How do you calculate enthalpy change?
2 Answers
Values of H (enthalpy) for particular reactants or reactions will always be given in the exercise. However, depending on the unit, you may be forced to either multiply H by moles (when unit is Kj/mol) or leave it as it is (when unit is Kj).
At first, you count H for finals, then for "ingredients" (by addition) and substract results. I think it is the easiest way.
EDIT:
That is one way. You are using the way of looking up enthalpy values in thermodynamic tables, like enthalpy of formation.
Other ways:
1. Using the heat capacity of a compound at a constant pressure (Cp) and integrating from one temperature to another, times dT.
The heat capacity may be experimentally determined (a fit line equation), or you can assume ideality for some gases and say that (N/2)R is equal to Cp, where R is 8.314472 J/mol*K and N is the degrees of freedom; 3 translational for monatomic gases, 3 translational and 2 rotational for diatomic linear gases, and 3 translational and 3 rotational for multiatomic nonlinear gases (vibrational and electronic contributions are negligible in many cases). (N/2)RT is the equipartition theorem, and the idea that (N/2)R = Cp assumes Cp varies negligibly with temperature.
EX: Water vapor would have Cp = (6/2)R = 3R. You probably won't learn this until sometime in college, but I thought I'd leave this here. Others:
-
Bond enthalpies. You can calculate bonds formed - bonds broken. This
is less accurate, though, because bond enthalpies are listed as
average bond enthalpies, and they depend on the particular
connectivity of nearby bonds. -
Hess's Law , building reaction mechanisms using enthalpies of reaction from thermodynamic tables.
More information is required as there are a few different ways to determine the enthalpy change of a chemical reaction. I am guessing you are looking at either
1) measuring by experiment using calorimetry
2) calculating using bond energies
3) calculating using standard heats of formation
Many chemistry teachers start with bond energies. In this case, the heat released (or the enthalpy of reaction ΔH) can be calculated by:
ΔH=∑(reactant bond energies)−∑(product bond energies)
A couple of things you should know
- breaking bonds requires energy (endothermic) and making bonds releases energy (exothermic) : if the ΔH value is negative, the overall reaction is exothermic; if positive, the overall reaction is endothermic
- you need a table of bond energy (enthalpy) values to solve problems
- you need to be able to determine bonding in molecules to solve problems
- bonding energies are averages, and you must consider what adjacent atoms might alter the bonding energy, such as electron-withdrawing groups.
Example
The reactants contain TWO H−H bonds @ 436kJ/mol
and ONE O=O bond @ 498kJ/mol
the products contain FOUR H−O bonds @ 463kJ/mol
ΔH=∑(reactant bond energies)−∑(product bond energies)
ΔH=2(436)+498−4(463)=−482kJ/mol
