1. Reaction Rates and Equilibrium
Chapter 18

2. Some Definitions: Entropy: Degree of disorder (S)
The entropy of a solid < liquid < gas
All processes create disorder (Law of Disorder)
Enthalpy: Heat energy (H)
Exothermic – losing heat energy (-DH)
Endothermic – gaining heat energy (+DH)

3. Three questions: Does a reaction happen? Does an equilibrium exist?
How fast will a reaction happen?

4. Does a reaction happen? Reactants Products
Spontaneous reactions move forward.
Enthalpy decreases (exothermic - DH).
and/or Entropy increases (+DS).

5. Exothermic Reaction Diagram

6. Does an equilibrium exist? …and how much product will form?
Equilibrium: point where the rate of the forward and reverse reactions are equal (Note: amounts of R & P aren’t necessarily equal)
This point can be determined using Le Chatelier’s Principle!
Reactants Products

7. Le Chatelier’s Principle
Every action has a reaction!
…or a disturbed system will return to equilibrium over time.

8. Three Possible Stressors: STRESSOR DEPENDS ON
Change in concentration
Change in temperature
Change in pressure
Substance and change
Exothermic or endothermic
Number of moles of gas of product / reactant

9. What would happen if we…
C (s) + H2O (g) + heat CO (g) + H2 (g)
Increase temperature
Increase pressure
Add more water
Add more carbon monoxide
Remove carbon
Lower temperature
Right 
Left 
N/a

10. How fast will a reaction happen?
Collision Theory:
Particles must collide with enough energy to react and form a bond.
Anything that increases the frequency of collisions will increase the rate.
BAM!

11. To Increase the Rate of a Reaction:
Increase temperature
Increase surface area
Increase concentration
Add a catalyst

12. Catalyst No catalyst Catalyst Added to but not used up in a reaction
Lowers the activation energy
No catalyst
Catalyst

13. Free Energy
Gibbs Free Energy Change (DG): Weighs the relative contributions of enthalpy and entropy to the overall spontaneity of a process.
DG = DH – TDS
DG < 0 Process is spontaneous
DG = 0 Process is at equilibrium
DG > 0 Process is nonspontaneous

14. You Try It
Which of the following reactions are spontaneous under standard conditions at 25°C?
a.AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq) DG° = –55.7 kJ
b.2 C(s) + 2 H2(g)  C2H4(g) DG° = 68.1 kJ
c.N2(g) + 3 H2(g)  2 NH3(g) DH° = –92 kJ; DS° = –199 J/K

15. CH 18 Test
March 26th and 27th