L5 - Hess' Law

Step 1 - Find Enthalpy Equations from the Chemistry Data Booklet

This reaction cannot be studied calorimetrically since the combustion of carbon produces carbon dioxide as well as carbon monoxide. However, the enthalpy of complete combustion for carbon and for carbon monoxide can be calculated by calorimetric measurements, and the enthalpy of formation for carbon monoxide can be determined
using Hess law as follows:

Step 2 - Rearrange the Enthalpy Equations

Rearrange these two equations, and then add them together to obtain the chemical equation for the formation of carbon monoxide. The first term in the formation equation for
carbon monoxide is 1 mol of solid carbon. Therefore, leave equation (1) unaltered so that C(s) will appear on the reactant side when we add the equations. However, we want 1 mol of CO(g) to appear as a product, so reverse equation (2) and divide each of its terms (including the enthalpy change) by 2.

Note that the sign of the enthalpy change in equation (2) has changed, since the equation has been reversed.

Step 3 - Add the Enthalpy Equations

Now add the reactants, products, and enthalpy changes to get a net reaction equation. Note that CO2(g) can be cancelled because it appears on both sides of the net equation. Similarly, 1/2 O2(g) can be cancelled from each side of the equation, resulting in:

While manipulating equations (1) and (2), you should check the desired equation and plan ahead to ensure that the substances end up on the correct sides and in the correct chemical amounts.