Answer to Question #316491 in General Chemistry for John

Question

Give examples that illustrate the use of standard heats of formation in calculating changes in enthalpy or ΔH of reactions.

Answer 1

Example Problem using an Enthalpy value:

CH₄(g) + 2O₂(g) -> CO₂(g) + 2H₂O(g) ΔH = -802 kJ

Given the above thermochemical equation for the combustion of methane, how much heat energy is released when 4.5 grams of methane is burned (in a constant pressure system)?

First determine the moles of methane: 4.5 g x 1 mole/16 g methane = 0.28125 mol CH4

Then multiply the amount of moles by the known per mole amount of Enthalpy shown: 0.28125 * -802 kJ = -225.56 kJ or -2.3e2 kJ

You may note that the units on the Enthalpy value are only shown as kJ and not kJ/mol in the reaction. This lack of the per mole unit is fairly common but the per mole unit is understood to be there even if it is not written, somewhat like a 1 coefficient in a balanced chemical reaction. Looking at the reaction above you can see that the enthalpy value for the reaction is for 1 mole of Methane or 2 moles of Oxygen or 1 mole of Carbon Dioxide etc.