1. Heat in Reactions

2. The study of heat in reactions is:
Thermochemistry,
a “branch” of Thermodynamics

3. Reactions involve energy because:
chemical bonds are
BROKEN (needs energy)
and FORMED (releases energy)

4. Heat is:
the energy transferred between objects due to a difference in temperature.
It flows from the higher temperature object to the lower temperature object.

5. Some reactions release energy:
EXOTHERMIC
C3H8 + 5O2 → 3CO2 + 4H2O + energy
C3H8 + 5O2 → 3CO2 + 4H2O kJ

6. Some reactions absorb energy:
ENDOTHERMIC
C + H2O + energy → CO + H2
C + H2O kJ → CO + H2

7. To study energy in reactions,
scientists use a concept called
ENTHALPY
symbolized H
and sometimes you will see it as ΔH

8. ENTHALPY is
the part of the energy of a substance due to the motion of its particles, or heat content.
H = E + PV

9. We cannot determine absolute H, rather we can determine change in H (DH). DH = Hproducts - Hreactants

10. (This works because pressure is essentially constant.)
For our purposes,
the heat absorbed or released during a chemical reaction is equal to the enthalpy change for the reaction
(This works because pressure is essentially constant.)

11. standard enthalpy change (DHo)
For easy comparisons,
define:
standard enthalpy change (DHo)
when reactants and products are in their standard states (at 25 oC and 1 atmosphere)

12. The sign of DH is informative.
When DH > 0, (positive)
energy is absorbed,
Endothermic reaction
When DH < 0, (negative)
energy is released,
Exothermic reaction