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.
Learn this properly: Calculating Rates of ReactionA student investigated how temperature affects the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid. As the reaction proceeds, a pale yellow precipitate of sulfur forms and the mixture turns cloudy. The student timed how long it took for a cross drawn under the flask to disappear, at several temperatures.
Explain, using collision theory, why increasing the temperature increases the rate of this reaction (i.e. why the cross disappears more quickly). (6 marks)
A student measured the volume of gas produced in a reaction every 10 seconds. The results are shown in the table.
| Time / s | Volume of gas / cm³ |
|---|---|
| 0 | 0 |
| 10 | 32 |
| 20 | 52 |
| 30 | 60 |
| 40 | 60 |
(a) Calculate the mean rate of reaction over the first 20 seconds, in cm³/s. (2 marks)
(b) Explain why the rate of reaction was fastest at the start and then slowed down. (2 marks)
The Haber process makes ammonia from nitrogen and hydrogen. The reaction is reversible and reaches a dynamic equilibrium:
N2(g)+3H2(g)⇌2NH3(g)
The forward reaction is exothermic. The industrial conditions used are about 450 °C and 200 atmospheres pressure, with an iron catalyst.
(a) A temperature higher than 450 °C would give a faster rate but a lower yield of ammonia. Explain why 450 °C is used rather than a much higher temperature. (2 marks)
(b) State the effect of the iron catalyst on the position of equilibrium. (1 mark)
A reversible reaction reaches equilibrium in a sealed container:
2SO2(g)+O2(g)⇌2SO3(g)
There are 3 moles of gas on the left-hand side and 2 moles of gas on the right-hand side.
(a) State what happens to the position of equilibrium (and therefore the amount of SO3) if the pressure is increased. (1 mark)
(b) Explain your answer to part (a) in terms of the number of moles of gas on each side. (2 marks)
A student reacted marble chips (calcium carbonate) with dilute hydrochloric acid. They repeated the experiment using the same mass of calcium carbonate as a fine powder instead of large chips, keeping everything else the same.
Explain why the reaction is faster with the powdered calcium carbonate. (2 marks)
A catalyst is often added to a reaction to speed it up.
State what is meant by a catalyst. (1 mark)