(a) A student investigates the reaction of small pieces of zinc with excess dilute sulfuric acid of three different concentrations. The time taken for each reaction to finish is recorded.
The three concentrations of the acid are:
- 0.2 mol/dm3
- 0.4 mol/dm3
- 0.8 mol/dm3
All other conditions stay the same.
Table 8.1 shows the time taken for each reaction to finish.
(i) Complete Table 8.1 by writing the concentrations in the first column.
(ii) Describe the effect on the time taken for the reaction to finish when the reaction is carried out in the presence of a catalyst. All other conditions stay the same.
(iii) Describe the effect on the time taken for the reaction to finish when larger pieces of zinc are used instead of small pieces of zinc. All other conditions stay the same.
(b) Dilute sulfuric acid is electrolysed using inert electrodes.
(i) Name the products at the positive and negative electrodes.
(ii) Choose from the list the metal used as an inert electrode. Draw a circle around your chosen answer.
calcium magnesium platinum sodium
(c) Zinc is a solid at room temperature. Describe the arrangement and separation of the particles in solid zinc.
▶️ Answer/Explanation
(a)(i) The concentrations should be ordered from slowest to fastest reaction time:
- 0.2 mol/dm3 – 92 s (slowest)
- 0.4 mol/dm3 – 46 s
- 0.8 mol/dm3 – 23 s (fastest)
This shows that higher acid concentrations lead to faster reaction times as there are more reactant particles available for collisions.
(a)(ii) The presence of a catalyst would decrease the time taken for the reaction to finish. Catalysts provide an alternative reaction pathway with lower activation energy, increasing the rate of reaction without being consumed.
(a)(iii) Using larger pieces of zinc would increase the time taken for the reaction to finish. Larger pieces have smaller surface area to volume ratio, meaning fewer zinc particles are exposed to the acid at any time, slowing the reaction.
(b)(i) Positive electrode (anode): oxygen
Negative electrode (cathode): hydrogen
During electrolysis of dilute sulfuric acid, water molecules decompose to form these gases.
(b)(ii) The correct inert electrode is platinum. Platinum is used because it doesn’t react with the electrolyte or products of electrolysis, unlike the other metals listed which would react.
(c) Arrangement: The particles in solid zinc are arranged in a regular, repeating pattern (crystalline structure).
Separation: The particles are closely packed together and touching, with strong metallic bonds between them.
This regular, close arrangement explains why zinc is dense and maintains a fixed shape at room temperature.
(a) A student investigates the reaction of small pieces of calcium carbonate with excess dilute hydrochloric acid of three different concentrations. The time taken for each reaction to finish is recorded.
The three concentrations of acid are:
- 0.5 mol/dm3
- 1.0 mol/dm3
- 2.0 mol/dm3
All other conditions stay the same.
Table 8.1 shows the time taken for each reaction to finish.
(i) Complete Table 8.1 by writing the concentrations in the first column.
(ii) Describe the effect on the time taken for the reaction to finish when the reaction is carried out at a lower temperature. All other conditions stay the same.
(iii) Describe the effect on the time taken for the reaction to finish when powdered calcium carbonate is used instead of small pieces of calcium carbonate. All other conditions stay the same.
(b) Molten calcium chloride is electrolysed using inert electrodes.
(i) Name the products at the positive and negative electrodes.
(ii) Choose from the list the substance that is used as an inert electrode. Draw a circle around your chosen answer.
graphite iodine magnesium phosphorus
(c) Carbon dioxide is a gas at room temperature. Describe the motion and separation of the particles in carbon dioxide gas.
▶️ Answer/Explanation
(a)(i) The completed table should be:
- 1.0 mol/dm3 – 32 s
- 0.5 mol/dm3 – 64 s
- 2.0 mol/dm3 – 16 s
This shows that higher concentrations lead to faster reactions (shorter times).
(a)(ii) At a lower temperature, the time taken for the reaction to finish would increase (longer time). This is because lower temperatures decrease the kinetic energy of particles, resulting in fewer successful collisions per second between reactant particles.
(a)(iii) Using powdered calcium carbonate instead of small pieces would decrease the time taken for the reaction to finish (shorter time). This is because powdering increases the surface area of the solid reactant, providing more contact points for collisions with acid particles.
(b)(i) The products are:
- Positive electrode (anode): Chlorine (Cl2)
- Negative electrode (cathode): Calcium (Ca)
This is because during electrolysis of molten calcium chloride, calcium ions (Ca2+) are reduced at the cathode to form calcium metal, while chloride ions (Cl–) are oxidized at the anode to form chlorine gas.
(b)(ii) The correct inert electrode is graphite. Graphite is commonly used as an inert electrode because it conducts electricity well and doesn’t react with the products of electrolysis.
(c) In carbon dioxide gas at room temperature:
- Motion: The particles move rapidly in random, irregular directions with no fixed pattern.
- Separation: The particles are far apart from each other relative to their size, with large spaces between them.
This explains why gases are compressible and can expand to fill their containers.