Unit F

Large K

Large K

• High concentration of products 

• Low concentration of reactants

• Forward rxn is favored

• Reaction lies to the right

Small K

• High concentration of reactants

• Low concentration of products

• Reverse rxn is favored

• Reaction lies to the left

K approximately 1

• Significant concentration of reactants & products

• Neither reaction is favored

K for the reverse reaction

• inverse of K for the forward reaction

• A + B ⇌  C + D K₁

• C + D ⇌   A + B 1/K₁

If you multiply coefficients in equation

• K will be raised to that power

• A + B ⇌   C + D K₁

• nA + nB ⇌   nC + nD K = K₁ⁿ

K for a net equation

• product of the K values for individual reactions

• Hess’s Law

• A + B ⇌  C + D K₁

• C + F ⇌  G + A K₂

• B + F ⇌  D + GK₃ = (K₁)(K₂)

If Q = K then

those values are actually at equilibrium.

If Q > K

the right side is too large and the system will shift to the left to reach equilibrium

If Q < K

the right side is too small and the system will shift to the right to reach equilibrium

Le Chatelier’s Principle

• If a system at equilibrium is disturbed the equilibrium will shift to minimize the disturbance

• A system at equilibrium tries to maintain that equilibrium

If the concentration of a reactant or product is increased,

• the reaction will consume what was added

• Add more reactants – shift to right

• Add more products – shift to left

If the concentration of a reactant or product is decreased,

• the reaction will replace what was removed

• Remove reactants – shift to left

• Remove products – shift to right

If the volume decreases,

pressure increases, the reaction will shift to the side with fewer moles of gas particles

If the volume increases,

pressure decreases, the reaction will shift to the side with more moles of gas particles

when concentration and pressure change, K…

does NOT change

when temperature changes, K…

DOES change

In Exothermic Reactions…

• N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat

• Heat is a product

• If the temperature increases, the reaction will shift to the left.

• If the temperature decreases, the reaction will shift to the right

In Endothermic Reactions…

• N₂O₄(g) + heat ⇌ 2NO₂(g) 

• Heat is a reactant

• If the temperature increases, the reaction will shift to the right

• If the temperature decreases, the reaction will shift to the left

Catalysts change the _______ of the reaction but not the ________ or _______________

RATE, K or equilibrium composition

+∆H

endothermic

-∆H

exothermic

+∆S

more disorder

-∆S

less disorder

+∆G

not thermodynamically favorable

-∆G

thermodynamically favorable

∆G = ∆H – T∆S

Convert S to kJ

∆G =  – RTlnK

R = 8.314 J mol-1K-1

K = e-∆G/RT

Crystallization

opposite of dissolving

Rate of dissolving =

rate of crystallization

Saturated solution

• in equilibrium with undissolved solute

• Maximum amount of solute is dissolved

• Excess may be present on bottom

Li+1

Soluble, no exceptions

Na+1

Soluble, no exceptions

K+1

Soluble, no exceptions

NH4+1

Soluble, no exceptions

NO3-1

Soluble, no exceptions

ClO3-1

Soluble, no exceptions

ClO4-1

Soluble, no exceptions

C2H3O2-1

Soluble, no exceptions

Cl-1

Soluble

Exceptions: Ag+1, Pb+2, Hg2+2

Br-1

Soluble

Exceptions: Ag+1, Pb+2, Hg2+2

I-1

Soluble

Exceptions: Ag+1, Pb+2, Hg2+2

SO4-2

Soluble

Exceptions: Ca+2, Sr+2, Ba+2, Pb+2

OH-1

Insoluble

Exceptions: Li+1, Na+1, K+1, NH4+1, Ca+2, Sr+2, Ba+2

S-2

Insoluble

Exceptions: Li+1, Na+1, K+1, NH4+1, Ca+2, Sr+2, Ba+2

CO3-2

Insoluble

Exceptions: Li+1, Na+1, K+1, NH4+1

PO4-3

Insoluble

Exceptions: Li+1, Na+1, K+1, NH4+1

SO3-2

Insoluble

Exceptions: Li+1, Na+1, K+1, NH4+1

K_sp

represents the extent to which an ionic solid dissolves in water

Molar solubility

moles/liter

Gram solubility

g/liter or g/100mL

When salts have same number of total ions…

Higher K_sp is more soluble

If salts have different numbers of total ions…

can’t compare K_sp directly

Common Ion Effect

Solubility of a substance is reduced when a substance containing a common ion is also present in the solution

• K_sp = [A⁺][X^-]

• Q = K

• at equilibrium

• solution is saturated

• precipitation = dissolving

• K_sp = [A⁺][X^-]

• Q < K

• less ions are present

• no precipitation

• K_sp = [A⁺][X^-]

• Q > K

• more ions are present

• precipitation

When mixing solutions a precipitate will form from…

• the salt with the lowest K_sp value first

• Can only compare directly when ion ratio is the same