Hydrogen cyanide is a highly poisonous, volatile liquid. One way in which it can be
prepared is by the reaction:
CH4 (g) + NH3 (g) → HCN (g) + 3 H2 (g)
Use Hess’ law and the following thermochemical equations to find the heat of the above
reaction at constant pressure.
N2 (g) + 3 H2 (g) → 2 NH3 (g); ∆H = –91,8 kJ
C (graphite) + 2 H2 (g) → CH4 (g); ∆H = –74,9 kJ
H2 (g) + 2 C (graphite) + N2 (g) → 2 HCN (g); ∆H = 270,3 kJ
To find the heat of the reaction CH4 (g) + NH3 (g) → HCN (g) + 3 H2 (g), we need to manipulate the given thermochemical equations to match the desired reaction.
First, let's reverse the first equation and multiply it by -1 and the second equation by 3 to get the desired reactants and products:
-NH3 (g) → N2 (g) + 3 H2 (g); ∆H = 91.8 kJ
3(C (graphite) + 2 H2 (g) → CH4 (g); ∆H = - (3)(74.9) = -224.7 kJ
Next, add the manipulated equations together to get the desired reaction:
3(C (graphite) + 2 H2 (g) → CH4 (g)) + -NH3 (g) → N2 (g) + 3 H2 (g) → 2NH3 (g) + 3H2(g)
-224.7 kJ + 91.8 kJ = -132.9 kJ
So, the heat of the reaction CH4 (g) + NH3 (g) → HCN (g) + 3H2 (g) at constant pressure is -132.9 kJ.