For this question, the enthalpy combustion of graphite(carbon) and the enthalpy combustion of hydrogen should be negative right? Because they are exothermic reactions?
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Thank you,
Lakshmi
PS question is here
Calculate the enthalpy of combustion of methane, given that the enthalpy of formation of methane is -74.6 kJ/mol and the enthalpy combustion of graphite and hydrogen are 393.4 kJ and 285.7 kJ/mol respectively.
yes you are right that they are negative enthalpies because their combustions are exothermic.
Enthalpy Combustion of graphite is a actually enthalpy of formation of CO2
Enthalpy Combustion of hydrogen is a actually enthalpy of formation of H2O
Reaction of combustion of methane is
CH4 +O2 ----> CO2 + 2H2O
(g) (g) (g) (g)
Now use formation enthalpies of the reactants and products to get enthalpy of above reaction.