1.
State what is meant by rate of reaction.
[1 mark]
- how quickly reactants are used up or products form
JD SCIENCE
Unit 5 answers — C5 Monitoring and controlling chemical reactions
Indicative marking points. Award marks for equivalent scientific or mathematical wording. Do not issue this sheet with the student worksheet.
2.
State what is meant by collision theory.
[1 mark]
- particles must collide with enough energy and the correct orientation
3.
State what is meant by activation energy.
[1 mark]
- minimum energy for a successful collision
4.
State what is meant by a catalyst.
[1 mark]
- increases rate by providing an alternative path with lower Ea
5.
State what is meant by equilibrium.
[1 mark]
- in a closed system, forward and reverse rates are equal
6.
State what is meant by Le Chatelier's principle.
[1 mark]
- if a change is made, the equilibrium shifts to oppose the change
7.
State what is meant by the Haber process.
[1 mark]
- N₂ + 3H₂ ⇌ 2NH₃, exothermic
8.
State what is meant by surface area.
[1 mark]
- for a solid, more area means more collisions
9.
State what is meant by a closed system.
[1 mark]
- no substances can enter or leave
10.
State what is meant by a reversible reaction.
[1 mark]
- can go forwards and backwards
11.
Grace is asked about rate of reaction in a OCR Chemistry paper.
(a) [2 marks]
- measured as quantity / time
(b) [3 marks]
- why rate is fastest at the start
12.
Grace is asked about collision theory in a OCR Chemistry paper.
(a) [2 marks]
- successful collisions cause reaction
(b) [3 marks]
- why a higher concentration increases rate
13.
Grace is asked about activation energy in a OCR Chemistry paper.
(a) [2 marks]
- affected by a catalyst
(b) [3 marks]
- why not all collisions succeed
14.
Grace is asked about a catalyst in a OCR Chemistry paper.
(a) [2 marks]
- not used up
(b) [3 marks]
- why industry uses catalysts
15.
Grace is asked about equilibrium in a OCR Chemistry paper.
(a) [2 marks]
- concentrations stay constant
(b) [3 marks]
- why it looks as if the reaction has stopped
16.
Grace is asked about Le Chatelier's principle in a OCR Chemistry paper.
(a) [2 marks]
- used to predict conditions
(b) [3 marks]
- why raising pressure favours the side with fewer gas moles
17.
A gas syringe collects 30 cm³ in 20 s. Mean rate?
[3 marks]
- Final answer: 1.5 cm³/s
18.
Mass falls by 0.80 g in 40 s. Mean rate of mass loss?
[3 marks]
- Final answer: 0.020 g/s
19.
From a graph, the tangent at t=0 has gradient 0.40 cm³/s. What does this represent?
[3 marks]
- Final answer: initial rate
20.
Concentration doubles and rate doubles. What is the order with respect to that reactant (A-level idea at GCSE: describe the effect)?
[3 marks]
- Final answer: rate is proportional to concentration
21.
A reaction finishes in 80 s at 20°C and 20 s at 30°C. By what factor has the rate increased?
[3 marks]
- time is 1/4 so rate ×4
- Final answer: 4
22.
Haber: if 10 mol N₂ and 30 mol H₂ react completely, mol of NH₃?
[3 marks]
- 1:3:2
- Final answer: 20 mol
23.
Grace carries out the rate required practical (marble and acid, or sodium thiosulfate and acid). This is a typical OCR required-practical style question.
(a) [2 marks]
- Independent: concentration / temperature / chip size
- Dependent: time to X on paper / volume of gas
(b) [2 marks]
- same volume of acid
- so the test is fair / only one variable is changed
(c) [2 marks]
- subjective end-point / gas leaks
24.
Grace carries out investigating the effect of a catalyst on hydrogen peroxide. This is a typical OCR required-practical style question.
(a) [2 marks]
- Independent: catalyst type
- Dependent: volume of O₂ / time
(b) [2 marks]
- same volume and concentration of H₂O₂
- so the test is fair / only one variable is changed
(c) [2 marks]
- catalyst not the same surface area
25.
Grace carries out drawing a rate graph from gas volume data. This is a typical OCR required-practical style question.
(a) [2 marks]
- Independent: time
- Dependent: volume
(b) [2 marks]
- same apparatus
- so the test is fair / only one variable is changed
(c) [2 marks]
- syringe sticks
26.
Grace carries out the disappearing-cross (thiosulfate) practical. This is a typical OCR required-practical style question.
(a) [2 marks]
- Independent: concentration or temperature
- Dependent: time for the cross to disappear
(b) [2 marks]
- same volume, same cross
- so the test is fair / only one variable is changed
(c) [2 marks]
- different people judge the end-point differently
27.
Compare a catalyst with an increase in temperature.
[4 marks]
- catalyst: lower Ea, not used up
- temperature: more particles above Ea and more collisions
28.
Compare a powder with a large lump of the same mass.
[4 marks]
- powder has larger SA
- faster rate
29.
Compare dynamic equilibrium with a finished irreversible reaction.
[4 marks]
- equilibrium: both directions continue
- irreversible: reactants used up
30.
Compare increasing pressure (Haber) with increasing temperature (Haber).
[4 marks]
- higher P: better yield, expensive
- higher T: worse yield, better rate
31.
Apply your knowledge of the Haber process to this situation: lower T favours yield but slower rate
[3 marks]
- lower T favours yield but slower rate
32.
Apply your knowledge of surface area to this situation: smaller SA
[3 marks]
- smaller SA