enthalpy

[starpaste]

For the equation change in enthalpy = bonds broken - bonds made. What if the compound makes a double bond. Meaning in the reactants there is a C--O bond, in the product it becomes C=O, do i add for bonds made the value for a single C-O bond (since 1 C--O bond already exists) or the value for a C=O (the final product)?

[hermanntrude]

assuming you've been given the data for the C-O and C=O bonds, I'd say you're safest using the values for C-O in "bonds broken" and C=O in "bonds formed". You'll probably also notice that the value for C=O is NOT twice the value for C-O

[starpaste]

the thing is that the C--O is not being broken to make to C==O bond, because I am comparing an intermediate to the product it makes, so only electrons are being shifted in the mechanism. wouldn't it be the value of c==o - c--o?

[hermanntrude]

There's a law which you may or may not have heard of, called hess's law. It states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. In other words, it doesnt matter if you measure the enthalpy change by breaking the C-O bond and then forming a C=O bond or if you just jump from C-O directly to C=O. the advantage of the former method is that you can do it using the numbers you've been given. essentially it works out the same.