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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Bjc51192 on April 05, 2011, 07:59:40 PM

Title: enthalopy
Post by: Bjc51192 on April 05, 2011, 07:59:40 PM

Given the following data:
2O3 → 3O2 ΔH = -430 kJ
O2 → 2O ΔH = +500 kJ
NO + O3 → NO2 + O2 ΔH = -203 kJ


Calculate ΔH for the reaction
NO + O → NO2


What I did first was I saw that they wanted O as a reactant instead of a product so I switched equation 2 around.

2O----->O2

we need O by itself so I divide by 2

O-------1/2O

I understand that we have to keep equation 3 the same. So I know im making a mistake in equation one.

I am dividing by 2

O3-------------->3/2O2

but the equation doesnt seem to be cancling out.

What exacly am I missing?

Title: Re: enthalopy
Post by: Schrödinger on April 06, 2011, 12:29:29 AM

Try this :
Reverse equations 1 and 2. Keep equation 3 the same. And do some intuitive algebra.

Title: Re: enthalopy
Post by: opti384 on April 06, 2011, 12:57:07 AM

If you fully understand the Hess's Law and do some practices, such kind of questions will be piece of cake.

Title: Re: enthalopy
Post by: Bjc51192 on April 06, 2011, 09:02:41 AM

Yeap got it ty. My problem is finding questions to do =/