1. CHEMISTRY 161
Chapter 6

2. THERMODYNAMICS study of the energy associated with change HEAT CHANGE
Physical
Why does water evaporate?
2. Chemical
Why do some molecules react and others do not?

3. study of the energy associated with chemical reactions
THERMOCHEMISTRY
study of the energy associated with chemical reactions
2 H2(g) + O2(g) → 2 H2O(l)
+ energy
Hindenburg 1937
Challenger 1986

4. ENERGY 1. Kinetic energy 2. Radiant energy 3. Potential energy
energy of a moving microscopic or macroscopic object
E = ½ m u2
2. Radiant energy
energy in form of photons (‘light’) (solar energy)
E = h  (h = Planck’s constant)
3. Potential energy
energy by changing object’s position in height
E = m g h (h = height)

5. LAW OF CONSERVATION OF ENERGY
4. Thermal Energy
energy associated with random motion of atoms and molecules
Ekin = ½ M u2 = 3/2 R T
5. Chemical Energy
energy stored in chemical bonds of substances
LAW OF CONSERVATION OF ENERGY
THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT

6. ENERGY vs. HEAT THERMOCHEMISTRY T2 T1
heat is spontaneous transfer of thermal energy two bodies
at different temperatures T1 > T2
spontaneous T2 T1
THERMOCHEMISTRY
study of the energy associated with chemical reactions
study of the heat change in a chemical system

7. REFERENCE SYSTEM CLASSIFICATION

9. THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT
SURROUNDINGS
SYSTEM
THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT

10. ENDOTHERMIC EXOTHERMIC 2 HgO(s) → O2(g) + 2 Hg(l)
heat
Surrounding
System
Surrounding
heat
System
ENDOTHERMIC
EXOTHERMIC
2 HgO(s) → O2(g) + 2 Hg(l)
2 H2(g) + O2(g) → 2 H2O(l)

11. O2(g) + 2 Hg(l) 2 H2(g) + O2(g) 2 HgO(s) 2 H2O(l) Energy Exothermic
(heat given off by system)
Endothermic
(heat absorbed by system)
2 HgO(s)
2 H2O(l)

12. DH = Hproducts - Hreactants
QUANTIFICATION
Enthalpy of Reaction
Enthalpy is the heat release at a constant pressure
(mostly atmospheric pressure)
DH = Hfinal - Hinitial
DH = Hproducts - Hreactants
Hfinal > Hinitial : DH > 0 ENDOTHERMIC
Hfinal < Hinitial : DH < 0 EXOTHERMIC

13. Hfinal > Hinitial Hfinal < Hinitial O2(g) + 2 Hg(l)
Energy
O2(g) + 2 Hg(l)
2 H2(g) + O2(g)
Hfinal > Hinitial
Hfinal < Hinitial
Exothermic
Endothermic
2 HgO(s)
2 H2O(l)

14. DH = Hfinal – Hinitial DH = + 6.01 kJ
Energy
H2O(l)
Hfinal > Hinitial
DH = Hfinal – Hinitial
DH = kJ
Endothermic
H2O(s)
H2O(s) → H2O(l) ΔH = kJ

15. DH = Hfinal – Hinitial DH = - 6.01 kJ
Energy
H2O(l)
Hfinal < Hinitial
DH = Hfinal – Hinitial
DH = kJ
Exothermic
H2O(s)
H2O(l) → H2O(s) ΔH = kJ

16. THERMOCHEMICAL EQUATIONS
H2O(l) → H2O(s) ΔH = kJ
Mol relationships
2. Enthalpy change

17. CH4(g) + 2 O2(g) → 2 H2O(l) + CO2(g) ΔH=-890.4 kJ
Calculate the heat evolved when combusting
24.0 g of methane gas.

18. DH = Hproducts - Hreactants
SUMMARY
Thermochemistry
Enthalpy of Reaction
heat released or absorbed by the system at a constant pressure
DH = Hproducts - Hreactants
Hfinal > Hinitial : DH > 0 ENDOTHERMIC
Hfinal < Hinitial : DH < 0 EXOTHERMIC

19. Homework
Chapter 6, p
problems