1. Unit 2.7 - Chemical equilibria
Reversible reactions
• Some reactions are irreversible like… 2H2(g) + O2(g) → 2H2O(l)
• There are many more, which are reversible , in other words they can easily go backwards or
forwards, like… H2(g) + I2(g) ⇌ 2HI(g)
• The ⇌ symbol indicates that the reaction is reversible
Dynamic equilibrium
•
If
a reaction is reversible then it does not go to completion
• Both the forward reaction (reactants → products) and the backward reaction (______________
→ _____________) can and do happen
• If the forward and backward reactions are happening at the same rate then the concentrations of
the reactants and products will stay the _____________
Dynamic equilibrium
The rates of the forward and reverse reactions are equal. Therefore, there is no further change in the
concentrations of the reactants and products.
Graph of concentration vs. time Graph of rate vs. time
Example - Formation of HI

2. For the reaction… H2(g) + I2(g) ⇌ 2HI(g)
To be at equilibrium, the rate of… H2(g) + I2(g) → 2HI(g) [forward]
Must be equal to the rate of… 2HI(g) → H2(g) + I2(g) [backward]
Case study - The Haber- Bosch process
• Perhaps the best-known reversible reaction
• Fixation of atmospheric nitrogen
• The product, ammonia (NH3), is used in fertilizer
and explosive manufacture
• Traditionally uses an iron catalyst
• Temperature 350- 500 °C
• High pressure 15- 25 MPa
Fritz Haber and Carl Bosch
reactants products
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
∆H = -92.4 kJ/mol
• High pressure is used to increase the ___________ of ammonia
• High temperature is used to increase the __________ of reaction
Le Chetaliers’s principle
“ If a chemical system at equilibrium experiences
a change in concentration, temperature,
volume, or partial pressure, then the equilibrium
shifts to counteract the imposed change and a
new equilibrium is established.”
In other words…
1. Effect of temperature
Remember… Henry Le Chetalier - and you
o Exothermic (-ve ∆H) reactions ___________ energy to the surroundings making them
thought Fritz Haber looked scary!
_________

3. o Endothermic (+ve ∆H) reactions ___________ energy from the surroundings making them
___________
Exothermic reactions
Increase temperature Decrease temperature
Favour the backward reaction, which is Favour the ______________ reaction, which is
endothermic, and move the position of _____thermic and move the position of
equilibrium towards the reactants equilibrium towards the ______________
Endothermic reactions
Increase temperature Decrease temperature
Favour the ____________ reaction, which is Favour the ______________ reaction, which is
_____ thermic, and move the position of _____thermic, and move the position of
equilibrium towards the ______________ equilibrium towards the ______________
Case study - The Haber- Bosch process
N2(g) + 3 H2(g) ⇌ 2 NH3(g) Forward reaction:
∆H = -92.4 kJ/mol (exothermic) Backward reaction:
• The effect of increasing the temperature would be…
• The effect of decreasing the temperature would be…
NB The reaction is done at a compromise temperature (350-500 °C) and uses a catalyst to
achieve a commercially viable rate of reaction. Lower temperatures would give a higher yield of
ammonia but the reaction would take much longer.
2. Effect of pressure
Again using the Haber-Bosch process as an example:
reactants products
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
moles of gas moles of gas
• As you can see there are __________ moles of gas on the left hand side of this equation
• Therefore, the forward reaction has the effect of _____________ the pressure (producing
more gas would increase the pressure)

4. • For the Haber-Bosch process high pressure favours the production of the _____________
ammonia as the equilibrium shifts in order to counteract the _____________ in pressure
• Lowering the pressure would favour the ________________ reaction
More moles of gaseous reactants
Increase pressure Decrease pressure
Favour the forward reaction and move the Favour the ______________ reaction, which is
position of equilibrium towards the reactants and move the position of equilibrium towards the
thus counteracting the initial increase ______________
More moles of gaseous products
Increase pressure Decrease pressure
NB This only applies to equilibrium
reactions involving gases.
Reducing the volume or adding more
gas to the same volume can increase
pressure.
3. Effect of concentration
The effect of altering the concentration of one or both of the
reactants applies to reactions that are carried out in solution. As the Haber-Bosch process involves
a gas-phase reaction we will have to find another example:
2CrO42-(aq) + 2H+(aq) ⇌ Cr2O72-(aq) + H2O(l)
Chromate(VI) Dichromate(VI)
Yellow Orange
Addition of excess acid
Adding excess acid causes the equilibrium to shift to the __________ to oppose the change
(increase in reactant ________________) and as more dichromate(VI) ions are produced, the
solution turns ________________
Addition of excess alkali

5. • Increasing the concentration of reactants favours the _________________ reaction
• Reducing the concentration of reactants favours the _________________ reaction
Effect of catalyst
• Catalysts are substances that
_______________ the rate of a
chemical reaction but remain
unchanged themselves
• They do this by providing an
alternative route with a lower
activation energy
• They have no effect on the position
of equilibrium
• They speed up both the forward and
backward reactions so equilibrium is reached faster
Nitrogen dioxide and dinitrogen tetroxide
2NO2(g) ⇌ N2O4(g) ∆H = -57 kJ/mol
brown colourless
Forward reaction:
Backward reaction:
• The forward reaction is _______thermic
• Increasing the temperature favours the _____________ reaction
• Decreasing the temperature favours the _____________ reaction
If we reduced the temperature we will observe…
If we increase the temperature we will observe…
Methane hydrate
Methane hydrate (also known as
methane clathrate and ‘fire ice’) is a solid
substance containing methane molecules
surrounded by cage-like structures of
water molecules. Huge deposits of
methane hydrate exist on deep-sea beds
around the world. Up to ten times the
current natural gas reserves may be
trapped in hydrates. Methane is also a
powerful greenhouse gas; any sudden

6. release could be catastrophic.
The methane in hydrates is in equilibrium with gaseous methane:
methane hydrate(s) ⇌ methane(g) + water(l) ∆H = +ve
Effect of increasing pressure on this equilibrium…
Effect of increasing temperature on this equilibrium…