Download - Chemical Equilibrium
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.