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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Zoloft on January 14, 2009, 04:41:12 PM

Title: Basic Question regarding Hess's Law
Post by: Zoloft on January 14, 2009, 04:41:12 PM

OK, so if I had C₃H₈(g) + 5O₂(g) ---> 3CO₂(g) + 4H₂O(g)

And I needed to find how much heat is released by burning 10 moles of propane, I would have to balance the equation, and in turn, that would give me like 10C₃H₈ +50O₂ ---> 30CO₂ + 40H₂0.

Anyway, I would then use Hess's equation and subtract the heat released by each reactant in their standard states (by multiplying by the moles per elements) from the heat released by their products.

Right?

Title: Re: Basic Question regarding Hess's Law
Post by: Astrokel on January 15, 2009, 03:05:13 AM

You are correct if you are using deltaH_f. Alternatively, if you have got enthalpy change of your combustion of per mole of propane, you could just multiply it by 10.

Title: Re: Basic Question regarding Hess's Law
Post by: Mitch on January 15, 2009, 03:15:41 AM

You can also factor out a 10 out of your coefficients. ;)