# How is Hess's law applied in calculating enthalpy?

##### 1 Answer

To develop a strategy for the order in which you add the various equations.

#### Explanation:

In a Hess’s Law problem, you have to develop a strategy for the order in which you add the various equations.

*EXAMPLE*

Determine the heat of combustion,

- C(s) + O₂(g) → CO₂(g);
#ΔH_f# = -393.5 kJ - S(s) + O₂(g) → SO₂(g);
#ΔH_f# = -296.8 kJ - C(s) + 2S(s) → CS₂(l);
#ΔH_f# = 87.9 kJ

*Solution*

Write down the target equation, the one you are trying to get.

CS₂(l) + 2O₂(g) → CO₂(g) + 2SO₂(g)

Start with equation 3, since it contains the first compound in the target (CS₂). We reverse equation 3 and its ΔH to get the CS₂ on the left and get equation A below.

Now we eliminate C(s) and S(s) in Equation A one at a time. Equation 1 contains C(s), so we write it as Equation B below.

We use Equation 2 to eliminate the S(s), but we have to double it and its ΔH. We then get equation C below.

Finally, we add equations A, B, and C to get the target equation, cancelling things that appear on opposite sides of the reaction arrows.

A. CS₂(l) → C(s) + 2S(s); -

B. C(s) + O₂(g) → CO₂(g);

C. 2S(s) + 2O₂(g) → 2SO₂(g);

CS₂(l) + 3O₂(g) → CO₂(g) + 2SO₂(g);