# Use Hess's Law and the following information to calculate the change in enthalpy for the reaction 2C + H2 -> C2H2?

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C2H2 + 5/2 O2 -> 2CO2 + H2O Entropy = -1299.6 kJ

C + O2 -> CO2 Entropy = -393.5 kJ

H2 + 1/2 O2 -> H2O Entropy = -285.8 kJ

C2H2 + 5/2 O2 -> 2CO2 + H2O Entropy = -1299.6 kJ

C + O2 -> CO2 Entropy = -393.5 kJ

H2 + 1/2 O2 -> H2O Entropy = -285.8 kJ

##### 1 Answer

#### Explanation:

You have listed entropy changes for the reaction. In fact, these are values for

We want

If I do the sum of the given reactions in this manner,

Which after cancelling gives:

Note here that

So here we have used the thermochemical equations as linear equations to give us the equation that we sought (and thus necessarily the thermodynamic parameters).