Calculation methods

[AlyssaYeh_1B]

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]

Method 1: When two reactions are given and both their ΔH_rxn are also given, you can add both reactions and their ΔH_rxn to get the new reaction's ΔH_rxn.
(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. ΔH_rxn = Σbond enthalpy _products - Σbond enthalpy _reactants

Method 3: Standard reaction enthalpies. ΔH^o is the reaction enthalpy in a standard environment and these are also usually given. From ΔH^o we can figure out ΔH_f^o. In the example given in class, the ΔH_rxn^o 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 ΔH_rxn^o by 2.

Method 4: ΔH_rxn^o = ΣΔH_f^o (products) - ΔH_f^o (reactants). Again, ΔH_f^o will usually be given in a situation like this.

[Brandi 2C]

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]

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]

How is method 4 different than method 3?

[chrisleung-2J]

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]

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