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, the temperature should be cooled to any temperature in the range from -33 to -196 °C. This cooling causes ammonia to condense into a liquid, allowing it to be separated from the remaining gases.

To calculate the energy change for the decomposition of hydrazine, use the bond energies:

1. Break the bonds in hydrazine:

   • 2 N–H bonds: $2 imes 391 = 782$ kJ
   • 1 N=N bond: 160 kJ
   • Total energy required to break bonds: 782 + 160 = 942 kJ

2. Form new bonds in products (N2 and H2):

   • 1 N≡N bond: 941 kJ
   • 2 H–H bonds: $2 imes 432 = 864$ kJ
   • Total energy released when forming bonds: 941 + 864 = 1805 kJ

3. Energy change = Energy required - Energy released: $E_{change} = 942 - 1805 = -81 ext{ kJ}$

The decomposition of hydrazine is exothermic because the total energy required to break bonds in the reactants is less than the energy released when new bonds are formed in the products. In simpler terms, energy is released when new bonds form, leading to overall energy output.