This question is about ammonia and a compound produced from ammonia - AQA - GCSE Chemistry - Question 6 - 2017 - Paper 3

To separate ammonia from the mixture of gases, cool the gas mixture to any temperature in the range of -33 to -196 °C. At this temperature, ammonia condenses into a liquid while hydrogen and nitrogen remain in gas form.

To calculate the energy change, we use bond energies:

1. For the bonds broken in hydrazine:

   • 2 N−H bonds: $2 \times 391 \text{ kJ} = 782 \text{ kJ}$
   • 1 N≡N bond: $941 \text{ kJ}$ Total energy required = $782 + 941 = 1723 \text{ kJ}$

2. For the bonds formed:

   • 1 N−N bond: $160 \text{ kJ}$
   • 2 H−H bonds: $2 \times 432 \text{ kJ} = 864 \text{ kJ}$ Total energy released = $160 + 864 = 1024 \text{ kJ}$

3. Energy change = Energy required - Energy released = $1723 - 1024 = 699 \text{ kJ}$

The decomposition of hydrazine is exothermic because the energy taken in to break bonds is less than the energy given out when bonds form. Breaking bonds requires energy input, while making bonds releases energy. In this case, the overall energy released in forming new bonds is greater than the energy consumed in breaking the original bonds.