Method #3 Clarification from Jan 27 lecture

[rihannasbestfriend]

For the last example in class about standard enthalpy of combustion of methane, the professor shows the numerical values for each enthalpy. Specifically for CH4, why is the value -75 and not +75? I thought the values are usually given as a positive value and we only turn the value negative when it is apart of the products, but CH4 is apart of the reactants.

[Rose A 1L]

I'm not sure if this is entirely correct, but I think that the table refers to the bond enthalpies for breaking bonds. in gases. This is just specific for the breaking of a bond at a specific temperature. The standard enthalpy of formation is what''s used in Method 3, so it's not about the breaking of bonds from the table, but the " formation of one mole of a substance from its elements in their most stable form". Therefore, method 3 is not using the table that professor was referring to with the positive values, but a different one.

[Julian Wang]

Hi Rose. Basically, in a table of values, all energy values are given with the same sign (either positive or negative) by convention. However, the most important thing to remember while doing enthalpy calculations is the fact that breaking bonds always require input of energy, and formation of bonds always releases an output of energy. Therefore, based on the convention of considering the system, breaking bonds will always involve positive energy change and formation of bonds will always involve negative energy change. There are multiple ways to summarize this in an equation, but as long as formation is signed negative and breaking is signed positive, the end result will be correct. Hope this helped!