Topic: EQUILIBRIUMDo Now:

• VIDEO CLIP

Equilibrium = BalanceNot necessarily equal

1 man and 1 man equal but not balanced

BALANCED!

Chemical equilibrium• Rate of forward reaction is EQUAL to the

Rate of the reverse reaction• Concentrations are not necessarily equal but

they are constant, unchanging

Time

Conc

entra

tion

NH3

H2

N2

N2(g) + 3H2(g) 2NH3(g)EQUILIBRIUM POINT

Dynamic Equilibrium

• macroscopic level –looks like nothing is happening

• microscopic level –lots going on

Phase Equilibriumphase changes are reversible processes• H2O(l) H2O(g)

• H2O(l) H2O(s)

same substance on both sides, only its phase is different

Liquid – Vapor Equilibrium

NOTE: anytime a gas is involved, the container must be sealed for

equilibrium to be reached- if not the gas will escape!

• Solution Equilibrium– saturated solution with some solid on

the bottom of the container.• As one molecule dissolves, one

molecule precipitates (forms solid).–Ex: Too much sugar in the

coffee.

- EX: CO2 in water

CO2(g) CO2(aq)

favored by high pressure & low temperature

Reversible Reactions• Most chemical reactions are reversible.

• Reactants react to form Products, then the Products react to form Reactants.

• N2(g) + 3H2 (g) 2NH3(g)

• N2 reacts with H2 to form NH3

• at the same time, NH3 is consumed and forms N2 and H2

Reversible Rxn vs Reaction that goes to completionNaOH + HCl --> NaCl + H2O

equilibrium can be changed or affected

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

• composition of equilibrium mixture will shift to accommodate these changes

Le Chatelier’s Principle• When a system at equilibrium is subjected

to a stress, the equilibrium will shift in the direction which tends to relieve the stress.– stress = change in concentration, volume,

pressure, or temperature

Increase Concentration• INCREASING the concentration causes the

equilibrium to shift AWAY from the substance that was added.

N2 + 3H2 2NH3

• Increase N2

At equilibrium

In order to get back to equilibrium, what has to happen?Need more products!

What happens to the amount of H2?Rxn shift to the RIGHT

N2 + 3H2 2NH3

• Increase NH3

Need more N2 and H2 so reaction shifts to the left

• DECREASING the concentration causes the equilibrium to shift TOWARD the substance that was removed.

N2 + 3H2 2NH3

• Decrease NH3

Need more NH3 so reaction shifts to the right

Changes in Temp

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

shifts to left – so endothermic rxn favored

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

shifts to right– so exothermic rxn favored

shift in the direction opposite of the “heat” term

!! PRESSURE !!

• Increase PRESSURE, shift toward the side of the equation that has LESS MOLES of GAS.

• Decrease Pressure, shift toward the side of the equation that has MORE MOLES of GAS.

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

VOLUME

• Increasing Volume decreases pressure.• Equilibrium will shift toward greater

number of moles of gas.

• Decreasing Volume increases pressure.• Equilibrium will shift toward smaller

number of moles of gas.

Reactions that go to completion – equilibrium can not exist

Formation of a gas which escapesH2CO3 (aq) CO2(g) + H2O (l)

Formation of waterHCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

Formation of a precipitate (insoluble compound)NaCl (aq) + AgNO3(aq) AgCl (s) + NaNO3 (aq)

A + B C + D

______[B]

______[C]

______[D]

______[A]

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

Stress

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