Equilibrium

• state of balance• condition in which opposing forces

exactly balance/equal each other• need 2-way or reversible situation• need a closed system

Dynamic Equilibrium

• macroscopic level –looks like nothing is happening

• microscopic level –lots going on

3 Kinds of Equilibria

• phase equilibrium – physical

• solution equilibrium – physical

• chemical equilibrium - chemical

Phase Equilibrium

phase changes are reversible processesphase changes are reversible processes• H2O(l) H2O(g)

• H2O(l) H2O(s)

same substance on both sides only its phase is different

Examples - Phase Equilibrium

• water & water vapor in sealed bottle• perfume in partially full, sealed flask• ice cubes & water in insulated container• dry ice & CO2(g) in a closed aquarium

Solution Equilibrium: Solids

• saturated solution = dynamic equilibrium

• dissolving & solidification occur at equal rates

Solid in Liquid

• NaCl(s) NaCl(aq)

• Favored a little bit by higher temperature

Solution Equilibrium: Gases

CO2 in water

CO2(g) CO2(aq)

favored by high pressure & low temperature

Reversible ReactionsReversible Reactions

• N2(g) + 3H2(g) 2NH3(g) • forwardforward:

–N2 & H2 consumed; NH3 produced

• 2NH3(g) N2(g) + 3H2(g) • reversereverse:

–NH3 consumed; N2 & H2 produced

Reversible Reactions: 1 Equation

• N2(g) + 3H2(g) 2NH3(g)

• forward reaction: reactants on L–read left to right

• reverse reaction: reactants on R –read in reverse: right to left

• reaction runs in both directions all the time

Time

Conc

entra

tion

NH3

H2

N2

N2(g) + 3H2(g) 2NH3(g)Why is this point Why is this point significant?significant?

Reaction Rate

• depends on concentration of reactants

• as concentration reactants ↓, rate forward reaction ↓

• as concentration product ↑, rate reverse reaction ↑

Chemical Equilibrium

• state in which forward & reverse rxns balance each other

• RateRateforwardforward rxnrxn = Rate = Ratereversereverse rxnrxn

• does this mean concentrations reactants/products are equal?

NO!NO!

Chemical Equilibrium

• Rateforward rxn = Ratereverse rxn

• at equilibrium: concentrations all species are constantconstant– stop changing– rarely ever equal

Reversible Reactions vs. Reactions that “Go to Completion”

If goal is to maximize product yield:• easier in reaction that goes to completion

– use up all reactants– left with only product

Reversible reactions are different• look at conc/time picture again

Time

Conc

entra

tion

NH3

H2

N2

N2(g) + 3H2(g) 2NH3(g)OriginalEquilibrium Point

Reversible Reactions

• once reach equilibrium, don’t produce any more product– bad news if product is what you’re selling

• can you change the equilibrium concentrations?

if so how can it be done? for example, how can you maximize product?

What you would really like to see…

lots of product created as fast as possible

New equilibrium point

• equilibrium can be changed or affected by:– any factor that affects forward and reverse

reactions differently

What factors affect rate of rxn?

• concentration/pressure (gases only)• temperature• presence of catalyst

Catalyst

• same effect on both forward & reverse reactions

• equilibrium reached more quickly, but “equilibrium point” not shifted

• equilibrium concentrations are same with or without catalyst

Concentration, Pressure, Temperature

• changes in concentration, pressure, temperature affect forward & reverse reactions differently

• composition of equilibrium mixture will shift to accommodate these changes

LeChatelier’s Principle

• “If system at equilibrium is subjected to stress, the system will act to reduce stress”

• stress = change in concentration, pressure, or temperature

• system tries to undo stress

System

• only only 2 possible actions2 possible actions• shift to shift to rightright & form more product

– forward reaction speeds upforward reaction speeds up more than reverse reaction

• shift to shift to leftleft & form more reactant– reverse reaction speeds upreverse reaction speeds up more than

forward reaction

A + B C + D (at equilibrium)

• If ↑ concentration A, how will system react?

• How does new equilibrium mixture compare to original equilibrium mixture?

• Use logic:– If you ↑ [A]: the system wants to ↓ [A]

• must use A up, so forward reaction speeds up

A + B C + D

DEC DEC ______INC left[B]

INC ______DEC DEC right[C]

______DEC INC INC left[D]

INC INC DEC ______right[A]

[D][C][B][A]Equil. Shift

Stress

Changes in Temp• exothermic reaction: A + B C + D + heat

– If ↑ temperature, system shifts to consumeconsume heat so shifts to left

• endothermic reaction: A + B + heat C + D– If ↑ temperature, system shifts to consume heat so

shifts to right

Changes in PressureN2(g) + 3H2(g) 2NH3(g)

• If ↑ pressure, system shifts to side with fewer moles of gas – left side: 4 moles of gas; right side: 2 moles – ↑ pressure causes shift to right

• If ↓ pressure, system shifts to side with more moles of gas– ↓ pressure causes shift to left

H2(g) + I2(g) 2HI(g)

• this system has 2 moles gas on left & 2 moles gas on right

• systems with equal moles gas on each side cannot respond to pressure changes so NO shift occurs