Question

How is Hess's law applied in calculating enthalpy?

Answer 1

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, `ΔH_c`, of CS₂, given the following equations?

1. C(s) + O₂(g) → CO₂(g); `ΔH_f` = -393.5 kJ
2. S(s) + O₂(g) → SO₂(g); `ΔH_f` = -296.8 kJ
3. 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); -`ΔH_f` = -87.9 kJ
B. C(s) + O₂(g) → CO₂(g); `ΔH_f` = -393.5 kJ
C. 2S(s) + 2O₂(g) → 2SO₂(g); `ΔH_f` = -593.6 kJ

CS₂(l) + 3O₂(g) → CO₂(g) + 2SO₂(g); `ΔH_c` = -1075.0 kJ