Chemical Equilibrium

Chemical EquilibriumStatic Equilibrum The entire system

is not moving Eg: A meter stick

that is suspended at its centre pf gravity.

Dynamic Equilibrum

The system is in motion.

Two opposing motions balance each other out.

Eg: a man on a threadmill.

Chemical Equilibrium Some reactions

go to completion. Eg: Magnesium

burning in air. 2Mg + O2 → 2MgO

Many chemical reactions don’t go to completion (equilibrum reactions).

Take place in both directions (reversable)

Eg: formation of ammonia from H2 + N2

3H2 + N2 2NH3

Dynamic equilibrium

Chemical Equilibrium At equilibrum the rate of the

forward reaction equals the rate of the reverse reaction.

The conc at equilibrum remains constant

Chemical Equilibrium Definition:

Chemical Equilibrum is a state of dynamic balance where the rate of the forward reaction equals the rate of the reverse reaction.

Le Chatelier’s Principle 1888 Henri Le Chatelier put forward a rule

which allowed chemists to predict the direction taken by an equilibrum rxn when the conditions of the rxn were changed.

RULE: If a stress is applied to a system at equilibrum, the system readjusts to relieve the stress applied.

Stress = change in condition of the reaction eg temp/conc/pressure

Catalyst=reaches equili quicker

Le Chatelier’s Principle When a stress is applied the

system will react to minimise the stress

Ie: return to its original equilibrum

Le Chatelier’s Principle Consider:

Each test tube is at equilibrum

Each test tube is at equilibrum

What happens when palced in ice?

Light Yellow (more dinitrogen tetraoxide made)

What happens when placed in hot water?

Darker Brown (more nitrogen dioxide made)

What were the stresses? The change in temperature hot and

cold. When placed in hot water it carried out

the reaction which would absorb heat When placed in cold water it carried out

the reaction which would create heat This is to minimise the effect of the

stress

Effects of Changing Pressure of Gases in rxns If pressure is increases the

equilibrium is shifter the side of the equation which has the smaller number of molecules.

Why? The smaller number of molecules will

occupy less volume thus reduce the pressure

Le Chatelier’s Principle

State Le Chatlier’s Principle: Le Chatlier’s Princilpe

states that if a stress is applied to a system at equilibrum the system will readjust to relieve/minimise the applied stress

Le Chatelier’s Principle Predicts: That in an all gaseous rxn an

increase in pressure will favour the rxn which takes place with reduction in volume ie, towards the side with the smaller number of molecules

Only affects equili rxns with unequal no’s of gaseous reactants and products

Equilibrium and Catalysts A catalyst speeds up the rate at

which equilibrium is reached but does not change the position of equilibrium. Increases both the rate of the forward

and reverse reactions

Mandatory Experiment To demonstrate Le Chatelier’s

Principle: A) Effect of

Temp Conc changes

What happens when Add HCl

Turns blue Add water

Turns red Place in hot water

Turns blue

To demonstrate Le Chatelier’s Principle:

B) Effect of Concentration

Originally the soln is orange. Where does the equilibrium lie?

What happens when

Add dilute NaOH Turns yellow

Add dilute HCl Turns orange

To demonstrate Le Chatelier’s Principle:

C) Effect of Concentration

Originally the soln is red. Where does the equilibrium lie?

What happens when

Add dilute HCl Turns blue

Add iron(III) chloride Turns red

Industrial Applications of Le Chatelier’s Principle Manufacture of ammonia by the

Haber Process Manufacture of sulfuric acid by the

contact process

The Haber Process Use: Fertilisers (80%), explosives

Cleaning agents Objective: Produce as much max

amount of NH3 as cheaply as possible.

What used: H2 from Natural Gas N2 from Air

The Haber Process

The amount of NH3 produced depends on: Temperature Pressure

Catalyst (Iron)

Achieving max yield

Pressure: Increased pressure at equilibrium

favours the production of NH3

Carried out at high pressure (200 atmospheres)

Pressure cant be too high: Expensive to build high pressure plants Problems like safety

Temperature: A lower temp will cause more

ammonia to be produced Temp cant be too low:

Rate of reaction will be to slow Not enough collisions having energy

= activation energy Temp used = 500°C

Catalyst: In absence of catalyst the rxn has

a high activation energy. Catalyst used because:

Brings system to equilibrium faster by lowering Eact

Rxn can now proceed at a lower temp reducing fuel costs

Thus: Le Chatelier’s Principle tells us that

the best conditions to produce NH3 in the Haber process is under conditions of: High pressure Low temperature

Manufacture of sulfuric acid by the contact process Manufactured by the Contact

Process H2SO4 used in the manufacture of

paints,detergents, fertilisers, plastics, fibres,car batteries.

Sulfur trioxide is the desired product as it dissolves in water to give H2SO4.

The Contact Process

Called the contact process as very close contact must be made between the two reactant gases and the catalyst.

The amount of SO3 produced depends on: Temperature Pressure

Catalyst Concentration

Temperature: A lower temp will cause more SO3 to

be produced Temp cant be too low:

Rate of reaction will be to slow Not enough collisions having energy =

activation energy Temp used = 450°C used as found the

catalyst works best at this temp.

Pressure: Increased pressure at equilibrium

favours the production of NH3

Carried out at a pressure just higher than atmospheric pressure.

Pressure cant be too high: Expensive to build high pressure plants Problems like safety

Concentration By removing the SO3 as it is

formed reduces its conc thus shifting the equilibrium to the right

Catalyst Brings the reaction to equilibrium

faster by reducing the activation energy.

The Equilibrium constant Calculations to find the conc of

products & reactants at equilibrm

System must be at equilibrium Temperature must stay constant

The Equilibrium Law

The Equilibrium constant Every equilibrum rxn has its own value

for Kc at a particular temperature The larger the value for Kc the futher the

equili is pushed towards the products. The smaller the value for Kc the futher

the equili is pushed towards the reactants.

We do not need to know the unit of Kc.