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# Hess's Law watch

1. Question is
a) write a equation for complete combustion of hydrogen
b) work out enthalpy change using h20 enthalpy of formation as (-285.5)

This is how I did it:

a) 2H2+O2=2H2O
b) -(0)+(-285.5*2)=-285.5*2= -571

I looked at the mark scheme and got Question a wrong for some reason and obviously if the equation is wrong, so is answer b.
What I want to know is why the equation has to be h2 + 1/2 O2 = H20 and not what I wrote.
Please don't write because it is enthalpy of formation because they don't tell you that until the next question
2. For enthalpy of comubstion it is when one mole of a substance combusts
3. (Original post by soup)
For enthalpy of comubstion it is when one mole of a substance combusts
Question a has nothing to do with enthalpy.
4. (Original post by nasira372)
Question a has nothing to do with enthalpy.
The key point in that user's post was 'one mole' - look at your equation again - can you see where you went wrong?

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