1. Reversible Reactions and Equilibrium
Ch. 18.2
Reversible Reactions
and
Equilibrium

2. Reversible Rxn
Reversible rxn: in which the conversion of reactants to products and products to reactants occur simultaneously.
2SO2 + O2  2SO3 (Forward rxn)
2SO3  2SO2 + O2 (Reverse rxn)
2SO2 + O2  2SO3
2SO2 + O SO3
(double arrow or 2 half arrows indicates reversible rxn)

3. Equilibrium Rates of forward and reverse reaction are equal.
No net change of concentration occurs in the system.
DOES NOT mean equal amounts of products and reactants

4. Middle picture is at equilibrium

5. Concentration of reactants (green & blue lines) and products(red and aqua lines) with time.

6. Le Chatelier’s Principle
If stress is applied to a system in dynamic equilibrium, the system changes to relieve that stress.
Stresses include:
Concentration changes
Temperature changes
Pressure changes (only affects gases)

7. Guide to Le Chatelier’s principle 2SO2(g) + O2(g) + heat  2SO3(g)
Add a substance or heat
Equilibrium shifts to opposite side of rxn
Remove a substance or heat
Equilibrium shifts to same side of rxn
Add (increase) pressure (decrease vol)
Equil. Shifts to side with fewest gas mc’s
remove (decrease) pressure (increase V)
Equil shifts to side with most gas mc’s

8. Effect of LeChatelier’s principle
Whichever side the equilibrium shifts to, the concentration increases on that side
If equil shifts to reactant side (left) reactant concentration increases
The side away from the shift, concentration decreases
If equil shifts to reactant side (left) product concentration decreases.

9. N2(g) + 3H2(g)  2NH3(g) + 22.0 kcal
Stress
Equil shift
[N2]
[H2]
[NH3]
heat
1. Add N2
right 
2. Add H2
3. Add NH3
4. Remove NH3
_______
5. Remove N2
__________
6. inc. temp
7. dec. temp
8. Increase pressure
9. Decrease pressure

10. 18.4
Entropy

11. Entropy Entropy: Measure of disorder in a system
Law of disorder: the natural tendency is for systems to go toward maximum disorder.

12. What factors promote Entropy?
Phase
Particle size
Number of products
Temperature (kinetic energy)

13. Phase
Solid  Liquid  Gas (most entropy)

14. Particle size
More particles (smaller) more entropy

15. Number of products
2 H2O  2 H O2
Dissolving increases entropy

16. Temperature
Increase of temperature increases kinetic energy which increases entropy

17. Entropy in Reactions 2 KClO3 (s)  2 KCl (s) + 3 O2 (g)
The more entropy the more likely a reaction will occur S = +# - more entropy
2 KClO3 (s)  2 KCl (s) + 3 O2 (g)
H2O (l)  H2O (s)
N2 (g) + 3 H2 (g)  2 NH3 (g)
HC2H3O2 (l)  H+ (aq) + C2H3O2- (aq)
Steam at 105 oC or steam at 115 oC

18. Free Energy
Free Energy: Energy that is available to do work (released by a chemical process/rxn)

19. Spontaneous Reaction:
Occurs naturally
Favors the formation of product
Releases free energy (combination of enthalpy and entropy)
Delta G = + # =nonspontaneous = Endergonic
Delta G = - # = spontaneous = Exergonic

20. Spontaneous Reactions
2 factors determine spontaneous rxns
Enthalpy (heat energy of reaction)
Exothermic rxns favor spontaneous rxns
Entropy (disorder)
Increase in entropy (disorder) favor spont. rxns

21. Is a reaction spontaneous?H S G
Spont
Dec (-)
Inc (+)
YES
Maybe NO ??