## Hess's Law

aphung1E
Posts: 101
Joined: Sat Aug 24, 2019 12:15 am

### Hess's Law

can someone explain hess's law? and when it is applied?

Myka G 1l
Posts: 100
Joined: Fri Aug 30, 2019 12:17 am

### Re: Hess's Law

Hess's Law states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. Therefore, you can use it to add enthalpy changes at each step of a multi step reaction to yield the total enthalpy change. To use Hess's law you have to manipulate the initial steps of the reaction to get certain substances to cancel out in order to achieve the end reaction. You also have to change the enthalpy values proportionally to how you manipulated the reactions.

Qiu Ya Wu 4I
Posts: 107
Joined: Fri Sep 20, 2019 12:17 am

### Re: Hess's Law

In order to use Hess's Law to solve for the change in enthalpy, you will have to know the changes in enthalpy for intermediate reactions that occur with the reactants given to form the end product.

Ryan Lee 1E
Posts: 50
Joined: Sat Aug 17, 2019 12:16 am

### Re: Hess's Law

Essentially, you'll be given multiple reactions and their enthalpy changes. You can then add those reactions in a sense or flip them so that parts of the left and right side cancel out. You will eventually end up with the equation you are asked to solve. You then add the manipulated enthalpy changes together to get the enthalpy change for the unknown reaction.

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

### Re: Hess's Law

So for Hess's Law, the sign of the resulting net enthalpy will tell if the sum of the reactions is endothermic or exothermic right?

sarahsalama2E
Posts: 164
Joined: Fri Aug 30, 2019 12:16 am

### Re: Hess's Law

Some additional important things to note are the following:
when you flip a reaction, you must flip the delta H.
when you multiply reactions by coefficients, you must also multiply the delta H by such value.

Trent Yamamoto 2J
Posts: 111
Joined: Sat Aug 17, 2019 12:18 am

### Re: Hess's Law

Hess's Law states that the enthalpy change at each step can be added to give the total enthalpy change, so you would use this when given the Delta H rxn is given for the different steps.

105335337
Posts: 103
Joined: Sat Aug 24, 2019 12:15 am

### Re: Hess's Law

In multi-step reactions, the enthalpies are able to be added together because enthalpy is a state function (meaning the path to get there doesn't matter, its just the value). So you need to just add the enthalpies together.

Michelle Xie 2B
Posts: 99
Joined: Sat Aug 17, 2019 12:18 am

### Re: Hess's Law

You add enthalpies together, reverse signs for reverse reactions, and multiply if you multiply the whole reaction.

Paul Hage 2G
Posts: 105
Joined: Thu Jul 25, 2019 12:17 am

### Re: Hess's Law

Micah3J wrote:So for Hess's Law, the sign of the resulting net enthalpy will tell if the sum of the reactions is endothermic or exothermic right?

That is correct. If delta H of the sum of the enthalpies of the multiple reactions is negative, the overall reaction is exothermic. If delta H of the sum of the enthalpies of the multiple reactions is positive, the overall reaction is endothermic.

Shivam Rana 1D
Posts: 106
Joined: Fri Aug 09, 2019 12:16 am

### Re: Hess's Law

Hess's law is used to find the change in enthalpy. It states that regardless of the pathway taken to reach a final product, the change in enthalpy will be the same. This is because enthalpy is a state function.

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

### Re: Hess's Law

Paul Hage 2G wrote:
Micah3J wrote:So for Hess's Law, the sign of the resulting net enthalpy will tell if the sum of the reactions is endothermic or exothermic right?

That is correct. If delta H of the sum of the enthalpies of the multiple reactions is negative, the overall reaction is exothermic. If delta H of the sum of the enthalpies of the multiple reactions is positive, the overall reaction is endothermic.

Sweet, thank you. Just wanted to be sure

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