AP Chemistry Unit 7: Equillbrium

ALMOST THERE 7/9!!!

Equilibrium expression

[Products]] / [Reactants]

characteristics of EQ expression

1. Includes only gaseous/aqueous molecules

2. DOES NOT include solids and pure liquids (e.g. H₂O)

3. rate of forward reaction = reverse reaction

Equilibrium constant Kc

The ratio of reactants to products at equilibrium

• K_c > 1 → more products than reactants (thermodynamically favorable = non spontaneous)

• K_c < 1 → more reactants than products (thermodynamically unfavorable = spontaneous)

K_eq when reaction is reversed

1/K_eq (reciprocal of original)

K_eq when reaction is multiplied by a coefficient C

K^C (raise power by the coefficient)

K_eq when 2 reactions are added

K_EQ1 x K_EQ2 → Multiply the equilibrium constants

Reaction Quotient (Q_c, Q_p)

relative concentrations/partial pressures of reacting species AT ANY TIME

• DOES NOT include solids and pure liquids (represented as 1)

• tends towards K_EQ

  • if Q > K_EQ → too many products → more reverse reaction

  • if Q < K_EQ → too many reactants → more forward reaction

Le Châtelier’s Principle

systems at equilibrium respond to external stresses by shifting the equilibrium to offset its effects

• leftward shift of EQ → more reverse reactions

• rightward shift of EQ → more forwards reactions

examples of external stresses

1. addition or removal of chemical species

2. change in temperature

3. change in pressure/volume

4. dilution of a reaction system

effect of addition/removal of chemical species

1. removal of products → more fwd rxn

2. addition of products → more reverse rxn

change in temperature effect on EQ

Higher Temp:

• Endothermic reaction → EQ shifts right (to use up heat)

• Exo → EQ shifts left

Lower Temp

• Endothermic reaction → EQ shifts left

• Exo → EQ shifts right (to release more heat)

treat heat as a reactant/product

change in volume/pressure effect on EQ

(Only in gas equation)

V↑ P↓ → more space = increase in number of molecules(shift to the side with less molecules)

V↓ P↑ → less space = decrease in number of molecules (shift to the side with more molecules)

dilution effect on EQ

dilution → decrease concentration of all dissolved species → EQ shifts towards more aqueous molecules

change in K_EQ

can change from a change in temperature

• since heat = energy, change in energy in the system can change the stable ratio of reactants and products

• a disturbance/stress to a system at EQ causes Q to differ from K

Solubility-product constant K_sp

can describe the dissolution of a salt

• type of equilibrium constant

• greater K_sp = better the salt dissolves (solubility)

• when K_sp «1 → insoluble

• when K_sp »1 → soluble

molar solubility

amount of solute that can be dissolved to make one liter of a saturated solution

• increases with rising temperatures

• higher temp = more energy to force the water molecules apart and make room for the solute ions

Common-Ion Effect

The solubility of a salt decreases when it is dissolved into a solution that already has one of the ions present in the salt

• the amount of the ion dissolved from the salt decreases

pH and Solubility

pH affects solubility of a salt if one of its ions is a weak acid/base

Free Energy Change (Gibbs Free Energy) (ΔG^o)

ΔG°diss = ΔH°diss - TΔS°diss

• Determines the spontaneity of a reaction

• uses change in enthalpy and entropy in

  • the breaking of intermolecular forces of solutes

  • preparation of solvent to receive solute

  • solvation (interaction of solute and solvent)

ΔG° > 1 → thermodynamically favorable (soluble)

ΔG° < 1 → thermodynamically unfavorable (insoluble)