## Calculation methods

AlyssaYeh_1B
Posts: 100
Joined: Sat Aug 17, 2019 12:16 am

### Calculation methods

Can someone provide a summary for all the methods we learned to calculate delta H? What are the notable differences between each one?

Posts: 50
Joined: Fri Sep 27, 2019 12:29 am

### Re: Calculation methods

Method 1: When two reactions are given and both their ΔHrxn are also given, you can add both reactions and their ΔHrxn to get the new reaction's ΔHrxn.
(See the example he gave in class)

Method 2: Use bond enthalpies. Bond enthalpies are always given on a chart. You can try to figure out what are the specific bonds broken (on the reactant side) and the bonds formed on the product side. Then sum up the bond enthalpies on each side of the reaction and subtract products from reactants. ΔHrxn = Σbond enthalpy products - Σbond enthalpy reactants

Method 3: Standard reaction enthalpies. ΔHo is the reaction enthalpy in a standard environment and these are also usually given. From ΔHo we can figure out ΔHfo. In the example given in class, the ΔHrxno for ethanol was given but the reaction created 2 molecules of ethanol, so to figure out the enthalpy formation for just one molecules of ethanol, we divide ΔHrxno by 2.

Method 4: ΔHrxno = ΣΔHfo (products) - ΔHfo (reactants). Again, ΔHfo will usually be given in a situation like this.

Brandi 2C
Posts: 52
Joined: Thu Feb 28, 2019 12:15 am

### Re: Calculation methods

Dr. Lavelle also stressed that the second method (using bond enthalpy) was the least accurate. I don't know if that was important but I thought it was worth noting since he said it a couple times in lecture.

Tahlia Mullins
Posts: 105
Joined: Thu Jul 25, 2019 12:15 am

### Re: Calculation methods

I remember method 4 being used a lot in AP Chem, so that might mean that it will be coming up pretty often.

andrewcj 2C
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### Re: Calculation methods

How is method 4 different than method 3?

chrisleung-2J
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### Re: Calculation methods

I believe that Method 3 and Method 4 above are essentially the same, and that Dr. Lavelle was using the example of ethanol formation to illustrate the formula stated as "Method 4"

Abigail_Hagen2G
Posts: 107
Joined: Fri Aug 09, 2019 12:17 am

### Re: Calculation methods

The second method uses averages, so it's the least accurate.

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

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