1. Thermodynamics

2. Every physical/chemical change is accompanied by change in energy
Thermodynamics: branch of chemistry that studies energy changes
specifically: changes in heat energy

3. Tells us if a reaction will occur 2 considerations:
Thermodynamics
Tells us if a reaction will occur
2 considerations:
enthalpy (heat energy)
entropy (chaos/randomness)

4. Enthalpy, H enthalpy: heat content of system at constant pressure
use symbol: H

5. changes in Enthalpy are measurable
cannot measure enthalpy content of system directly
can measure changes in enthalpy! symbol = H
H = Hfinal – Hinitial = Hproducts - Hreactants

6. net gain in energy Endothermic Process: energy absorbed
Hfinal > Hinitial
so Hfinal – Hinitial
result: H is positive

7. net loss in energy Exothermic Process: energy released
Hfinal  Hinitial
so Hfinal – Hinitial
result: H is negative
[ see footnote to table I]

8. energy of universe is conserved
Which arrow represents an endothermic change?
? exothermic change B
environment A
energy can move between system and the environment
system A B

9. Change in Energy choose how measure energy change
depends on how set up experiment
monitor the system
monitor the environment (this is easier)

10. Energy lost = Energy gained
how do you know energy has moved?
can measure energy gained/lost by environment
equals energy lost/gained by system
change in temperature!

11. reaction is carried out in water in styrofoam cup
- cup is the universe!
temperature of water is monitored
- water is the environment!

12. Q = mCT Q = energy change m = mass of water
c = specific heat of water
T = temperature change = Tf – Ti

13. different types of H’s
H of dissolving: heat of solution
H of phase change:
heat of fusion/heat of vaporization
H of reaction: heat of reaction
categorized by rxn type

14. Table I: Heats of Reaction
rxns #1-6: combustion rxns H: heat of combustion
rxns #7-18: formation reactions (synthesis)
H: heat of formation
rxns #19-24: dissolving equations
H: heat of solution

15. energy depends on amount
example:
it takes more energy to heat up water in bathtub than to make a cup of tea

16. CH4(g) + 2O2(g)  CO2(g) + 2H2O (l) H = -890.4 kJ
1 mole of methane + 2 moles of oxygen →
1 mole of carbon dioxide gas & 2 moles of liquid water
reaction is exothermic (negative sign for ΔH)
890.4 kJ energy released per mole of CH4(g) burned

17. Energy depends on amount
CH4(g) + 2O2(g)  CO2(g) + 2H2O (l) H = kJ
burn 2 moles of CH4(g) with 4 moles of O2(g), get 2 times as much energy out
remember stoichiometry!
(2)(890.4 kJ) = kJ is released

18. Phase Change: Energy depends on direction
endothermic
exothermic
gas
melting/fusion
boiling/ vaporization
sublimation
condensation
freezing
deposition PE
liquid
solid

19. Reactions: Energy depends on direction too!
N2(g) + 3H2(g)  2NH3(g) H = kJ
If look at reverse reaction, then need to reverse sign of H
2NH3(g)  N2(g) + 3H2(g) H = 91.8 kJ

20. Thermochemical Equations
balanced chemical equations
show physical state of all reactants & products
energy change can be given in 2 ways
1. energy term written as reactant or product OR
2. H is given right after equation

21. Exothermic Rxn: energy = product
4Fe(s) + 3O2(g)  2Fe2O3(s) H = kJ OR
4Fe(s) + 3O2(g)  2Fe2O3(s) kJ
exothermic if (-)

22. Endothermic Rxn: energy = reactant
NH4NO3(s)  NH4+1(aq) + NO3-1(aq) H = 27 kJ OR
NH4NO3(s) + 27 kJ  NH4+1(aq) + NO3-1(aq)
endothermic if (+)

23. Changes of State H2O(s)  H2O(l) Hf = 334 J/g at 0oC
H2O(l)  H2O(s) Hf = -334 J/g at 0oC
energy is absorbed when water melts &
energy is released when water freezes!
H2O(l)  H2O(g) Hv = J/g at 100oC
H2O(g)  H2O(l) Hv = J/g at 100oC
energy is absorbed when water evaporates &
energy is released when water condenses!