1. 12.2 Heat and Enthalpy changes

2. Enthalply Def: (H) heat content of a system at constant pressure
Is essentially energy with a small term added that takes pressure and volume into account

3. Enthalply Change Symbolized as ΔH. Δ is the Greek symbol for “change.”
ΔH = H products – H reactants
The sign ΔH in an endothermic reaction is positive (+) and for an exothermic reaction is negative (-).

4. Enthalply Change
This calculation does depend on conditions such as temperature, pressure and states.
As a result, the standard for comparison is at 1 atmosphere (atm) of pressure and at 25 °C.
The substance must also be in its “standard” or purist form at 1 atm
For example, carbon at 1 atm can exists as diamond or graphite, but graphite is more stable.

5. Standard Enthalply Change
Is symbolized by ΔH°.
All reactants and products should be in their standard states.
Examples
C3H8 + 5 O2  3 CO2 + 4 H2O where ΔH° = 2043 kJ
C + H2O  CO + H2 where ΔH° = +113 kJ

6. Using Enthalply Changes
See sample problem on pg. 386
Complete practice problems 1 and 2 on page 386.