Hess's Law

Moderators: Chem_Mod, Chem_Admin

Christian Wooten 3A
Posts: 19
Joined: Fri Sep 25, 2015 3:00 am

Hess's Law

Postby Christian Wooten 3A » Wed Jan 06, 2016 8:29 pm

So to my knowledge, there were three different ways to solve the problem, but does it matter which one you use? Thanks!

Francesca Genera 3F
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: Hess's Law

Postby Francesca Genera 3F » Wed Jan 06, 2016 9:16 pm

I believe today in lecture Dr. Lavelle said that you could use any of the methods and get the same result, but we have only gone through two of the three ways.

Spencer Johnson 1C
Posts: 9
Joined: Fri Sep 25, 2015 3:00 am

Re: Hess's Law

Postby Spencer Johnson 1C » Wed Jan 06, 2016 9:58 pm

Dr. Lavelle said the method we used would depend on the information given to us in the problem.

004594950
Posts: 28
Joined: Fri Sep 25, 2015 3:00 am

Re: Hess's Law

Postby 004594950 » Wed Jan 06, 2016 10:42 pm

It does't matter which method you use. As long as you get the correct answer. But Lavelle didn't go over the 3rd method today in lecture.

Chem_Mod
Posts: 18884
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 716 times

Re: Hess's Law

Postby Chem_Mod » Thu Jan 07, 2016 12:34 pm

Method of Hess's Law can be used when you can derive the reaction of interest from other reactions and their GIVEN enthalpies.

Method of bond enthalpy should be used when the problem gives you bond enthalpies. It is not guaranteed to give the same answer as method 3 because bond enthalpies are only approximations (the precise energy of a bond in a molecule depends on more than just two atoms).

Method of standard enthalpies of formation is the default method and can always be done by looking up values in the Appendix. On exams, necessary values will be given.

chemgal4L
Posts: 19
Joined: Fri Sep 25, 2015 3:00 am

Re: Hess's Law

Postby chemgal4L » Fri Jan 08, 2016 10:51 am

When you use the method of using the bond energies, do you add the left side of the equation to the right side, or subtract the right side from the left side?

Chi-Yun 1B
Posts: 13
Joined: Fri Sep 25, 2015 3:00 am

Re: Hess's Law

Postby Chi-Yun 1B » Fri Jan 08, 2016 11:08 am

chemgal4L wrote:When you use the method of using the bond energies, do you add the left side of the equation to the right side, or subtract the right side from the left side?


If you are calculating enthalpy using bond energies, it is [H(final)-H(initial). In other words, the energy used to break the reactant bonds minus the energy of the bonds formed.

GiselleMartinez_1B
Posts: 9
Joined: Mon Jan 26, 2015 2:17 pm

Re: Hess's Law

Postby GiselleMartinez_1B » Fri Jan 08, 2016 4:57 pm

In the CR, when solving a problem using the Hess's Law method the two equations are presented in such a way where you can add the equations vertically, but he cancels out the reactants in one equation that appears in the product of the other, is this something that can always be done? Will there always be something to cancel out?

Arsheeta Kumar 1B
Posts: 19
Joined: Fri Sep 25, 2015 3:00 am

Re: Hess's Law

Postby Arsheeta Kumar 1B » Sat Jan 09, 2016 2:31 pm

GiselleMartinez_1B wrote:In the CR, when solving a problem using the Hess's Law method the two equations are presented in such a way where you can add the equations vertically, but he cancels out the reactants in one equation that appears in the product of the other, is this something that can always be done? Will there always be something to cancel out?


Usually, there should be an intermediate to cancel out, because the enthalpies of the two equations will be equal to that of the third reaction you get from cancelling a product/reactant out. Hess's Law states that you don't need the intermediate steps to figure out the total enthalpy, which is why the enthalpies are the same for multiple steps versus one step.


Return to “Phase Changes & Related Calculations”

Who is online

Users browsing this forum: No registered users and 2 guests