If the combustion of 1g of graphite produces 20.7 kJ of heat, what will be molar enthalpy change? Give the significance of the sign also.

Question

Philoid · Accepted Answer

o The heat of combustion ∆H_c of graphite (i.e. carbon) is given as = 20.7 kJ for 1g of graphite (C).

1 mole of Carbon = 12 g

Hence the molar enthalpy change (enthalpy for 1 mole )

= (20.7 × 12 )= 248.4 KJ mol^-1

Since the heat is evolved, the actual molar enthalpy change

= -248.4 KJ mol^-1

o In combustion reactions, heat is always evolved i.e. it is an exothermic reaction. Hence the sign of ∆H for the reaction will always be negative (for the process to occur).