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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