Q. 41

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.

Answer :

o The heat of combustion ∆Hc 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).


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