when to change sign of enthalpies

Moderators: Chem_Mod, Chem_Admin

405318478
Posts: 90
Joined: Sat Jul 20, 2019 12:16 am

when to change sign of enthalpies

Postby 405318478 » Sun Feb 09, 2020 11:52 pm

Hi, i'm looking at question 4 on the Pizza Rolls review sheet. When do I know to change the sign of the enthalpies given?

Eugene Chung 3F
Posts: 142
Joined: Wed Nov 15, 2017 3:03 am

Re: when to change sign of enthalpies

Postby Eugene Chung 3F » Sun Feb 09, 2020 11:54 pm

If you mean question 4a, you would flip the sign if u flip the chemical equation.

Rida Ismail 2E
Posts: 139
Joined: Sat Sep 07, 2019 12:16 am

Re: when to change sign of enthalpies

Postby Rida Ismail 2E » Sun Feb 09, 2020 11:55 pm

For a Hess’s law equation anything you do to the chemical equation you need to do to the Delta H as well

Ariana Iranmahboub1G
Posts: 114
Joined: Fri Aug 09, 2019 12:17 am

Re: when to change sign of enthalpies

Postby Ariana Iranmahboub1G » Sun Feb 09, 2020 11:58 pm

If you are using the reverse reaction, you would flip the sign of delta H.

205154661_Dis2J
Posts: 109
Joined: Wed Sep 18, 2019 12:21 am

Re: when to change sign of enthalpies

Postby 205154661_Dis2J » Sun Feb 09, 2020 11:58 pm

If you mean 4A, we change the sign of enthalpies when we reverse the equation. For example, we reversed equation 2 because we wanted Vodka on the product side since we have it on the product side in the overall recipe. We then multiplied it by two since we want 2 Vodka on the product side to match the 2 Vodka in the overall recipe. I hope that makes sense!

Daria Azizad 1K
Posts: 116
Joined: Thu Jul 25, 2019 12:15 am

Re: when to change sign of enthalpies

Postby Daria Azizad 1K » Mon Feb 10, 2020 12:34 am

Flipping the sign accounts for flipping the equation because we have changed from an endo to exo or exo to endo rxn.

Micah3J
Posts: 100
Joined: Tue Oct 08, 2019 12:16 am

Re: when to change sign of enthalpies

Postby Micah3J » Mon Feb 10, 2020 1:17 am

By flipping the equation it would mean that you are just taking the reverse reaction instead right? So then basically the reactants are on the products side in order to cancel things out?

Amy Kumar 1I
Posts: 114
Joined: Thu Jul 11, 2019 12:15 am

Re: when to change sign of enthalpies

Postby Amy Kumar 1I » Mon Feb 10, 2020 2:52 am

Yes, taking the reverse reaction allows the products and reactants to switch sides so that things can get canceled out as needed.


Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

Who is online

Users browsing this forum: No registered users and 1 guest