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DSE Chemistry: Chemical Reactions and Energy — Practice Questions & Answers
Exothermic and endothermic reactions, enthalpy change, energy-level diagrams, bond enthalpy calculations, and finding energy changes by calorimetry.
312 practice questions available for this topic — here are 10 with full answers and explanations.
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Practice questions with answers
1
Multiple choice · Hard
The standard enthalpy changes of combustion are H2(g) -286, C(graphite) -394 and CH3CH2OH(l) -1371 kJ mol-1. The standard enthalpy change of formation of CH3CH2OH(l) is
-275 kJ mol-1
+275 kJ mol-1
-3017 kJ mol-1
+691 kJ mol-1
Tap an answer to check it.
Why Using a Hess cycle: 2(-394) + 3(-286) - (-1371) = -275 kJ mol-1.
2
Multiple choice · Medium
Hess's law allows an enthalpy change to be found indirectly because the overall enthalpy change of a reaction is
independent of the reaction route
always equal to zero
dependent on the rate of reaction
dependent on the catalyst
Tap an answer to check it.
Why It is independent of the path taken between reactants and products.
3
Multiple choice · Easy
For an exothermic reaction, the sign of the enthalpy change is
negative
positive
zero
always +57 kJ mol-1
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Why Exothermic reactions release energy and have a negative enthalpy change.
4
Fill in the blank · Medium
In a Hess cycle, when a reaction is reversed the magnitude of its enthalpy change stays the same but its
Check answer
Answer:
sign
Why Reversing a step flips the sign of DH.
5
Fill in the blank · Hard
From the combustion data, the standard enthalpy change of formation of ethanol is
Check answer
Answer:
-275 / minus 275
Why 2(-394) + 3(-286) - (-1371) = -275.
6
Fill in the blank · Easy
Hess's law follows from the principle of conservation of
Check answer
Answer:
energy
Why Energy is neither created nor destroyed.
7
Multiple choice · Hard
Which of the following pairs of reactants would react in water to give out the largest amount of heat?
1 mol of H2SO4 and 2 mol of KOH
1 mol of HCl and 1 mol of KOH
1 mol of (COOH)2 and 2 mol of KOH
1 mol of CH3COOH and 1 mol of KOH
Tap an answer to check it.
Why 1 mol H2SO4 with 2 mol KOH gives a strong-acid/strong-base neutralisation forming 2 mol of water.
8
Multiple choice · Medium
The enthalpy change of neutralisation between a strong acid and a strong alkali is almost constant because the reaction is essentially
H+(aq) + OH-(aq) -> H2O(l)
a redox reaction
an endothermic reaction
a precipitation reaction
Tap an answer to check it.
Why It is always H+(aq) + OH-(aq) -> H2O(l) regardless of the acid or alkali used.
9
Multiple choice · Medium
Neutralising a weak acid with a strong alkali releases slightly less heat than a strong acid because
energy is absorbed to ionise the weak acid
the weak acid is endothermic
no water is produced
the weak acid is more concentrated
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Why Some energy is needed to ionise the weak acid fully.
10
Fill in the blank · Medium
In a calorimetry experiment, the heat released by a reaction is found from q = m c
Check answer
Answer:
DT / delta T / temperature change
Why m is the mass of solution and c its specific heat capacity.
Key terms in Chemical Reactions and Energy
Enthalpy change: The heat energy absorbed or released by a reaction at constant pressure, symbol deltaH.
Exothermic reaction: A reaction that releases heat to the surroundings, giving a negative deltaH.
Endothermic reaction: A reaction that absorbs heat from the surroundings, giving a positive deltaH.
Heat of reaction: The enthalpy change for a reaction as written in its balanced equation.
Heat of neutralisation: The heat released when an acid and an alkali react to form one mole of water.
Heat of combustion: The heat released when one mole of a substance is completely burnt in oxygen.
Heat of formation: The heat change when one mole of a compound forms from its elements in standard states.
Heat of solution: The heat change when one mole of a solute dissolves completely in a solvent.
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