Hess Law

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Jorja De Jesus 2C
Posts: 121
Joined: Sat Jul 20, 2019 12:15 am

Hess Law

Postby Jorja De Jesus 2C » Wed Feb 12, 2020 2:47 pm

How do you do use Hess's law when only the main reaction is given?

Paige Lee 1A
Posts: 136
Joined: Sat Sep 07, 2019 12:16 am

Re: Hess Law

Postby Paige Lee 1A » Wed Feb 12, 2020 2:58 pm

Given the main reaction, find the sum of the entropies, enthalpies, etc. of the reactants and products

Gabriel Ordonez 2K
Posts: 113
Joined: Sat Jul 20, 2019 12:15 am

Re: Hess Law

Postby Gabriel Ordonez 2K » Thu Feb 13, 2020 2:58 pm

You'll need to get the overall change in enthalpy for each equation used. If you're adding equations together to calculate the overall reactionary enthalpy, you'll need to scale and/or flip your equations with the calculated enthalpies to find the final answer.

Eunice Nguyen 4I
Posts: 100
Joined: Sat Aug 17, 2019 12:17 am

Re: Hess Law

Postby Eunice Nguyen 4I » Thu Feb 13, 2020 4:28 pm

For Hess's Law, you need to get the total sum of the entropies/enthalpies/Gibbs Free Energy (only because they're state functions/extensive properties) of the products and then subtract from it the total sum of the reactants. Don't forget to include the coefficients.

Emily Lo 1J
Posts: 50
Joined: Thu Sep 19, 2019 12:16 am

Re: Hess Law

Postby Emily Lo 1J » Sun Feb 16, 2020 1:31 pm

Usually, you'll be given a couple or more equations that are related to the main one given and then you'll want to algebraically solve to get the main equation through the other ones. Whatever changes made on those equations you make onto the delta H's. Then you add them up to get the net change in enthalpy.

Suraj Doshi 2G
Posts: 100
Joined: Fri Aug 02, 2019 12:15 am

Re: Hess Law

Postby Suraj Doshi 2G » Sun Feb 16, 2020 1:34 pm

Depending on what you are solving for(enthalpy, entropy... etc) find the values for both the products and reactants, making sure to multiply by coefficients if necessary and to find the total, sum the products and then sum the reactants. Subtract the reactants from the products for the final answer.

Posts: 111
Joined: Wed Sep 18, 2019 12:19 am

Re: Hess Law

Postby gabbymaraziti » Sun Feb 16, 2020 2:02 pm

In order to use Hess's Law, you should be given multiple reactions so that you can add/subtract them as needed. Maybe if you aren't given multiple equations, you could use the reaction enthalpies for the formation of all the products and reactants, and create the equations yourself (ie creating an equation that includes C3H8 could be found by considering the combustion of C3H8 to form CO2 and H20).

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