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.
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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