What is Hess's law of heat summation?

1 Answer
Jun 23, 2014

Hess's law of heat summation states that the total enthalpy change during a reaction is the same whether the reaction takes place in one step or in several steps.

www.docbrown.infowww.docbrown.info

For example, in the above diagram,

ΔH1=ΔH2+ΔH3=ΔH4+ΔH5+ΔH6.

In Hess's Law calculations, you write equations to make unwanted substances cancel out.

Sometimes you have to reverse an equation to do this, and you reverse the sign of ΔH.

Sometimes you have to multiply or divide a given equation, and you do the same thing to the ΔH.

EXAMPLE

Determine the heat of combustion, ΔHc, of CS₂, given the following equations.

  1. C(s) + O₂(g) → CO₂(g); ΔHc = -393.5 kJ
  2. S(s) + O₂(g) → SO₂(g); ΔHc = -296.8 kJ
  3. C(s) + 2S(s) → CS₂(l); ΔHf = 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. It contains the first compound in the target (CS₂).

We have to reverse equation 3 and its ΔH to put the CS₂ on the left. We get equation A below.

A. CS₂(l) → C(s) + 2S(s); -ΔHf = -87.9 kJ

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

B. C(s) + O₂(g) → CO₂(g); ΔHc = -393.5 kJ

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

C. 2S(s) + 2O₂(g) → 2SO₂(g); ΔHc = -593.6 kJ

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

A. CS₂(l) → C(s) + 2S(s); -ΔHf = -87.9 kJ
B. C(s) + O₂(g) → CO₂(g); ΔHc = -393.5 kJ
C. 2S(s) + 2O₂(g) → 2SO₂(g); ΔHc = -593.6 kJ

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

Hope this helps.