Some reactions go to completionSome reactions go to completion A precipitate formsA precipitate forms A gas forms A gas forms

CHCH4 4 (g) + O(g) + O22 (g) (g) CO CO22 (g) + 2H (g) + 2H22O(g)O(g) Some are reversibleSome are reversible

The reactants can reformThe reactants can reform

3Fe(s) + 4H3Fe(s) + 4H22O(g) O(g) FeFe33OO44(s) + 4H(s) + 4H22(g)(g)

Chemical equilibriumChemical equilibrium

Rate of forward reaction = rate of reverse Rate of forward reaction = rate of reverse reaction.reaction.

The concentration of reactants and The concentration of reactants and products remain unchanged.products remain unchanged.

LeChatelier’s PrincipleLeChatelier’s Principle

If stress is applied to a system at If stress is applied to a system at equilibrium, the system will try to equilibrium, the system will try to adjust to reduce the stress.adjust to reduce the stress.

Stresses could beStresses could be TemperatureTemperature Pressure (for gases)Pressure (for gases) ConcentrationConcentration

HH22COCO33(aq) CO(aq) CO22(aq) + H(aq) + H22O(l)O(l)

<1% >99%<1% >99%

If more COIf more CO22 is added, equilibrium shifts is added, equilibrium shifts to the left to minimize the stress. to the left to minimize the stress.

If COIf CO2 2 is removed, equilibrium shifts is removed, equilibrium shifts right.right.

ConcentrationConcentration

Removing products shifts the Removing products shifts the equilibrium to create more product equilibrium to create more product (right).(right).

Adding a product shifts equilibrium Adding a product shifts equilibrium toward the reactants (left).toward the reactants (left).

““ttoward a oward a ttake-away”ake-away” ““aaway from an way from an aaddition”ddition”

TemperatureTemperature

Increasing temperature causes Increasing temperature causes equilibrium to shift in the direction that equilibrium to shift in the direction that absorbs heat.absorbs heat.

2SO2SO22(g) + O(g) + O22(g) 2SO(g) 2SO33(g) + heat(g) + heat

add heat, shifts leftadd heat, shifts left

remove heat, shifts rightremove heat, shifts right

Opposite shift for endothermic reactionsOpposite shift for endothermic reactions

exothermic

PressurePressure

Only affects gases with unequal number Only affects gases with unequal number of moles of reactants and products.of moles of reactants and products.

Increasing pressure, shifts equilibrium to Increasing pressure, shifts equilibrium to side with fewer moles.side with fewer moles.

NN22(g) + 3H(g) + 3H22(g) 2NH(g) 2NH33(g)(g)

4 moles of reactants 2 moles of product4 moles of reactants 2 moles of product

Increase pressure, shift rightIncrease pressure, shift right

decrease pressure, shift leftdecrease pressure, shift left

What effect do the following What effect do the following changes have on equilibrium?changes have on equilibrium?

PClPCl55(g) + heat PCl(g) + heat PCl33(g) + Cl(g) + Cl22(g)(g)

a.a. Addition of ClAddition of Cl22

b.b. Increase in pressureIncrease in pressure

c.c. Increase temperatureIncrease temperature

d.d. Removal of PClRemoval of PCl33 as it forms as it forms

left

right

left

right

Equilibrium constantEquilibrium constant

HH22(g) + I(g) + I2 2 (g) 2HI(g)(g) 2HI(g)

Rate of forward reaction = Rate of forward reaction = kkff[I[I22][H][H22]]

Rate of reverse reaction = Rate of reverse reaction = kkrr[HI][HI]22

At equilibrium At equilibrium kkff[I[I22][H][H22] = ] = kkrr[HI][HI]22

or or kkff = = [HI][HI]2 2 equilibrium constant Kequilibrium constant Keqeq

kkrr [I [I22][H][H22]]

mmA + A + n n B B s s P + P + r r QQ

KKeqeq = = [P][P]ss[Q][Q]rr

[A][A]mm[B][B]nn

If KIf Keqeq<1, equilibrium is established <1, equilibrium is established before much product is formedbefore much product is formed

If KIf Keqeq is large the reaction approaches is large the reaction approaches completion.completion.

Sample problemsSample problems

Write the equilibrium expression forWrite the equilibrium expression for

4HCl + O4HCl + O22 2Cl 2Cl22 + 2H + 2H22OO

KKeqeq = = [Cl[Cl22]]22[H[H22O]O]22

[HCl][HCl]44[O[O22]]

Determine the KDetermine the Keqeq if the concentrations if the concentrations are [HCl]=.200M, [Oare [HCl]=.200M, [O22]=.500M, ]=.500M, [Cl[Cl22]=.32M, and [H]=.32M, and [H22O]=.42M.O]=.42M.

KKeqeq= 22.6= 22.6

What is the equilibrium concentration What is the equilibrium concentration of SOof SO33 in the following reaction if in the following reaction if [SO[SO22] and [O] and [O22] are 0.0500M and K] are 0.0500M and Keqeq = = 85.0?85.0?

2SO2SO22 + O + O22 2SO 2SO33

KKeqeq= = [x] [x]22 [0.0500][0.0500]22[0.0500][0.0500]

XX22=0.0106=0.0106

X=0.103MX=0.103M