Chemical Equilibrium

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Chemical
Equilibrium
Static equilibrium

Entire system not moving
Dynamic Equilibrium


The two opposing motions balance
each other out
Constantly moving in both
directions
Chemical Equilibrium




2Mg + O2 = 2MgO this reaction is
completely converted to products
Reactions that do not go to
completion
N2 +3H2 = 2NH3
This reaction is reversible so its
indicated using the two way arrow
sign
Chemical Equilibrium


Is a state of dynamic balance
where the rate of the forward
reaction equals the rate of the
reverse reaction
Diagram 17.2 pg 233
EQUILIBRIUM REACTIONS
Initially, there is no backward reaction but, as products form, it speeds up and
provided the temperature remains constant there will come a time when the backward
and forward reactions are equal and opposite; the reaction has reached equilibrium.
FASTEST AT THE START
NO BACKWARD REACTION
FORWARD REACTION SLOWS DOWN
AS REACTANTS ARE USED UP
BACKWARD REACTION
STARTS TO INCREASE
In an equilibrium reaction, not all the
reactants end up as products; there is not a
100% conversion.
BUT IT DOESN’T MEAN THE REACTION
IS STUCK IN THE MIDDLE
AT EQUILIBRIUM THE BACKWARD
AND FORWARD REACTIONS ARE
EQUAL AND OPPOSITE
Le Chateliers Principle

If a stress is applied to a system at
equilibrium, the system readjusts
to relieve the stress applied

A stress is
Change in concentration of
reactants or products
Change in pressure
Change inTemperature



Le Chateliers Principle
Predicts

In a gaseous reaction a increase in
pressure will favour the reaction
which takes place with reduction in
volume ie towards the side with
smaller number of molecules
Le Chateliers Principle
If a stress is applied to a system at equilibrium,
the system adjusts to relieve the stress
CoCl42- +
6H2O
Co(H2O)62+
+
4Cl-
1.
Add the cobalt chloride crystals to water, state which side the equilibrium
favours and state how you came to this conclusion?
2.
Add HCL, State and Explain what you observed
3.
Add Water, State and explain what you observed
4.
Heat in Beaker of hot water, State and explain what you observed
Le Chatelier’s Principle
If a stress is applied to a system at equilibrium, the
system adjusts to relieve the stress
Cr2O72- +
orange
H 2O
2CrO42-
+
2H+
yellow
1. Add sodium dichromate to deionised water, note colour observed
and explain
2. Add NaOH and note observation and explain
3. Add HCL, Note observation and explain
Le Chateliers Principle
If a stress is applied to a system at equilibrium, the
system adjusts to relieve the stress
FeCl3
+
CNS-
Fe(CNS)2-
+
3Cl-
1. Add iron (III) chloride and potassium thiocyanate, note colour
and explain why
2. Add HCL, note colour change and explain why
3. Add Iron (III) chloride, note colour change and explain why
The Haber Process
Is one of the most important industrial process
there is.
It is the process by which ammonia [ NH3 ] is
made.
Ammonia is used to make nitric acid [ HNO3 ] and
ammonium nitrate fertiliser [ NH4NO3 ] by
neutralizing ammonia with the nitric acid
NPK
Haber process
Manufacture of ammonia NH3 for the fertilizer industry
N2
+
2 NH3
3 H2
2
NH3
Kc =
N2
x
H2
3
H= - 92.4
Haber process
Manufacture of ammonia NH3 for the fertilizer industry
N2
+
3 H2
2 NH3
H= - 92.4
Le Chatelier’s principle predicts the yield of NH3 is maximised by
1. Low temperature
2. High pressure
Actual temp used is 500oC (as lower temp reduces the rate )
Pressure used is 200 ATM, Catalyst iron
HABER PROCESS
N2(g) + 3H2(g)
Conditions
Pressure
Temperature
Catalyst
2NH3(g)
: DH = - 92 kJ mol-1
20000 kPa (200 atm)
380-500°C
iron
Equilibrium theory favours
low temperature
exothermic reaction - higher yield at lower temperature
high pressure
decrease in number of gaseous molecules
Kinetic theory favours
high temperature
greater average energy + more frequent collisions
high pressure
more frequent collisions for gaseous molecules
catalyst
lower activation energy
Compromise conditions
Which is better?
A low yield in a shorter time
or
a high yield over a longer period.
The conditions used are a compromise with the catalyst
enabling the rate to be kept up, even at a lower temperature.
Pressure
affects
reactions with
gases
Catalyst
No effect on equilibrium
• It just reduces the time it takes to reach the
its chemical equilibrium
• reaches equilibrium faster but yield
unchanged
Contact Process
• Sulfuric acid is used to manufacture a
variety of substances (paints, detergents,
fertilisers, plastics, fibres, car batteries)
• The name Contact Process comes from the
fact that one of the stages in the
manufacture involves passing sulfur dioxide
gas (obtained by burning sulfur in air) and
oxygen gas over a catalyst.
• Need very close contact between the two
gases
Vanadium
Pentoxide
Sulfur Dioxide+ oxygen
Sulfur trioxide
V2O5
2SO2 + O2
2SO3
H = -196 Kj/mol
Sulfer trioxide is then reacted with water to form sulfuric
acid.
Platinum can be used as catalyst but it is easily
poisened by impurities
What conditions do you think are ideal to improve
percentage yield?
Actual conditions
• Pressure just above Atm pressure
• Low temperatures slow reaction rate so a
compromise of 450 deg Celsius
The Equilibrium Constant
• Mathematical equation to find equilibium
• Law of Chemical Equilibrium
• Consider the reaction in which a moles of of
Reactant A react with b moles of reatant B
to form c mole of product C and d moles of
reactant D at Equilibrium
• aA + bB
cC + dD
Equilibrium Constant
•aA + bB
c
d
•Kc = [C] [D]
a
b
•
[A] [B]
cC + dD
Products
Reactants
•Value of K depends on the concentrations of reactants and
products
•It is only affect by temperature ( not affected by concentration and
pressure)
• N2 (g) + 3H2 (g)
• Kc = [NH3]2
•
[N2][H2]3
2NH3 (g)
Kc constant for this reaction is 46. This tells us that the
Numerator(products) is larger than the Denominator
( reactants) therefore the reaction favours the products.
The greater the Kc value the more the reaction favours the
Products.
Equilibrium constant
• The value of Kc is temperature
dependent
• If there is the same number of molecules in
the forward and reverse reactions, then the
volume of the container may be ignored,
hence concentration calculated as moles per
litre
• It is not changed by pressure or
concentration because it will shift to relieve
the stress
No need to know the volume when the number of
molecules is equal on both sides!!
Some cases
X= -b +/- √b2-4ac
2a
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