Hess Law

Moderators: Chem_Mod, Chem_Admin

Sophia Dinh 1D
Posts: 100
Joined: Thu Jul 25, 2019 12:15 am

Hess Law

Postby Sophia Dinh 1D » Sun Mar 15, 2020 7:39 am

To clarify, you add all the equations together but multiply the values?

ayushibanerjee06
Posts: 177
Joined: Thu Jul 11, 2019 12:16 am
Been upvoted: 1 time

Re: Hess Law

Postby ayushibanerjee06 » Sun Mar 15, 2020 7:41 am

Yes you add all of them up at the end. If you reverse a reaction, it is negative. You multiply/divide H by whatever you manipulate the reaction. Remember that you can do this because enthalpy is a state function!

Alan Cornejo 1a
Posts: 98
Joined: Fri Aug 30, 2019 12:15 am

Re: Hess Law

Postby Alan Cornejo 1a » Sun Mar 15, 2020 7:46 am

ayushibanerjee06 wrote:Yes you add all of them up at the end. If you reverse a reaction, it is negative. You multiply/divide H by whatever you manipulate the reaction. Remember that you can do this because enthalpy is a state function!


This is correct thank you for the clarification.

isha dis3d
Posts: 101
Joined: Wed Sep 30, 2020 9:47 pm

Re: Hess Law

Postby isha dis3d » Wed Jan 20, 2021 3:34 pm

You add up the values of the enthalpies and "cancel" out the reactants if those reactants are the same as those on the product side of another equation. If in case the coefficients are different, the equations can be combined algebraically. If one side of a reaction had say, 4H2 on the reactants side while another equation has 5H2 on the product side, the combined reaction would have H2 on the product side.

Samudrala_Vaishnavi 3A
Posts: 93
Joined: Wed Sep 30, 2020 9:34 pm

Re: Hess Law

Postby Samudrala_Vaishnavi 3A » Fri Jan 22, 2021 7:37 am

For the Hess Law, we basically have to manipulate the equation so that it matches the specific final reaction we want. The real first step is to write the chemical formula for what the question wants and to balance it. Then, take the other formula given and create a formula in which the stoichiometric coefficients and reactant match with the reactant in the problem and do the same for a product. Then, combine the change in enthalpy for both equations and write the equation so that all the products and reactants of the new manipulated equations. Whatever changes you do to these equations, do to the changes in enthalpy. Cross out any similar products or reactants. The third equation is usually used to get rid of some of the other leftover products or reactants not represented in the needed chemical forumla.

Libby Dillon - 1A
Posts: 94
Joined: Fri Oct 02, 2020 12:17 am
Been upvoted: 1 time

Re: Hess Law

Postby Libby Dillon - 1A » Fri Jan 22, 2021 12:51 pm

Hess's Law has to do with enthalpies being a state function - meaning they can be treated as a simple value that can be added. If a reaction is reversed (products become reactants and vice versa), then the sign of the reaction enthalpy switches (negative/positive). If a reaction is multiplied by a scalar, the reaction enthalpy is multiplied by the same number. If two reactions are added together, their enthalpies are also added together to get a net reaction enthalpy.

DMaya_2G
Posts: 103
Joined: Wed Sep 30, 2020 9:58 pm

Re: Hess Law

Postby DMaya_2G » Fri Jan 22, 2021 1:00 pm

You would cancel out the reactants if those reactants are the same as the one's that are on the product side of another equation. The equations could be combined algebraically if the coefficients are different. Also, I think, if necessary you can flip the chemical equation, which you would also have to switch the sign of the delta H. Ultimately, you add up the values of the enthalpies.

rita_debbaneh2G
Posts: 96
Joined: Wed Sep 30, 2020 9:57 pm

Re: Hess Law

Postby rita_debbaneh2G » Sun Jan 24, 2021 6:10 pm

As far as I know, if you're not multiplying by anything or reversing the reactions, just add the values together. If you reverse a reaction, add the opposite value of the reaction enthalpy. If you multiply by a coefficient, multiply the reaction enthalpy with the same coefficient.

apurva-3E
Posts: 142
Joined: Wed Sep 30, 2020 9:59 pm

Re: Hess Law

Postby apurva-3E » Sun Jan 24, 2021 7:20 pm

Yes.

Abhinav Behl 3G
Posts: 40
Joined: Wed Nov 18, 2020 12:25 am

Re: Hess Law

Postby Abhinav Behl 3G » Sun Jan 24, 2021 7:54 pm

When you flip a reaction, its delta H value will change signs. When you multiply a reaction by a number, multiply the delta H value by that same number. After doing all the necessary adjustments, add up all the delta H values together.

Edison Tham 3D
Posts: 45
Joined: Mon Jun 17, 2019 7:25 am
Been upvoted: 1 time

Re: Hess Law

Postby Edison Tham 3D » Sun Jan 24, 2021 7:57 pm

Yes! With Hess's Law, you would add up all the ∆H values. You would put a negative sign if the equation is switched around and you would multiply it if the coefficients of a given reaction are different.


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 3 guests