## Methods to Calculate Enthalpies

Charisse Vu 1H
Posts: 101
Joined: Thu Jul 25, 2019 12:17 am

### Methods to Calculate Enthalpies

How do we know which method to use when calculating the total enthalpy of a reaction?

Brian J Cheng 1I
Posts: 115
Joined: Thu Jul 11, 2019 12:15 am

### Re: Methods to Calculate Enthalpies

Depends on the information given and the setup of the question. For example: if multiple rxn equations are given with their respective enthalpy changes, then you might suspect Hess's Law. If bond energies are given, then you might use bond energies.

sarahforman_Dis2I
Posts: 109
Joined: Sat Aug 17, 2019 12:18 am

### Re: Methods to Calculate Enthalpies

Charisse Vu 1H wrote:How do we know which method to use when calculating the total enthalpy of a reaction?

Just like someone said above, it depends on what the problem gives you. Hess's method can be used if you are given all of the chemical equations of the intermediates with their respective changes in enthalpy. If you are given the bond enthalpies of reaction (not to be confused with standard enthalpy), you can draw out the lewis structures of the reactants and products to see how many of each bonds are being broken and formed. You can then add the sum of the reactants and products. If you are not given the bond enthalpies (you are not given a table), you can calculate the standard reaction enthalpy though using the standard enthalpies of formation of the products and reactants. When using the standard enthalpies of formation, you sum the enthalpies of formation for the products and then subtract the sum of the enthalpies of formation for the reactants.

In short, if you are given the bond enthalpies (or the amount of energy needed to break a bond) you add the bonds broken and bonds formed (making sure to flip the sign of the bond enthalpy for the products). If you are given the standard enthalpies of formation, sum the products and reactants, and then subtract the reactant sum from the product sum. I hope this helps!

805097738
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Joined: Wed Sep 18, 2019 12:20 am

### Re: Methods to Calculate Enthalpies

Brian J Cheng 1I wrote:Depends on the information given and the setup of the question. For example: if multiple rxn equations are given with their respective enthalpy changes, then you might suspect Hess's Law. If bond energies are given, then you might use bond energies.

what exactly are bond energies?

Amy Pham 1D
Posts: 103
Joined: Fri Aug 09, 2019 12:15 am

### Re: Methods to Calculate Enthalpies

805097738 wrote:
Brian J Cheng 1I wrote:Depends on the information given and the setup of the question. For example: if multiple rxn equations are given with their respective enthalpy changes, then you might suspect Hess's Law. If bond energies are given, then you might use bond energies.

what exactly are bond energies?

I think what was meant to be said was "bond enthalpies." If the bond enthalpies for the bonds broken and formed during the reaction are given, you could add up all the enthalpies with the appropriate amounts and signages (positive for bonds broken and negative for bonds formed) to find the overall total enthalpy of the reaction.

Ruby Richter 2L
Posts: 103
Joined: Thu Jul 25, 2019 12:17 am

### Re: Methods to Calculate Enthalpies

Bonds being broken and bonds forming also have different energies, as one is exothermic and one is endothermic i believe

Verity Lai 2K
Posts: 100
Joined: Sat Aug 24, 2019 12:18 am

### Re: Methods to Calculate Enthalpies

It depends on what the information gives you. If there is a table of bonds and their enthalpies, that indicates a bond enthalpy problem, if it gives you the delta H for multiple reactions, you will probably will have to use Hess’s law and for standard enthalpy of formation it will probably ask for that specifically. Or you can tell if they provide the equation and all the reactants are in their most stable form (ex. O2(g), N2(g), etc.).

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