Chemical Equilibrium. In principle, every chemical reaction is reversible: Capable of moving in...

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Chemical Equilibrium

THE CONCEPT OF EQUILIBRIUM In principle, every chemical reaction is

reversible: Capable of moving in the forward or

backward direction. Some reactions are easily reversible…Some are not!

Ie) Making water: 2H2 + O2 2H2O Breaking it up: 2H2O 2H2 + O2

EQUILIBRIUM SYSTEMSWhat is Chemical Equilibrium? When the concentrations of all

reactants and products of a chemical reaction remain constant over time

Any chemical in a closed system will eventually reach chemical equilibrium

WHAT IS EQUILIBRIUM IN TERMS OF CHEMICAL KINETICS??

Remember Kinetics?

Equilibrium occurs when the RATE of the forward reaction is exactly equal to the RATE of the reverse reaction.

DYNAMIC EQUILIBRIUM

LET’S TAKE A CLOSER LOOK AT THE DECOMPOSITION OF DINITROGEN TETROXIDE

Let’s evaluate the reversible reaction:

•At room temperature, colourless N2O4 decomposes to brown NO2

-.

•At some time, the colour stops changing and we have a mixture of the two.

THE EQUILIBRIUM CONDITION The point where the colour stops changing is

equilibrium. At equilibrium, as much N2O4 reacts to

form NO2- as NO2

- reacts to re-form N2O4 The double arrow implies the process is

dynamic.

Dynamic equilibrium occurs when the rate of formation of products is the same as the rate of re-formation of reactions.

DYNAMIC EQUILIBRIUM has reactants and products that are in

constant motion. (i.e. – DYNAMIC) is reversible. can be reached from either direction.

forward or reverse

THERE ARE THREE TYPES OF EQUILIBRIA; Phase equilibria

A dynamic equilibrium between different physical states of a pure substance in a closed system.

Such as when the rate of evaporation equals the rate of condensation.

H2O(l) D H2O(g)

THERE ARE THREE TYPES OF EQUILIBRIA; Solubility equilibria

A dynamic equilibrium between a solute and a solvent in a saturated solution in a closed system.

The rate of dissolving equals the rate of crystallization.

Both dissolved and undissolved solute must be present.

NaCl(s) D Na+(aq)

+ Cl - (aq)

THERE ARE THREE TYPES OF EQUILIBRIA; Chemical equilibria

A dynamic equilibrium between reactants and products in a closed system.

The reaction must be reversible. N2O4(g) + 58 kJ/mol D 2NO2 (g)

(Colourless) (Brown)

THE EQUILIBRIUM PROCESS

In a quantitative reaction the reaction consumes all of the reactants and goes to completion.

In open systems the products are removed or escape and an equilibrium does not occur. A steady state can be

created if the supply of reactants is sufficient or replenished.

Reactants

Steady state

THE EQUILIBRIUM PROCESS

NH4Cl (s)

NH3 (g) HCl (g)NH3 (g) HCl (g)NH3 (g)

HCl (g)

A quantitative reaction in an open system results in the reactants being consumed and no equilibrium state is established.

NH4Cl (s) NH3 (g)+ HCl (g)

NH 4Cl

NH 3 +

HCl

NH4Cl(s) g NH3(g) + HCl(g) The ammonium chloride completely reacts

as the gas products escape into the atmosphere in an open system.

THE EQUILIBRIUM PROCESS

When a quantitative reaction is enclosed in a closed system the products build up and may begin to react in the reverse direction.

NH4Cl(s) f NH3(g) + HCl(g) The increased quantity of ammonia

and hydrogen chloride combine to produce ammonium chloride.

THE EQUILIBRIUM PROCESS In the closed system, a quantitative

reaction may produce a competition between the forward and reverse reaction.

This competition results in a chemical equilibrium.

NH4Cl(s) D NH3(g) + HCl(g)

THE EQUILIBRIUM PROCESS

NH3 (g) HCl (g)

Some quantitative reactions in a closed system result in the products becoming involved in competing reverse reaction and an equilibrium state being established.

NH4Cl (s)⇄ NH3 (g)+ HCl (g)NH4Cl (s)

NH4Cl (s)+

NH4Cl

NH

3 +

HCl

NH

3 + HC

lNH

4 Cl

THE EQUILIBRIUM PROCESS

As one observes the concentrations of the reactants and products in the closed system, one finds at equilibrium, the concentrations become stable.

This stability in concentrations defines the equilibrium state.

THE EQUILIBRIUM PROCESS

Time (s)

Con

cent

ratio

n (m

ol/L

)

NH4Cl(s) g NH3(g) + HCl(g)

NH4Cl(s)

NH3(g)HCl(g)

No equilibrium is established in an open system.

A quantitative reaction.

THE EQUILIBRIUM PROCESS

Time (s)

Con

cent

ratio

n (m

ol/L

)

NH4Cl(s) D NH3(g) + HCl(g)

NH4Cl(s)

NH3(g)

HCl(g)

NH4Cl(s)

NH3(g)

HCl(g)

Equilibrium established in a closed system

Time (s)

Con

cent

ratio

n (m

ol/L

)

THE EQUILIBRIUM PROCESS

NH4Cl(s) D NH3(g) + HCl(g)

NH4Cl(s) The reduction in NH4Cl is proportional to

NH3(g)

HCl(g)

the rise in product as predicted by the balanced chemicalequation

-X

+X