1. Equilibrium: a rate of reaction perspective
Forward reaction
A + B → C + D
Backward reaction
A + B ← C + D
Equilibrium results
A + B ↔ C + D
Ca2+(aq) + 2HCO3-(aq) ↔ CaCO3(s) + CO2(aq) + H2O (l)

2. Equilibrium is dynamic
All systems at equilibrium involve opposing processes in balance:
Chemical reactions, dissolving, phases, diffusion through membranes
Rate forward = rate reverse

3. Equilibrium constant kf//kr = Keq kf//kr = [Ay]/[Ax]
Consider the reaction Ax ↔ Ay
At equilibrium,
Ratef = Rater
Ratef = kf[Ax]; Rater = kr[Ay]
kf[Ax] = kr[Ay]
kf//kr = [Ay]/[Ax]
kf//kr = Keq

4. General expression for reactions
aA + bB ↔ cC + dD
Products
Reactants

5. Not all products and reactants are included
Ignore all pure solids and liquids – they do not have concentrations in the sense of mol/L
Consider
MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)

6. Not all products and reactants are included
Ignore all pure solids and liquids – they do not have concentrations in the sense of mol/L
Consider
MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)

7. Significance of Keq
Keq > 100: products much greater than reactants – almost complete reaction
Keq < .01: products much less than reactants – very little reaction
.01< Keq <100 products and reactants in similar concentrations

8. Calculations – putting numbers in
Consider the reaction
2HI(g) ↔ H2(g) + I2(g)
What is the value of Keq if [HI] = 0.54 M, [H2] = [I2] = 1.72 M?

9. Units of Keq…
The units of Keq depend on the ratios of products and reactants in the expression
Unitless
M-2

10. Upsetting the applecart
What happens to the equilibrium when changes are made?
Le Chatelier’s Principle
If a stress is placed on a system at equilibrium, the system will respond by changing its position to minimize the stress

11. Changes in composition
Consider the reaction at equilibrium
2HI(g) ↔ H2(g) + I2(g)
What happens if additional H2(g) is added?
The system responds by trying to reduce the amount of added material; H2 is converted into HI – the equilibrium shifts away from the point of change

12. In general: Other effects; Add products: products → reactants
aA + bB ↔ cC + dD
Add reactants: reactants → products
Other effects;
Temperature
Pressure

13. Temperature and equilibrium
N2(g) + 3H2(g) = 2NH3(g) + heat
Reaction is exothermic
Supply heat: equilibrium adjusts to disperse heat
Less NH3 is made
Endothermic reactions will show opposite response (why we heat endothermic reactions)

14. Pressure and equilibrium
2HI(g) ↔ H2(g) + I2(g)
2 moles reactants → 2 moles products
No overall pressure change
N2(g) + 3H2(g) = 2NH3(g)
4 moles reactants → 2 moles products
Increase pressure drives reactants → products