EQUILIBRIUM 2 REACTION YIELDS. Equilibrium Very few reactions proceed unhindered to completion. Some begin reversing as soon as products are present.

EQUILIBRIUM 2

• REACTION YIELDS  

Equilibrium

• Very few reactions proceed unhindered to completion.

• Some begin reversing as soon as products are present.

• Examples of reversible reactions– Melting ice block

• H2O (s) H2O (l)

– Ni-Cad rechargeable batteries 

Equilibrium

• Chemical reactions that consist of two opposing processes (forward and reverse reactions) will eventually reach an equilibrium.

• The state of equilibrium is characterized by the forward and reverse reactions proceeding at the same rate

• i.e. reactions do not stop ‑ we have a dynamic situation

Dynamic Equilibrium

• Characterized by the following criteria1. amounts and concentrations of substances

remain constant

2. total gas pressure remains constant

3. temperature remains constant

4. the reaction is incomplete (all substances involved in the reaction are present)

Equilibrium

rate

timeEquilibrium first established

N2 + 3H2 2NH3

2NH3 N2 + 3H2

Variation of the rates of the forward and reverse reactions with time

The Equilibrium Law

• For the general equilibrium xA + yB pC + qD

• It can be stated

= constant = K[C]p [D] q

[A]x [B]y

Equilibrium Law

• K allows for the evaluation of the concentration fraction at any time.

• When the system is at equilibrium the concentration fraction is constant ‑ so called the equilibrium constant (K).

• For a particular reaction, K is constant for all equilibrium mixtures (provided temperature remains constant)

Information From The Equilibrium Constant

• If K is about 104 to 10–4 there will be significant amounts of both reactants and products present at equilibrium

• If K is very large (> 104) the equilibrium mixture consists mostly of products

• If K is very small (< 10–4 ) the equilibrium mixture consists mostly of reactants

Le Chatelier's Principle 

• Whenever a change is made to a system at equilibrium, the equilibrium position will shift to partially oppose the change 

Disturbing Equilibrium

• There are 4 major means of disturbing a system at equilibrium

1. Adding or removing a reactant or product

2. Changing the pressure by changing the volume (gases only)

3. Dilution (for solutions only)

4. Changing the temperature

 Disturbing Equilibrium

• Addition of a catalyst will increase both the rate of the forward and reverse reactions equally

• It will simply reduced the time taken to reach equilibrium.

Effect of Temperature on Equilibria

• As temperature INCREASES– For exothermic reactions, value of K decreases

and amounts of products decrease– For endothermic reactions, value of K increases

and amounts of products increase

Effect of Temperature on Equilibria  

• The value of K depends on temperature• When stating a value of K, the temperature

at which the constant was calculated must also be stated

• Temperature is the only change that can be made to a system at equilibrium that will actually change the equilibrium constant (ie K is temperature dependant)

Consider the Reaction

• N2 (g) + 3H2 (g) 2NH3 (g)

Effect on Equilibrium of Adding / Removing Reactant or

Product• N2 (g) + 3H2 (g) 2NH3 (g)

Effect of Adding Nitrogen

• Causes the rate of the forward reaction to increase

• More ammonia is formed [NH3] increases

• This causes the rate of the back reaction to increase to re form more N2 and H2

Effect of Adding Nitrogenco

ncen

trat

ion

time

[NH3]

[N2]

[H2]

Initialequilibrium

Effect of Adding Nitrogen

timeInitialequilibrium

Nitrogen added

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Nitrogen

timeInitialequilibrium

Nitrogen added

System returns to equilibrium

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Nitrogen

timeInitialequilibrium

Nitrogen added

System returns to equilibrium New equilibrium

established

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Hydrogen

timeInitialequilibrium

Hydrogen added

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Hydrogen

timeInitialequilibrium

Hydrogen added

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Hydrogen

timeInitialequilibrium

Hydrogen added

System returns to equilibrium

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Hydrogen

timeInitialequilibrium

Hydrogen added

System returns to equilibrium New equilibrium

established

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Product

• Leads to Formation of more Reactants

• A nett back reaction occurs

Effect of Adding Ammonia

timeInitialequilibrium

[NH3]

[N2]

[H2]

conc

entr

atio

n

Effect of Adding Ammoniaco

ncen

trat

ion

timeInitialequilibrium

Ammonia added

[NH3]

[N2]

[H2]

Effect of Adding Ammoniaco

ncen

trat

ion

timeInitialequilibrium

Ammonia added

System returns to equilibrium

[NH3]

[N2]

[H2]

Effect of Adding Ammoniaco

ncen

trat

ion

timeInitialequilibrium

Ammonia added

System returns to equilibrium New equilibrium

established

[NH3]

[N2]

[H2]

Effect of Changing Reactant / Product

• Addition of Reactant leads to more Products being formed (Nett Forward Reaction)

• Addition of Product leads to more Reactants being formed (Nett Back Reaction)

• Removal of Reactant leads to less Products being formed (Nett Back Reaction)

• Removal of Product leads to less Reactants being formed (Nett Forward Reaction)

Changing Pressure

• Pressure can be changed by increasing or decreasing the volume of the container while keeping the temperature constant.

