energy
5 exam questions worth 10 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is activation energy?
Choose one answer.
Correct Not quite: the answer is The minimum energy needed for particles to react
Mark scheme
D
Model answer
D: the minimum energy particles need to react when they collide.
On a reaction profile, how can you tell that a reaction is endothermic?
Mark scheme
- the products are at a higher energy than the reactants (1)
Model answer
The products line is above the reactants line.
Explain, in terms of bonds, why a reaction is exothermic.
Mark scheme
- more energy is released when new bonds are made (1)
- than is needed to break bonds in the reactants (1)
Model answer
More energy is released making the bonds in the products than is taken in breaking the bonds in the reactants.
Describe how to draw a reaction profile for an exothermic reaction. Include labels.
Mark scheme
- axes labelled energy and progress of reaction (1)
- products line lower than reactants (1)
- curve rising above reactants (hump) (1)
- activation energy arrow from reactants to top of hump / energy change arrow from reactants to products (1)
Model answer
Draw energy against progress of reaction, with reactants higher than products, joined by a curve with a hump. Label the activation energy from the reactants to the top of the hump, and the energy given out from the reactants down to the products.
Suggest why a fuel such as methane does not burn until it is lit, even though combustion is exothermic.
Mark scheme
- the reaction has an activation energy (1)
- a spark / flame provides the energy needed to start breaking bonds (1)
Model answer
Combustion has an activation energy. The flame or spark supplies the energy needed to start breaking the bonds; after that, the reaction releases enough energy to keep going.
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