Why is enthalpy change expressed in kJ mol^-1? Shouldn't it just be kJ? I ask this because when you calculate enthalpy change, you use delta H = heat of combustion * moles of fuel. This should give you units of kJ, not kJmol^-1?
Thermochemistry can be quite tricky to understand at the start (better to be getting it done now than at the end of the year, you have time to clear up any difficulties).
The equation used is actually change in energy/moles = delta h. The units will be xkJ/ymol resulting in the enthalpy change is kJ/mol.
There needs to be a reference. kJ would be worthless in giving us any useful information. Enthalpy change can be viewed as how much energy is released when 6.02 x 10
23 of the reaction occurs (one mole). We have the combustion of octane 2C
8H
18 + 25O
2 --> 16CO
2 + 18H
2O delta h= -10000kJ/mole. What this means is that when 2 atoms of octane react with 25 atoms of oxygen producing 16 atoms of CO2 and 18 atoms of water, occurring 6.02 x 10
23 times, the total energy released (or energy difference between bonds) will be -10000kJ. Therefore the delta h of the reaction is going to be -10000kJ/mol, as for every 1 mole of a reaction occurring -10000kJ of energy will be released.
Similarly, why do you double the enthalpy change when you double a thermochemical equation (to balance it, for example). Shouldn't the units of kJmol^-1 already take into account the new mole amount? Doesn't doubling the enthalpy change quadruple the enthalpy change?
I've covered most of this above about it being per 1 mole of the reaction occurring. The molar enthalpy of octane is -5000kJ. What this means is that 1 mole of octane undergoing complete combustion; This reaction occurring C8H18 + 12.5O2 --> 8CO2 + 9H2O 6.02 x 10
23 times will release 5000kJ.
When we write the proper thermochemical equation everything has doubled. So wouldn't it be fair to say that twice as much energy is going to be output?
Compare; C8H18 + 12.5O2 --> 8CO2 + 9H2O to 2C
8H
18 + 25O
2 --> 16CO
2 + 18H
2O
occurring 6.02 x 10
23 times, there being twice as many reactants means that the energy change is going to be 2x greater.