4. Chemical Equilibrium
A reversible reaction can go in either the forward or reverse directions A + B C D +
Equilibrium is dynamic as both the forward and reverse reactions continuously occur
At Chemical equilibrium the rate of forward reaction and rates of reverse reaction are equal

6. CONCENTRATION CHANGE IN A REACTION
As the rate of reaction is dependant on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated
FASTEST AT
THE START
THE STEEPER THE GRADIENT, THE FASTER THE REACTION
SLOWS DOWN AS REACTANTS ARE USED UP
TOTAL CONVERSION TO PRODUCTS
In an ordinary reaction; all reactants end up as products; there is 100% conversion

7. 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

9. The conditions and properties of a system at equilibrium are summarized below:
1. The system must be closed, meaning no substances can enter or leave the system.
2. Equilibrium is a dynamic process. Even though we don’t observe any changes, both the forward and reverse reactions are still taking place.
3. The rates of the forward and reverse reactions must be equal.
4. The amounts of reactants and products do not have to be equal. However, after equilibrium is attained, the amounts of reactants and products (their concentration) will remain constant.

10. DYNAMIC EQUILIBRIUM IMPORTANT REMINDERS
• a reversible chemical reaction is a dynamic process
• everything may appear stationary but the reactions are moving both ways
• the position of equilibrium can be varied by changing certain conditions
Trying to get up a “down” escalator gives an excellent idea of a non-chemical situation involving dynamic equilibrium.
Summary When a chemical equilibrium is established ...
• both the reactants and the products are present at all times
• the equilibrium can be approached from either side
• the reaction is dynamic - it is moving forwards and backwards
• the concentrations of reactants and products remain constant

11. THE EQUILIBRIUM LAW VALUE OF Kc
“If the concentrations of all the substances present at equilibrium are raised to the power of the number of moles they appear in the equation, the product of the concentrations of the products divided by the product of the concentrations of the reactants is a constant, provided the temperature remains constant”
for an equilibrium reaction of the form...
aA bB cC dD
then (at constant temperature) [C]c . [D]d = a constant, (Kc) [A]a . [B]b
where [ ] denotes the equilibrium concentration in mol/L
Kc is known as the Equilibrium Constant
VALUE OF Kc
AFFECTED by a change of temperature
NOT AFFECTED by a change in concentration of reactants or products
a change of pressure
adding a catalyst

12. The value of the equilibrium constant for any reaction does not depend on the starting concentrations, so the equilibrium constant has the same value regardless of the initial amounts of each reaction component.
It does, however, depend on the temperature of the reaction.

13. Kc = 1 neither reactants nor products are favoured
Large Kc > 1 products are "favoured" they dominate the mixture
Kc = 1 neither reactants nor products are favoured
Small Kc < 1 reactants are "favoured" they dominate the mixture
Kc = [Products]
[Reactants]
Kc give a ratio of the product to reactants
Kc at a particular temperature is constant

15. Equilibrium Constant The value of Kc is only applies at equilibrium
The value of Kc is temperature dependent i.e. constant at a particular temperature
It is not changed by pressure or concentration of reactant or products because it will shift to relieve the stress
Units of Kc depends on relative no. of moles. If no. of moles equal on both sides then Kc has no units.

16. Some cases X= -b +/- √b2-4ac 2a
If there is the same number of molecules in the forward and reverse reactions, it is possible to calculate the equilibrium constant even if the volume of the equilibrium mixture is unknown
No need to know the volume when the number of molecules is equal on both sides!!
Some cases
X= -b +/- √b2-4ac 2a

17. The position (i. e. rate of forward vs
The position (i.e. rate of forward vs. reverse) of an equilibrium is temporary affected by many things. This means that the relative concentrations of reactant and products can differ.
The equilibrium position will shift to try and make things equal again. This means the reverse reaction will speed up a bit, until the concentrations are where they want to be, and then it will return to a constant dynamic equilibrium - forward and reverse reactions being the same. 

19. LE CHATELIER’S PRINCIPLE
”When a change is applied to a system in dynamic equilibrium, the
system reacts in such a way as to oppose the effect of the change.”
If you do something to a reaction that is in a state of equilibrium, the equilibrium position (not Kc) will change to oppose what you have just done (in order to maintain same Kc value)

21. FACTORS AFFECTING THE POSITION (not value) OF EQUILIBRIUM
CONCENTRATION
The equilibrium constant is not affected by a change in concentration at constant temperature. To maintain the constant, the composition of the equilibrium mixture changes.
If you increase the concentration of a substance, the value of Kc will theoretically be affected. As it must remain constant at a particular temperature, the concentrations of the other species change to keep the constant the same.

24. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM
CONCENTRATION
example CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) H2O(l)
the equilibrium constant Kc = [CH3COOC2H5] [H2O] = (at 298K)
[CH3CH2OH] [CH3COOH]
increasing
[CH3CH2OH] - will make the bottom line larger so Kc will be smaller
- to keep it constant, some CH3CH2OH reacts with CH3COOH
- this reduces the value of the bottom line and increases the top
- eventually the value of the constant will be restored
decreasing
[H2O] - will make the top line smaller
- some CH3CH2OH reacts with CH3COOH to replace the H2O
- more CH3COOC2H5 is also produced

25. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM
SUMMARY
REACTANTS PRODUCTS
INCREASE CONCENTRATION OF A REACTANT
EQUILIBRIUM MOVES TO THE RIGHT
THE EFFECT OF CHANGING THE CONCENTRATION ON THE POSITION OF EQUILIBRIUM
DECREASE CONCENTRATION OF A REACTANT
EQUILIBRIUM MOVES TO THE LEFT
INCREASE CONCENTRATION OF A PRODUCT
DECREASE CONCENTRATION OF A PRODUCT
Predict the effect of increasing the concentration of O2 on the equilibrium position
2SO2(g) + O2(g) SO3(g) EQUILIBRIUM MOVES TO RHS
Predict the effect of decreasing the
concentration of SO3 on the equilibrium position EQUILIBRIUM MOVES TO RHS

26. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM
PRESSURE
When studying the effect of a change in pressure, we consider the number of gaseous molecules only.
The more particles you have in a given volume, the greater the pressure they exert.
If you apply a greater pressure they will become more crowded (i.e. they are under a greater stress). However, if the system can change it will move to the side with fewer gaseous molecules - it is less crowded.
No change occurs when equal numbers of gaseous molecules appear on both sides.
INCREASE PRESSURE
MOVES TO THE SIDE WITH FEWER GASEOUS MOLECULES
DECREASE PRESSURE
MOVES TO THE SIDE WITH MORE GASEOUS MOLECULES
THE EFFECT OF PRESSURE ON THE POSITION OF EQUILIBRIUM
Predict the effect of an increase of pressure on the equilibrium position of..
2SO2(g) + O2(g) SO3(g) MOVES TO RHS :- fewer gaseous molecules
H2(g) + CO2(g) CO(g) + H2O(g) NO CHANGE:- equal numbers on both sides

28. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM
TEMPERATURE
• temperature is the only thing that can change the value of the equilibrium constant.
• altering the temperature affects the rate of both backward and forward reactions
• it alters the rates to different extents
• the equilibrium thus moves producing a new equilibrium constant.
• the direction of movement depends on the sign of the enthalpy change.
REACTION TYPE DH
INCREASE TEMP
DECREASE TEMP
EXOTHERMIC -
TO THE LEFT
TO THE RIGHT
ENDOTHERMIC +
Predict the effect of a temperature increase on the equilibrium position of...
H2(g) + CO2(g) CO(g) + H2O(g) DH = kJ mol-1 moves to the RHS
2SO2(g) + O2(g) SO3(g) DH = - ive moves to the LHS

30. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM
CATALYSTS
Catalysts work by providing an alternative reaction pathway involving a lower activation energy.

32. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM
CATALYSTS
An increase in temperature is used to speed up chemical reactions but it can have an undesired effect when the reaction is reversible and exothermic.
In this case you get to the equilibrium position quicker but with a reduced yield because the increased temperature moves the equilibrium to the left.
In many industrial processes a compromise temperature is used (see Haber and Contact Processes). To reduce the problem one must look for a way of increasing the rate of a reaction without decreasing the yield i.e. with a catalyst.
Adding a catalyst DOES NOT AFFECT THE POSITION OF EQUILIBRIUM. However, it does increase the rate of attainment of equilibrium. This is especially important in reversible, exothermic industrial reactions such as the Haber or Contact Processes where economic factors are paramount.

