Hess' Law
If
you can 'build' a reaction by adding together other reactions, then
the enthalpy of the total reaction is the sum of the enthalpies of
the reactions you added together.
Here's an example:
Q: Given these reactions and enthalpies at 25ºC:
Here's an example:
Q: Given these reactions and enthalpies at 25ºC:
What
is the enthalpy of reaction, ΔH(reaction),
for
A: We can combine the reactions above, so that they ADD to the target reaction.
Some
of the products and reactants cancel, because they're on both side of
the equation. When we flip an equation (so the products become
reactants and vice versa), we change the SIGN of the enthalpy
(positive become negative and vice versa).
