Hydrogen cyanide (HCN) is a colourless liquid with a characteristic almond-like odour. It can be pro — Physical Chemistry Chemistry Question
Synthesis of hydrogen cyanide
Hydrogen cyanide (HCN) is a colourless liquid with a characteristic almond-like odour. It can be produced when sufficient energy is supplied to numerous systems containing hydrogen, nitrogen, and carbon. Today, only the processes starting from hydrocarbons and ammonia are of economic importance. Two main HCN production processes are:
• Degussa (BMA) process: CH4(g) + NH3(g) → HCN(g) + 3 H2(g)
• Andrussow process: CH4(g) + NH3(g) + 3/2 O2(g) → HCN(g) + 3 H2O(g)
Both processes take place at temperatures above 1 000 °C and at near standard pressure. Both of them require the use of special platinum catalysts.
Calculate a change in enthalpy ΔrHm at 1 500 K for the reactions which take place in the Degussa process (BMA process) and in the Andrussow process, respectively. Use the data on the enthalpy of formation ΔfHm given in the table below.
Compound ΔfHm(1 500 K), kJ mol−1
CH4(g) −90.3
NH3(g) −56.3
HCN(g) 129.0
H2O(g) −250.1
H2(g) 0
O2(g) 0
Model Answer
Degussa process (BMA process):
ΔrHm = − ΔfHm(CH4) − ΔfHm(NH3) + ΔfHm(HCN) + 3 ΔfHm(H2)
ΔrHm = [− (−90.3) − (−56.3) + 129.0 + 3 × 0] kJ mol−1 = 275.6 kJ mol−1
Andrussow process:
ΔrHm = − ΔfHm(CH4) − ΔfHm(NH3) − 3/2 ΔfHm(O2) + ΔfHm(HCN) + 3 ΔfHm(H2O)
ΔrHm = [− (−90.3) − (−56.3) − 3/2 × 0 + 129.0 + 3 × (−250.1)] kJ mol−1 = −474.7 kJ mol−1
Which process (Degussa BMA or Andrussow) requires the use of an external heater to keep the reaction system at 1 500 K? Why?
Model Answer
An external heater has to be used in the Degussa process (BMA process) because the reaction is endothermic.
Calculate the equilibrium constant K of the reaction which takes place in the Degussa process (BMA process) at the temperatures of 1 500 K and 1 600 K. The standard change in Gibbs free energy for this reaction at 1 500 K is ΔrGm (1 500 K) = −112.3 kJ mol−1. Assume that the reaction enthalpy at 1 500 K is constant over a temperature range from 1 500 K to 1 600 K. Is the result in accordance with Le Chatelier’s principle?
Model Answer
The result is in accordance with the Le Chatelier’s principle because the reaction is endothermic and therefore an increase in temperature shifts the equilibrium toward products (in other words, the equilibrium constant increases).
Referring to the Le Chatelier’s principle, estimate whether the equilibrium constant K of the reaction in the Andrussow process increases or decreases when the temperature changes from 1 500 K to 1 600 K.
Model Answer
The equilibrium constant of the reaction in the Andrussow process decreases with an increase in temperature because the reaction is exothermic.