Enthalpy is defined as how much heat a substance has at a given temperature and pressure, and is symbolized by the symbol H. This temperature and pressure is usually STP. Although there is no way to measure the absolute enthalpy of a substance, changes in enthalpy can be measured.
The change in enthalpy for a reaction is known as the heat of reaction and has symbol δH. δH is negative for all exothermic reactions and positive for all endothermic reactions.
The states (gas, liquid, solid) of all products and reactants must be stated in any reaction involving enthalpy. Note how the state of the product in the following two reactions affects δH.
|H2 +1/2O2→H2O (gas) δH = - 241.8 kJ|
|H2 +1/2O2→H2O (liquid) δH = - 285.8 kJ|
The heat of formation is defines as the δH The next term you need to be familiar with is the standard heat of formation. It is defined as the δH for a reaction that produces 1 mole of compound from its constituent elements. It has its own special symbol, δHf. When solving enthalpy problems, you can find the heat of reaction using the following formula:
|δH = δHf (products) - δHf (reactants)|
Problem: Find δH for the reaction of sulfur dioxide with oxygen to form sulfur trioxide given the following heats of formation:
|δHfSO2 = - 296.8 kJ / mole|
|δHfSO3 = - 395.7 kJ / mole|
|2SO2(g) + O2(g)→2SO3(g)|
|δH (reactants) = + = - 593.6 kJ|
|δHf products = = - 791.4 kJ|
|δH = δHF (products) - δHF (reactants) = - 791.4 kJ - (- 593.6 kJ) = - 197.8 kJ|