3.2.2 - Reaction rates

Question 1

The graph shows that rate of reaction decreases over time. Explain why, in terms of collision theory. (2)

Answer 1

Rate
- (Acid) concentration decreases
Collisions
- Fewer collisions per second / less frequent collisions

Question 2

Explain how a catalyst increases the rate of reaction. (2)

Answer 2

• Catalyst lowers the activation energy (by providing an alternative route)
• A greater proportion of molecules have more energy greater than/equal to activation energy

Question 3

N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = −92 kJ mol−1
Describe and explain the effect of increasing the pressure on the rate of this reaction. (2)

Answer 3

• (Increase in pressure) increases the rate and because molecules are closer together
• So there are more collisions per unit time

Question 4

Using the Boltzmann distribution model, explain why the rate of a reaction increases in the presence of a catalyst. (2)

Answer 4

• Catalyst provides a lower activation energy
• Ec shown below Ea on Boltzmann distribution
• More molecules/particles/collisions have energy above activation energy (with catalyst) / greater area under curve above activation energy

Question 5

Explain why use of the catalyst reduces energy demand and benefits the environment. (2)

Answer 5

• Lower temperatures / less heat / less thermal energy
• Less fossil fuels / oil / coal / gas / non-renewable fuels
• Reduces CO2 emissions

Question 6

A student was asked to carry out an experiment to determine the initial rate of reaction of zinc and hydrochloric acid. The student plans to collect a total of about 72 cm3 of hydrogen at RTP and to use an excess of zinc. The student selects the following apparatus:
* the apparatus shown in the diagram
* 100 cm3measuring cylinder
* stop clock
* 2 decimal place balance
Outline how the student could carry out the experiment and explain how the results could be processed graphically. (6)

Answer 6

1 - Method:
- Measure mass of (excess) zinc (using
2 decimal place balance)
- Measure volume of hydrochloric acid (using measuring cylinder)
- Mix zinc and acid in flask
- Measure gas volume at time intervals

2 - Calculations:
- Moles of hydrogen 72/24000 = 0.00300 mol
- Minimum mass of zinc0.003 × 65.4 = 0.20 g
- Moles of hydrochloric acid
Zn + 2HCl → ZnCl2 + H2
0.00300 × 2 = 0.00600 mol
- volume / concentration of acid If [HCl(aq)] = 0.1 mol
dm-3 appropriate
- volume of acid = 0.006 × 1000/0.1
= 60 cm3

3 - Processing results:
- Plot a graph of volume against time
- Draw a tangent at t = 0
- Gradient of tangent = initial rate
- Gradient = volume / time

Question 7

Using a Boltzmann distribution, explain how a catalyst increases the rate of a chemical reaction. (2)

Answer 7

• Catalyst lowers the activation energy (by providing an alternative route)
• (With a catalyst a) greater proportion of molecules with energy greater than activation energy
  /(With a catalyst a) greater proportion of molecules with energy equal to the activation energy
  /(With a catalyst there is a) greater area under curve above the activation energy

Question 8

State two ways that the use of catalysts helps chemical companies to make their processes more sustainable and less harmful to the environment. (2)

Answer 8

• Lower temperatures / lower pressures (can be used)
• Lower energy demand / uses less fuel / reduces CO2 emissions (different reactions can be used with) greater atom economy /less waste /can reduce use of toxic solvents
• Can reduce use of toxic reactants (catalysts are often enzymes) generating specific products

Question 9

The student plots a graph of total mass (reagents + container) against time. Outline a method that could be used to obtain the results that are plotted on the graph. Your answer should include the apparatus required and the procedure for the experiment. (3)

Answer 9

• Flask / beaker , balance and stopwatch / stop clock
• Records mass at time intervals
• Time interval quoted between 10-50s

Question 10

CO2(g) + 3H2(g) ⇌ CH3OH(g) + H2O(g) ΔH = −49 kJ mol−1
Describe and explain the effect of increasing the pressure on the reaction rate. (2)

Answer 10

• Increased rate and greater concentration of molecules / more molecules per (unit) volume
• More collisions per second / more frequent collisions

Question 11

Explain why the use of a catalyst can reduce the demand for energy. (1)

Answer 11

• Allows reactions to take place at lower temperatures

Question 12

Explain how increasing the temperature increases the rate of reaction. (1)

Answer 12

• (At a higher temperature) more molecules have energy above
  activation energy / greater area under the curve above the activation energy