Edexcel GCSE Combined Science: Chemistry: Rates of Reaction and Energy Changes
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Marble chips (calcium carbonate) react with dilute hydrochloric acid to produce carbon dioxide gas. A student found that the reaction goes faster when:
- the temperature of the acid is increased, and
- the marble is used as a fine powder instead of large chips.
Explain, using collision theory, why increasing the temperature and increasing the surface area each make this reaction faster. (6 marks)
In a core practical investigating rate of reaction, a student reacted excess marble chips with dilute hydrochloric acid and measured the total volume of carbon dioxide gas collected over time. The results are shown below.
| Time / s | Volume of CO₂ collected / cm³ |
|---|---|
| 0 | 0 |
| 10 | 28 |
| 20 | 46 |
| 30 | 58 |
| 40 | 60 |
| 50 | 60 |
(a) Calculate the mean rate of reaction, in cm³/s, over the first 20 seconds. Show your working and give the unit. (2 marks)
(b) State and explain what the readings between 40 s and 50 s tell you about the reaction. (2 marks)
A catalyst can be used to speed up a chemical reaction without being used up itself.
(a) State, in terms of activation energy, how a catalyst increases the rate of a reaction. (1 mark)
(b) Hydrogen peroxide decomposes to form water and oxygen. A small amount of manganese(IV) oxide catalyst makes this reaction much faster. Explain why the catalyst can be reused for many reactions. (2 marks)
When a student added ammonium nitrate to water in a beaker and stirred, the temperature of the mixture fell from 21 °C to 13 °C.
(a) State whether this change is exothermic or endothermic, and give a reason for your answer using the temperature data. (2 marks)
(b) In terms of bond breaking and bond making, explain what must be true about the energy changes in an endothermic reaction. (1 mark)
Some reactions are reversible. A reversible reaction is often written using the symbol ⇌, for example:
A + B ⇌ C + D
Explain what is meant by a reversible reaction. (2 marks)
On a reaction profile diagram, the activation energy is shown as an energy barrier between the reactants and the products.
Define the term activation energy. (1 mark)