Representing Enthalpy Changes
It is often difficult to determine whether a chemical change is exothermic or endothermic by just looking at the equation.
NO + NO2 à N2O + O2 exothermic or endothermic?
To reduce any confusion chemists have 4 different ways of communicating information regarding changes in the enthalpy of a system.
1) thermochemical equations with energy included within the equation
- a thermochemical equation is a regular chemical equation (all states included) with the addition of an energy term ( units of kJ)
- the heat of the reaction would be included on the reactant side if the reaction is endothermic and on the product side if the reaction is exothermic
H2O(l) + 285.8 kJ à H2(g) + ½ O2(g)
Mg(s) + ½ O2(g) à MgO(s) + 601.6 kJ
- the treatment of heat energy as either a product or a reactant will return during Le Chatelier’s principle
2) thermochemical equations with energy outside of the equation as a DH value
- the heat of the reaction will be stated as a DH value to the right of the equation and the sign will indicate whether it is exothermic or endothermic
- a positive DH is endothermic and a negative DH is exothermic
CO(g) + 2 H2(g) à CH3OH(l) DH = - 128.6 kJ
SO2(g) à S(g) + O2(g) DH = + 297.0 kJ
- don’t forget you must ALWAYS include the sign on DH
- if the equation changes, the DH value must as well
6 SO2(g) à 6 S(g) + 6 O2(g) DH = + 1782.0 kJ
3) special molar enthalpies of reaction
- some reactions are so common that they are described with just a name or a symbol
- each of these reactions occurs under SATP conditions and will have the symbol DHo
- the two standard heats of reaction we will be using are a) the standard heat of formation (DHof) and b) the standard heat of combustion (DHocomb)
a) the standard heat of formation is the enthalpy of a reaction when one mole of material is created from its constituent elements, all of which are in their standard states (the states you would find them under SATP conditions)
Na(s) + 1/2 H2(g) + C(s) + 3/2 O2(g) à 1 NaHCO3(s)
- don’t forget that some elements will exist as diatomics ( HOFBrINCl)
b) the standard heat of combustion is the enthalpy of a reaction when one mole of a hydrocarbon is combusted to produce carbon dioxide and water
1 CH4(g) + 2 O2(g) à CO2(g) + 2 H2O(l)
- use whatever numbers you need to balance the equation, but make sure that there is a 1 in front of the hydrocarbon
4) potential energy diagrams
PE of reactants
energy lost (DH)
PE of products
course of the reaction
PE of products
energy gain (DH)
PE of reactants
course of the reaction
Write a thermochemical equation (1st type) for the combustion of 2 moles of C4H10(g).