Write a balanced formation equation at standard conditions for the compound HCN.

The formation of hydrogen cyanide, HCN is formed by reacting hydrogen gas, carbon and nitrogen gas, expressed in the following expression below:

½ H2 (g) + C (s) + ½ N2 (g) → HCN (g) ΔHof = 135 kJ

At standard conditions, hydrogen and nitrogen exist as gases while carbon is solid. This is also an endothermic reaction due to the positive ΔHof