The 3 methods

[Adrian C 1D]

We learned that there were 3 methods to calculate the enthalpy. Does this mean we can use whatever method interchangeably, or is based upon the question??

[David S]

In theory, with enough reference tables, you could use whatever method, but in certain problems and situations, given info will make one method more preferable than another.

For example, If I want to calculate the enthalpy of a reaction, and that reaction is a sum of other reations whose enthalpies we are given, it would be easiest to just add together enthalpies of constituent reactions than doing Method 2 with bond enthalpies.

Also good to note is that Method 2 (bond enthalpies) is generally not as accurate as the other two because bond enthalpy values are average values.

[Bianca Barcelo 4I]

Can someone clarify how to use bond enthalpy to calculate the enthalpy change?

[Aarti K Jain 1L]

For bond enthalpy, you first have to recognize which bonds are broken and which are formed. Then you add the bond enthalpies for the bonds that are broken and do the same for the bonds that are formed. From there, subtract the bond enthalpy of the formed bonds from the bond enthalpy of the broken bonds, which will give you the total bond enthalpy.

[Maya_Peterson1C]

I believe that it depends on the type of information given in the problem. However, I recall Dr. Lavelle saying that Method 1 would give the most reliable and accurate answer.

[Theodore_Herring_1A]

If you have all the necessary information than yes, but usually you will only be given enough information for one of the methods.