## Questions on Exothermic and Endothermic Reactions

### 2018 O Level, MCQ 16

Which statement explains why the complete combustion of methane is an exothermic reaction?

A. More bonds are broken than are formed
B. More bonds are formed than are broken
C. The total enthalpy of the bonds broken is greater than the total enthalpy of the bonds formed.
D. The total enthalpy of the bonds broken is less than the total enthalpy of the bonds formed.

Bond breaking requires energy while bond forming releases energy. If a reaction is exothermic, it releases energy on the whole. This means that less energy is required for bond breaking. In other words, the total enthalpy of the bonds broken is less.

### 2018 O Level, Open-ended A6

The industrial production of ammonia involves two stages.

Stage 1: methane gas reacts with steam to form hydrogen gas

CH4(g) + H2O(g) ⟶ CO(g) + 3H2(g), ΔH = +210 kJ/mol

Stage 2: the hydrogen gas then reacts with nitrogen gas to form ammonia

N2(g) + 3H2(g) ⟶ 2NH3(g), ΔH = -92 kJ/mol

Stage 1 proceeds at 800 °C while stage 2 proceeds at 450 °C.

Suggest two reasons why the reaction in stage 2 requires less heating.

Firstly, stage 2 operates at a lower temperature of 450 °C compared to stage 1 at 800 °C.

Secondly, the reaction in stage 2 is exothermic, while the reaction in stage 1 is endothermic. The exothermic reaction releases heat, which helps to maintain the temperature at 450 °C even without much external heating.

## Enthalpy Change Questions on Energy Profile

### 2019 O Level, MCQ 15

A. The forward reaction is endothermic. The activation energy is different for the forward and reverse reactions.
B. The forward reaction is endothermic. The activation energy is the same for the forward and reverse reactions.
C. The forward reaction is exothermic. The activation energy is different for the forward and reverse reactions.
D. The forward reaction is exothermic. The activation energy is the same for the forward and reverse reactions.

For the forward reaction, the products of nitrogen gas and hydrogen gas have a higher energy level than the reactant. Therefore, energy were absorbed to form the product, which means that the reaction is endothermic.

The activation energy of the forward reaction is measured from the energy level of ammonia to the top of the ‘hump’. This is greater than the activation energy of the backward reaction, which is measured from the energy level of nitrogen gas and hydrogen gas to the top of the ‘hump’. Therefore, the two activation energies are different.

### 2018 O Level, MCQ 17

When steam is passed through excess hot coke a reaction occurs and the coke cools:

C(s) + H2O(g) ⟶ CO(g) + H2(g)

The activation energy for the reaction is X.

Which energy profile diagram for this reaction is correct?

The question tells us that the chemicals “cool” during the reaction. The decreasing of temperature reflects an endothermic reaction, whereby heat energy is absorbed. The energy absorption causes the products to have a higher energy level than the reactants.

For the activation energy, it is always measured from the reactants. Therefore, B and D are wrong as they measure the activation energy from the products instead.

## Enthalpy Change Questions on Calculations

### 2019 O Level, MCQ 16

Which statement(s) is/ are correct?

1. Octane produces more heat than hydrogen or methane for each gram of fuel burned.
2. On complete combustion, 1 g of hydrogen produces 286 kJ of energy.
3. When methane is completely combusted, the energy required for bond breaking is less than the energy released in bond forming.

Statement 1 is wrong. If you remember from your textbook, hydrogen is “the most efficient source of energy” that provides more juice per unit mass. If you have forgotten, you can instead calculate the heat released per gram. To do so, divide the enthalpy of combustion by the molar mass of each fuel.

• Heat released per gram of hydrogen = -286 ÷ (2×1) = -143 kJ/mol
• Heat released per gram of methane = -889 ÷ (12 + 4×1) = -55.6 kJ/mol
• Heat released per gram of octane = -5330 ÷ (12×8 + 18×1) = -46.8 kJ/mol

Statement 2 is wrong. While the enthalpy is stated to be -286 kJ, that is for 1 mol of H2. As 1 mol of H2 weighs 2 g, the energy released by 1 g of hydrogen is instead -286 ÷ 2 = -143 kJ/mol.

Statement 3 is correct. The negative enthalpy suggests that the reaction is exothermic. Thus, less energy is absorbed during bond breaking.
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