1.  Heat of reaction is the quantity of heat
released or absorbed during a chemical
reaction
 Exothermic rx releases heat
 Heat of products are less than heat of reactants
 Endothermic rx absorbs heat
 Heat of products are greater than heat of reactants

2. Enthalpy
 Enthalpy is the heat absorbed or released
during a chemical rx
 Represented by symbol H
 H = Hproducts – Hreactants
 H of exothermic rx is negative
 H of endothermic rx is positive
 Hess’s Law
 Overall enthalpy change in a reaction is equal to
the sum of enthalpy changes for the individual
steps in the process

3. Entropy
 Tendency of universe is to become less orderly
(or more randon)
 Entropy defined as degree of randomness of
molecules
 Represented by symbol S
 Example: MOST solids are more orderly than
liquids and MOST liquids are more orderly
than gases.
 Generally, solids have low entropy and
gases have high entropy

4. Combined Enthalpy and
Entropy
Called free energy
Represented by symbol G
 G is the difference between the change in
enthalpy H and the product of *temperature in K
and the entropy change
 G = H – T S
If G is negative, the reaction is spontaneous.
If G is positive, the reaction will not occur.
* K = oC + 273