Hess's Law

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J_Rodriguez_Dis2k
Posts: 25
Joined: Fri Sep 25, 2015 3:00 am

Hess's Law

Postby J_Rodriguez_Dis2k » Sun Jan 29, 2017 10:17 pm

what are your guys approach to Hess's law ?

Muhammad A Nasir
Posts: 9
Joined: Wed Sep 21, 2016 2:55 pm

Re: Hess's Law

Postby Muhammad A Nasir » Sun Jan 29, 2017 10:40 pm

Well there are many things we can do to approach Hess's Law. First I would make sure to see that the equations are balanced because one might never know that the equations are balanced and you need to multiply the change in enthalpy by that coefficient. Also this isn't true for every situation but when you multiply by negative make sure to reverse the reactants and products and change the sign in enthalpy.

Good luck.

Faizal_Mamani_1F
Posts: 10
Joined: Wed Sep 21, 2016 2:58 pm

Re: Hess's Law

Postby Faizal_Mamani_1F » Sun Jan 29, 2017 11:23 pm

Additionally adding on to the answer above, make sure to multiply the equation when needed and therefore multiply the enthalpy too.

Matthew Leung 1G
Posts: 7
Joined: Wed Sep 21, 2016 2:57 pm

Re: Hess's Law

Postby Matthew Leung 1G » Mon Jan 30, 2017 12:33 am

For me, I like to think of cancelling things one at a time, and multiplying only if I see fractions.

Kevin Tam 1J
Posts: 23
Joined: Wed Sep 21, 2016 2:59 pm

Re: Hess's Law

Postby Kevin Tam 1J » Mon Jan 30, 2017 8:04 am

To add on, I would say that you would want to always refer back to your "goal" equation. Compare your "goal" equation to the equations that are given to you, and, as aforementioned in the previous comments, multiply the equations by a coefficient that will ensure that you end up with the "goal" equation. One caveat is that one of more of the components in the "goal" equation may be present in multiple given equations. In that case, you have to do a little more manipulation.

Take, for example, Question 5 from the Quiz 1 Prep in the Course Reader.

The goal equation, when balanced is: N2H4 + H2 --> 2NH3

The given equations should be manipulated as follows:

I see that the first equation should be left alone because one of the reactants, N2H4, is present in the "goal" equation.
Since both the second and thrid equations have H2 in them, you can foresee that flipping the second equation would, in the end, cancel the H2 to give you 1 H2 on the left side, which matches the "goal" equation.
As for the third equation, you can leave this one alone, because the 2NH3 is present in the "goal" equation.

Hope this helps.


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