Question

What percentage of resulted Hydrogen in the reaction: CH4 (g) + NH3 (g) --> HCN (g) + 3H2 (g) must be burnt, with formation if H2O (g), to ensure the quantity of heat absorbed in the reaction?

(the answer is: approximately 35%)

Heat of formation : Hf (kJ/mol)

CH4 (g) = -74,7
NH3 (g) = -46,2
HCN (g) = 130,5
H2O (g) = -241,6

Answer 1

CH4 (g) + NH3 (g) --> HCN (g) + 3H2 (g) -------------->1

Heat of reation= Heat of formation of Products - Heat of formation of Reactants

Heat of reation= 3*(Heat of formation of H2) + 1*(Heat of formation of HCN) - 1*(Heat of formation of CH4)-1*(Heat of formation of NH3)

= 3*0 + 1*(130.5) - 1*(-74.7) - 1*(-46.2)

= 250.9 KJ/mol

Burning reaction of H2

H2(g)+ 1/2O2(g) = H2O(g) ---------------------->2

Heat of burning of H2 = Heat of formation of H2O - Heat of formation of H2 - 1/2*Heat of formation of O2

Heat of burning of H2 = -241 - 0 - 1/2*0 = -241KJ/mol

Let x mol of H2 is burnt ,so heat of burning x mole of H2 = x*(-241)KJ

Since we have to ensure the heat absorbed in the reaction 1 is recovered in reaction 2

Therefore, x*241=250.9 => x=250.9/241 => x = 1.041

Number of moles of H2 formed in reaction 1 is 3 and number of moles of H2 consumed in burning is 1.041

Therefore % of H2 burnt = (1.041/3)*100 = 34.7%(ANS)