Page 40 - ACE YR IGCSE A TOP APPROACH TO CHEM

Enthalpy change = –824 kJ/mol [1] Correct energy level change [1]
(410 × 5) + 347 + 336 + 365 + (496 × 3) = 4586 kJ/mol Correct label [1]
(805 × 4) + (365 × 6) = 5410 kJ/mol Arrow pointing up [1]
Enthalpy change = energy taken in to break the
bonds – energy given out to form the bonds (d) (i) The minimum energy needed by the
= 4586 – 5410 particles [1]
= –824 kJ/mol to start a chemical reaction by collision [1]
(c) Exothermic [1] (ii) The catalyst reduces the activation energy.
The energy given out to form the bonds [1]
is greater than [1]
the energy taken in to break the bonds [1]
6 Chemical Reactions
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3 (a) V [1]
Part 1: Multiple-choice Questions
– + 1 D
(b) (i) Magnesium is more reactive. [1] Keep the concentration of reactants the same will not
More reactive metals will tend to lose electrons change the rate of reaction.
to form ions. 2 C
(ii) Mg ➞ Mg + 2e [1] Based on the graph, experiment 2 has a higher rate of
2+
reaction. Therefore, the molecules in experiment 2 have
(c) (i) Hydrogen [1] more kinetic energy.
In the electrolyte, there are H and Na . The reactions experiments 1 and 2 complete at the
+
+
Na is more reactive. It will stay as an ion in same volume of gas showing that the total volume of
+
the solution. H will gain electrons to form gas collected is the same. This is because the volume of
+
hydrogen gas at the positive pole. reactants has no change.
+
(ii) 2H + 2e ➞ H [1] When a catalyst is used, the rate of reaction increases.
2 The catalyst lowers the activation energy, thus the
(d) (i) Any one from: [1] reaction completes at a higher rate. If a catalyst is added,
• Change the magnesium strip to a more the line should be above experiment 1.
reactive metal e.g. calcium, lithium
• Change the copper strip to a less reactive 3 B
metal e.g. silver, gold The difference between the two experiments is the size
The further apart the metals are in the of magnesium used. In experiment 2, the powdered
reactivity series, the higher the voltage will be. magnesium used has a bigger surface area than the
(ii) Decreases [1] magnesium ribbon. Bigger surface area promotes a
higher rate of reaction.
Zinc is less reactive than magnesium. So, the line plotted for experiment 2 is above the line
(e) (i) Copper, lead, iron, zinc, magnesium [1] plotted for experiment 1 on the graph.
(ii) 0.63 V [1] This shows that the reaction completes at a higher rate /
took shorter time to collect the same amount of gas.
1.1 – 0.47 = 0.63 V
4 (a) Energy taken in to break the bonds 4 A
The smaller the size of a substance, the higher its surface

= 1941 kJ/mol [1] area. The higher surface area exposes more particles for
Energy given out to form the bonds the reaction.
= 1616 kJ/mol [1] 5 D
Enthalpy change = +325 kJ/mol [1] If the bung is not replaced by cotton wool, no gas will
945 + (498 × 2) = 1941 kJ/mol escape. Therefore, the reading on the balance will not show
2 × (201 + 607) = 1616 kJ/mol any changes. The rate of reaction cannot be calculated.
Enthalpy change = energy taken in to break the
bonds – energy given out to form the bonds 6 C
= 1941 – 1616 A catalyst is used to lower the activation energy.
= +325 kJ/mol Therefore, the equilibrium can be achieved faster.
Increasing the temperature will always increase the rate
(b) Endothermic [1] of reaction. (Do not confuse between equilibrium and
(c) rate of reaction.)
N + 3H L 2NH 3
2
2
This symbol “ L ” shows it is a reversible reaction.
2NO 2
Based on the equation, the mole ratio of the reactants
to the product is 4 : 2. A decrease in pressure favours the
direction that produces more gas molecules.
N 2 + 2O 2 Therefore, lowering the pressure will reduce the yield of
ammonia.

Cambridge IGCSE TM
152 Ace Your Chemistry

Answers.indd 152 3/4/22 3:54 PM