Thermochemistry with Equation Stoichiometry

Key Questions

  • Answer:

    Consider an exothermic combustion reaction. The amount of heat may be treated as a stoichiometric product, precisely dependent on the amount of hydrocarbon combusted.


    Methane combustion drives our civilization to a very great degree:

    #CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O, DeltaH = -890# #kJ# #mol^-1#.

    The quoted enthalpy of combustion is per mole of reaction as written. You don't have to know these; you do have to know how to balance the equation. Because this energy is associated with the combustion of 1 mol of methane, I could also treat the evolved energy as a reagent or product in the reaction:

    i.e. #CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O + 890# #kJ#.

    It would be on the reactant side if the reaction was endothermic. In other words the minus sign denotes evolution of heat. Alternatively, #890# #kJ# of heat are evolved from the above reaction, so I am treating energy as a product just as carbon dioxide and water are (as indeed it is; of course the heat is a consequence of the formation of water and carbon dioxide bonds). If less than 16 g (1 mol) methane are combusted, the heat evolved will diminish stoichiometrically. Does this address your question?