heat question

[joonkimdds]

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calculate the heat of combusion of C2H4.
delta Hf are given for each.

when it asks to calculate the heat, do we assume that it's always asking for the heat of reaction? There can be many other kinds of heat such as heat of formation or heat applied to find specific heat....etc.

so from the question above, how do we know that it's asking for the heat of reaction which is sum of Hf product - sum of Hf reactants.

 

[tranv117]

you're pretty much right. for a question like this, just write out the balanced equation.

C2H4 + 3O2(g) --> 2 CO2 + 2H2O

dH = (2*HfH2O+2*HfCO2) - (HfC2H4) --> Hf of O2 is zero since Hf of a molecule in standard state is 0.