Calculation methods

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Calculation methods

Postby AlyssaYeh_1B » Fri Jan 24, 2020 9:52 pm

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

Anuradha S 1F
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Re: Calculation methods

Postby Anuradha S 1F » Fri Jan 24, 2020 10:04 pm

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
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Re: Calculation methods

Postby Brandi 2C » Fri Jan 24, 2020 10:16 pm

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
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Re: Calculation methods

Postby Tahlia Mullins » Fri Jan 24, 2020 10:21 pm

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

Postby andrewcj 2C » Sat Jan 25, 2020 1:38 am

How is method 4 different than method 3?

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Re: Calculation methods

Postby chrisleung-2J » Sat Jan 25, 2020 8:08 pm

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"

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Re: Calculation methods

Postby Abigail_Hagen2G » Sat Jan 25, 2020 9:28 pm

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

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