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### Hess's Law

Posted: Mon Jan 27, 2020 6:27 pm
can someone explain hess's law? and when it is applied?

### Re: Hess's Law

Posted: Mon Jan 27, 2020 7:23 pm
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.

### Re: Hess's Law

Posted: Mon Jan 27, 2020 7:53 pm
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.

### Re: Hess's Law

Posted: Mon Jan 27, 2020 9:55 pm
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.

### Re: Hess's Law

Posted: Mon Jan 27, 2020 10:48 pm
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?

### Re: Hess's Law

Posted: Mon Jan 27, 2020 10:53 pm
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.

### Re: Hess's Law

Posted: Tue Jan 28, 2020 2:10 pm
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.

### Re: Hess's Law

Posted: Tue Jan 28, 2020 2:24 pm
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.

### Re: Hess's Law

Posted: Tue Jan 28, 2020 4:55 pm
You add enthalpies together, reverse signs for reverse reactions, and multiply if you multiply the whole reaction.

### Re: Hess's Law

Posted: Tue Jan 28, 2020 5:05 pm
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.

### Re: Hess's Law

Posted: Tue Jan 28, 2020 5:56 pm
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.

### Re: Hess's Law

Posted: Tue Jan 28, 2020 6:35 pm
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