33. In Summary Stress Change Increase in concentration of X
Reaction that removes X is favoured
Decrease in concentration of X
Reaction that forms X is favoured
Increase in temperature
Endothermic reaction is favoured
Decrease in temperature
Exothermic reaction is favoured
Increase in Pressure
Reaction that produces fewer molecules is favoured
Decrease in Pressure
Reaction that produces more molecules is favoured
Adding a Catalyst
No Change

34. Le Chateliers Principle - Experiments
If a stress is applied to a system at equilibrium, the system adjusts to relieve the stress
CoCl42-
Blue
H2O
Co(H2O)62+
Pink +
4Cl- +
Add the cobalt chloride crystals to water, state which side the equilibrium favours and state how you came to this conclusion?
Add HCL, State and Explain what you observed
Add Water, State and explain what you observed
Heat in Beaker of hot water, State and explain what you observed = blue

35. CrO42- yellow Cr2O72- Le Chatelier’s Principle - Experiments
If a stress is applied to a system at equilibrium, the system adjusts to relieve the stress
CrO42-
yellow
Cr2O72-
orange +
H2O
2H+ +
Add sodium dichromate to deionised water, note colour observed and explain
Add NaOH and note observation and explain
Add HCL, Note observation and explain

36. Fe(CNS)2- FeCl3 red yellow Le Chateliers Principle -Experiments
If a stress is applied to a system at equilibrium, the system adjusts to relieve the stress
FeCl3
yellow
Fe(CNS)2-
red +
CNS- +
3Cl-
Add iron (III) chloride and potassium thiocyanate, note colour and explain why
Add HCL, note colour change and explain why
Add Iron (III) chloride, note colour change and explain why

37. Haber process 2 NH3 N2 3 H2 NH3 N2 H2
Manufacture of ammonia NH3 for the fertilizer industry
2 NH3 N2
3 H2 H= + 2
NH3
Kc = 3 N2 H2 x

38. HABER PROCESS N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1
Conditions Pressure kPa (200 atmospheres)
Temperature °C
Catalyst 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.

39. IMPORTANT USES OF AMMONIA AND ITS COMPOUNDS
HABER PROCESS
IMPORTANT USES OF AMMONIA AND ITS COMPOUNDS
MAKING
FERTILISERS 80% of the ammonia produced goes to make fertilisers such as
ammonium nitrate (NITRAM) and ammonium sulphate
NH HNO3 ——> NH4NO3
2NH H2SO4 ——> (NH4)2SO4
NITRIC ACID ammonia can be oxidised to nitric acid
nitric acid is used to manufacture... fertilisers (ammonium nitrate)
explosives (TNT)
polyamide polymers (NYLON)

40. Haber process
Manufacture of ammonia NH3 for the fertilizer industry
2 NH3 N2
3 H2 H= +
Le Chatelier’s principle predicts the yield of NH3 is maximised by
Low temperature
High pressure
Actual temp used is 500oC (as lower temp reduces the rate )
Pressure used is 200 ATM

41. 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

42. Sulfur Dioxide+ oxygen Sulfur trioxide
Vanadium
Pentoxide
Sulfur Dioxide+ oxygen Sulfur trioxide
V2O5
2SO2 + O SO 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?

43. Contact process Manufacture of Sulfuric acid H2SO4 2 SO3 2SO2 O2+
Le Chatelier’s principle predicts the yield of SO3 is maximised by
Low temperature
High pressure
Actual temp used is 450 oC (as low temp reduces the rate )
Pressure used is 1 ATM

44. explain what is meant by a reversible reaction
8 Chemical Equilibrium
explain what is meant by a reversible reaction
explain what is meant by dynamic equilibrium
explain what is meant by chemical equilibrium
state the equilibrium law (Kconly)
write expressions for Kc
perform calculations involving equilibrium constants (Kc)
state Le Chatelier’s principle
use Le Chatelier’s principle to predict the effect (if any ) on equilibrium position of concentration, pressure, temperature and catalyst
perform simple experiments to demonstrate Le Chatelier’s principle using the following equilibrium mixtures
CoCl42– + 6H20 Co(H20) Cl–
(to demonstrate the effects of both temperature changes and concentration changes on an equilibrium mixture)
Cr2O72– + H2O 2CrO42– + 2H+
Fe3+ + CNS– Fe(CNS)2+
(to demonstrate the effects of concentration changes on an equilibrium mixture)
discuss the Industrial application of Le Chatelier’s principle in the catalytic oxidation of sulfur dioxide to sulfur trioxide and in the Haber process