Untitled - Kelly Walsh High School

102 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Quick Tip
Don’t Forget!
Watch your signs in all the thermodynamic calculations. They are extremely
important.
The H is dependent upon the state of matter. The enthalpy change would be
different for the formation of liquid water instead of gaseous water.
Pay close attention to the state of matter for your reactants and products and
choose the corresponding value in your tabulated enthalpies.
6-3 Hess’s Law
We can measure enthalpies of reaction using a calorimeter. However, we can
also calculate the values. Hess’s law states that if we express a reaction in a
series of steps, then the enthalpy change for the overall reaction is simply the
sum of the enthalpy changes of the individual steps. If, in adding the equations
of the steps together, it is necessary to reverse one of the given reactions, then
we will need to reverse the sign of the H. In addition, we must pay particular
attention if we must adjust the reaction stoichiometry.
It really doesn’t matter whether or not the steps which are used are the actual ones
in the mechanism (pathway) of the reaction because H reaction is a state function, a
function that doesn’t depend on the pathway, only the initial and final states.
Given the following information:
C(s) O2(g) l CO2(g) H 393.5 kJ
H2(g) (1/2) O2(g) l H2O(l) H 285.8 kJ