If the combustion of 1g of graphite produces


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.


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

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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