Page 42 - ACE YR IGCSE A TOP APPROACH TO CHEM

(e) (i) A substance that will oxidise another The ratio of the number of molecules of the
substance [1] reactants to the products = 2 : 2
itself being reduced [1] Increasing the pressure will not affect the yield.
(ii) Potassium manganate(VII) [1] (f) Achieve the equilibrium faster [1]
MnO ➞ Mn 2+
–
4
Mn is reduced to Mn . It changes from 5 (a) The reaction can go forward [1]
7+
2+
purple to colourless. and backward [1]
The reaction is reversible.
3 (a) A chemical reaction that obtains energy [1]
from light [1] (b) Endothermic [1]
Energy is taken in [1]
(b) The uncovered part is exposed to light. [1] to break the bonds [1]
Silver bromide on the film decomposes, [1]
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turns into silver which is black. [1] (c) Turns blue [1]
2AgBr ➞ 2Ag + Br 2 6 (a) A chemical reaction in which a more reactive
(c) (i) Photosynthesis [1] metal displaces [1]
(ii) Chlorophyll [1] a less reactive metal from its compounds [1]
2+
2+
Without chloroplasts, photosynthesis cannot (b) Zn(s) + Cu (aq) ➞ Zn (aq) + Cu(s)
be carried out. Correct formula [1]
Correct state symbols [1]
4 (a) Reaction 2 [1]
The energy needed to break the bonds is lower (c) Blue to colourless [1]
[1] Copper(II) sulfate is blue. Zinc sulfate is colourless.
than the energy given out to form the bonds. [1] Zinc is dissolved. [1]
The “–” sign shows that the forward reaction is Zinc is a solid before the reaction starts. It will
exothermic. dissolve when reacting with copper(II) sulfate.
(b) A brown solid is formed. [1]
CO + H 2 O Copper is a brown solid that does not dissolve in a
salt solution.
(d) Copper(II) sulfate is the oxidising agent. [1]
An oxidising agent is a substance that will oxidise
CO 2 + H 2
another substance and itself being reduced.
It gains electrons. [1]
Correct energy level change [1] Electrons are gained in reduction.
Correct label [1] (e) Concentration
Arrow pointing down [1]
Energy is released in exothermic reactions.
The total energy of the products is less than the
total energy of the reactants.
(c) The forward reaction is endothermic. [1]
Higher temperature favours the endothermic
reaction. [1]
Shift the equilibrium to the right. [1] Time / s
The “+” sign shows that the forward reaction is Below the original line [1]
endothermic. Stop at the same place [1]
(d) Increase the temperature [1] (f) More particles have enough energy to react. [1]
The forward reaction is endothermic for reaction Collision rate increases. [1]
3. Increasing the temperature favours the More successful collisions / The energy gained
endothermic reaction.
is greater than the activation energy. [1]
Decrease the pressure [1]
CH (g) + H O(g) L CO(g) + 3H (g) 7 (a) Gain electrons [1]
2
4
2
2+
The ratio of the number of molecules of the Zn gains electrons to form Zn.
reactants to the products = 2 : 4 (b) Zn + 2e ➞ Zn
2+
Lowering the pressure favours the side of Correct formula [1]
equilibrium with more gas molecules.
Balanced equation [1]
(e) The number of molecules is the same on both (c) (i) Carbon [1]
sides of equilibrium. [1] A reducing agent is a substance that will reduce
CO(g) + H O(g) L CO (g) + H (g)
2 2 2 another substance and itself being oxidised.
Cambridge IGCSE TM
154 Ace Your Chemistry

Answers.indd 154 3/4/22 3:54 PM