1. Objectives:
Why do chemical and physical processes release or absorb energy?
How can these energy values be used stoichiometrically?

2. Energy Transfers in Chemistry
The energy transferred as heat (at constant P) during physical or chemical processes is called Enthalpy!
Energy Transfers in Chemistry
Physical processes involve energy transfers:
Phase changes: H2O (s)  H2O (l)
E is absorbed
Dissolving: solute (s)  solute (aq)
E can be released or absorbed, depends
Chemical processes involve energy transfers:
H2O  H2 + O2
E is absorbed
C2H5OH + O2  H2O+ CO2
E is released

3. Energy Transfers in Chemistry
The energy transferred as heat (at constant P) during physical or chemical processes is called Enthalpy!
Energy Transfers in Chemistry
Physical processes involve energy transfers:
Phase changes: H2O (s)  H2O (l)
Enthalpy of fusion
E is absorbed
Dissolving: solute (s)  solute (aq)
E can be released or absorbed, depends
Enthalpy of solution
Chemical processes involve energy transfers:
H2O  H2 + O2
E is absorbed
Enthalpy of reaction or
C2H5OH + O2  H2O+ CO2
E is released
Reaction enthalpy

4. Enthalpy basics: The enthalpy change for a process is written as ∆H
For chemical rxn’s: ∆Hr or ∆rH or ∆Horxn or ∆Hrxn
The units are usually kJ/mol or kJ/mol-rxn
The sign of ∆H indicates the direction of the heat flow
-∆H = heat is released to surroundings (exothermic)
+∆H = heat is absorbed from surroundings (endothermic)

5. Where do enthalpy changes come from ?!?!