Using Hess' Law, how do you calculate the standard heat of formation of Copper(I) Oxide given the following data?

CuO(s) -> Cu(s)+ 1/2 O_2 Delta H = 157.3 kJ/mol
4CuO(s) -> 2Cu_2O(s) + O_2(g) Delta H 292.0 kJ/mol

1 Answer
Michael
Apr 17, 2016

You can do it like this:

Explanation:

Hess' Law states that the overall enthalpy change of a process is independent of the route taken.

In thermodynamics we are interested in initial and final states.

You need to construct a Hess Cycle using the information given:

MF Docs

You can see that, in energy terms, the color(blue)("blue") route is equal to the color(red)("red") route as the arrows start and finish in the same place. This is in accordance with Hess' Law.

So we can write:

(4xx157.3)+DeltaH=292

:.DeltaH=-337.2"kJ"

Enthalpy of formation refers to the formation of 1 mole of a substance from its elements in their standard states under standard conditions.

We have found DeltaH for:

4Cu+2O_2rarr2Cu_2O +O_2

Which is the same as:

4Cu+O_2rarr2Cu_2O

This refers to the formation of 2 moles of copper(I) oxide. We need the enthalpy change for the formation of 1 mole of copper(I) oxide.

:.DeltaH_f[Cu_2O]=(DeltaH)/2=-337.2/2=-168.6"kJ/mol"

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