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Hess's Law (Method 1)

Posted: Mon Jan 28, 2019 1:16 pm
by Daisylookinland
In Method 1, why is it necessary to add the rxns to get the net rxn? Why would you not simply just add together the two deltaHs?

Re: Hess's Law (Method 1)

Posted: Mon Jan 28, 2019 1:42 pm
by Nicklas_Wright_1A
You can do that if you want. Just make sure to add the right reactions. Sometimes a question will give you delta H for the reverse reaction instead and you will need to account for that.

Re: Hess's Law (Method 1)

Posted: Mon Jan 28, 2019 9:18 pm
by 904936893
In addition to that, sometimes you'll need to multiply one of the delta H's if the final reaction requires two moles of one of the substances, and the given delta H is for one mole. So you have to make sure that the reactions match with the amount of moles required in the final reaction, if that makes sense.

Re: Hess's Law (Method 1)

Posted: Mon Jan 28, 2019 10:20 pm
by Angela Cong 3C
they're almost similar to math equations. Whatever you do to one you have to do to another, so if the rxn in the second equation is multiplied by two or divided by two you much also do the same to the enthalpy. It will be more convenient to keep track of if you add them together

Re: Hess's Law (Method 1)

Posted: Mon Jan 28, 2019 10:30 pm
by Vicky Lu 1L
Since we do not know the delta H of the net reaction, we have to rely on the delta Hs of the intermediate reactions that can lead us to the net reaction. It is not necessarily adding the two delta H as we have to make sure first the you use the intermediate reactions correctly that would lead you the delta H of the net reaction. Sometimes you need to flip the reaction, so that that reaction's delta H will be reverse/the reverse sign of the value of the delta H. Other times, the coefficient of the reaction is manipulated to get the necessary coefficient in the net reaction so the delta H can also multiplied by the coefficient.