• Need to examine 2 examples

Changing Pressure

• 2SO2(g) + O2(g) 2SO3(g)

– 3 gas particles 2 gas particles

• A nett forward reaction – involves a reduction in the number of gas particles,

– so a reduction in pressure

• A nett back reaction– Involves an increase in the number of gas particles

– So an increase in pressure

Changing Pressure

• 2SO2(g) + O2(g) 2SO3(g)

– 3 gas particles 2 gas particles

• Using Le Chatelier’s Principle• An increase in pressure will lead to

– Be adjusted by a reduction in pressure– A nett forward reaction will occur increasing

the amount of sulphur trioxide present at equilibrium

Changing Pressure•2SO2(g) + O2(g) 2SO3(g)

SO2 5

O2 3

SO3 1

TOTAL 9

Changing Pressure•2SO2(g) + O2(g) 2SO3(g)

SO2 1

O2 1

SO3 5

TOTAL 7

Increased pressureNett forward

reaction

Changing Pressure

• N2O4(g) 2NO2(g)

– 1 gas particles 2 gas particles– Colourless Brown

• A nett forward reaction – involves an increase in the number of gas particles, – so an increase in pressure

• A nett back reaction– Involves a decrease in the number of gas particles– So a decrease in pressure

Changing Pressure

• N2O4(g) 2NO2(g)

• An equilibrium mixture of the gases was compressed

• Initially darkened - [NO2] increases

• Then colour of gas mixture fades– Nett backward reaction

Changing Pressure

• N2O4(g) 2NO2(g)

conc

entr

atio

n

Initial equilibrium

[N2O4]

[NO2]

time

Changing Pressure

• N2O4(g) 2NO2(g)

conc

entr

atio

n

Increase of pressure

Initial equilibrium

time

[N2O4]

[NO2]

Changing Pressure

• N2O4(g) 2NO2(g)

conc

entr

atio

n

Increase of pressure

Initial equilibrium

System returns to equilibrium

time

[N2O4]

[NO2]

Changing Pressure

• N2O4(g) 2NO2(g)

conc

entr

atio

n

timeIncrease of pressure

Initial equilibrium

System returns to equilibrium

New equilibrium established

[N2O4]

[NO2]

Adding an inert gas

• Total pressure of equilibrium system can be changed without changing the volume of the container by adding an inert gas

• There is no increase in concentrations of reactants or products

• No change in equilibrium

Dilution

• When dilution occurs, a net reaction results which produces the greater number of particles

• The effect of diluting the solution by adding water is– A net reaction in the direction that produces

more particles

Dilution

• Fe3+(aq) + SCN–

(aq) Fe(SCN)2+(aq)

– 2 particles in soln 1 particle in soln

• Dilution of this equilibrium will result in a nett back reaction

• Results in an increase of [Fe3+] and [SCN–]

Change in Temperature

• Using Le Chatelier’s Principle• Exothermic reaction can be written as

– Reactants Products + energy– Heating increases the energy of the substances– Principle says the reaction will oppose an

increase in energy by removing energy– A nett back reaction occurs– Less product and more reactants now present

Change in TemperatureExothermic

A + B C + D

conc

entr

atio

n

[A]

[B]

[C]

[D]

Initial equilibrium

time

Change in TemperatureExothermic

A + B C + D

conc

entr

atio

n

[A]

[B]

[C]

[D]

Initial equilibrium

Temperature increases

Temperature increases

System returns to equilibrium

time

Change in TemperatureExothermic

A + B C + D

time

conc

entr

atio

n

[A]

[B]

[C]

[D]

Initial equilibrium

Temperature increases

Temperature increases

System returns to equilibrium

New equilibrium established

Change in TemperatureEndothermic

A + B C + D

conc

entr

atio

n

[A]

[B]

[C]

[D]

Initial equilibrium

time

Change in TemperatureEndothermic

A + B C + D

conc

entr

atio

n

[A]

[B]

[C]

[D]

Initial equilibrium

Temperature increases

Temperature increases

System returns to equilibrium

time

Change in TemperatureEndothermic

A + B C + D

conc

entr

atio

n

[A]

[B]

[C]

[D]

Initial equilibrium

Temperature increases

Temperature increases

System returns to equilibrium

timeNew equilibrium established

Adding a Catalyst

• Catalysts increase the rate of the reaction

• They affect both forward and back reaction equally

• Do NOT change the position of the equilibrium

• Do NOT change the equilibrium constant