Download 2.4 Chemical equilibria

2.9
Chemical equilibria
a. demonstrate an understanding that chemical equilibria are dynamic
b. deduce the qualitative effects of changes of temperature, pressure and
concentration on the position of equilibrium, eg extraction of methane from
methane hydrate
c. interpret the results of simple experiments to demonstrate the effect of a
change of temperature, pressure and concentration on a system at
equilibrium, eg
i. iodine(I) chloride reacting with chlorine to form iodine(III) chloride, or
ii. N2O4 ⇌2NO2.
Connector - Explain the terms:
- Reversible reaction
- Dynamic equilibrium
Crowe2009
Dynamic equilibrium
A reversible reaction is where products can, under
appropriate conditions, turn back into reactants.
• There will be a range of conditions over which both
the forward and backward reaction will take place
and this can lead to a state of balance with both
reactants and products present in unchanging
amounts.
• This is called a dynamic equilibrium.
A
B
these combine
A B
these decompose
Dynamic equilibrium
Equilibrium – because of the unchanging amounts
Dynamic – because reaction is still occurring
It is rather like the situation where a man is walking the
wrong way along a moving pavement or escalator.
Neither have stopped but the man could remain in the same
place for ever!
The symbol  is used to mean dynamic equilibrium.
The man stays in the same place!
These reactions are reversible, but under the conditions
normally used, they become one-way reactions. The
products aren't left in contact with each other, so the reverse
reaction can't happen.
Demo –
chromate(VI)/dichromate(VII) reversible reaction
2CrO42- (aq) + 2H+ (aq)
yellow
Cr2O72- (aq) + H2O (l)
orange
Conc. soln. of Potassium dichromate
2M HCl
2M NaOH
250ml conical flask
Reversible reactions happening in a closed system
A closed system is one in which no substances are either added to
the system or lost from it. Energy can, however, be transferred in or
out at will.
Initially the iron and steam will react:
Once iron(III) oxide and hydrogen have begun to form, they too will react:
Since nothing can escape, eventually the rate of the first reaction will be
the same as that of the second and a DYNAMIC EQUILIBRIUM is set up.
Dynamic Equilibria
There are two reactions here, the forward reaction
(left to right), and the reverse reaction (right to left).
What are the reactants of the forward reaction?
What are the products of the forward reaction?
What are the reactants of the reverse reaction?
What are the products of the reverse reaction?
Dynamic Equilibria
There are two reactions here, the forward reaction (left to
right), and the reverse reaction (right to left).
At equilibrium, the rate of each reaction will be the same.
What effect will this have on the amounts of A, B, C and D?
Remember both reactions are still happening, but because
they are doing so at the same rate the amounts of reactants
and products remain constant. (It’s a bit like going up an
escalator the wrong way, and remaining in the same position.)
At the beginning of the reaction, the concentrations of A
and B were at their maximum. That means that the rate of
the reaction was at its fastest.
As A and B react, their concentrations fall. That means
that they are less likely to collide and react, and so the
rate of the forward reaction falls as time goes on.
In the beginning, there isn't any C and D, so there can't be
any reaction between them. As time goes on, though, their
concentrations in the mixture increase and they are more
likely to collide and react.
With time, the rate of the reaction between C and D
increases:
Eventually, the rates of the two reactions will become
equal. A and B will be converting into C and D at exactly
the same rate as C and D convert back into A and B
again.
At this point there won't be any further change in the
amounts of A, B, C and D in the mixture. As fast as
something is being removed, it is being replaced again by
the reverse reaction. We have reached a position of
dynamic equilibrium.
A summary
A dynamic equilibrium occurs when you have a reversible
reaction in a closed system. Nothing can be added to the
system or taken away from it apart from energy.
At equilibrium, the quantities of everything present in the
mixture remain constant, although the reactions are still
continuing. This is because the rates of the forward and the
back reactions are equal.
If you change the conditions in a way which changes the
relative rates of the forward and back reactions you will
change the position of equilibrium - in other words, change
the proportions of the various substances present in the
equilibrium mixture.
Le Chatelier's Principle
If a dynamic equilibrium is disturbed by changing the
conditions, the position of equilibrium moves to
counteract the change.
Suppose you have an equilibrium established between
four substances A, B, C and D.
What would happen if you changed the conditions
by increasing the concentration of A?
What would happen if you changed the conditions
by increasing the concentration of A?
What would happen if you changed the
conditions by decreasing the concentration of A?
If a dynamic equilibrium is disturbed by changing the
conditions, the position of equilibrium moves to
counteract the change.
What would happen if you changed the
conditions by decreasing the concentration of A?
Pressure
This applies to gas reactions.
Here the rule depends upon the number of gas
molecules on each side of the equation
Get more gas molecules in backward direction
2NO2(g)

N2O4 (g)
Get less gas molecules in forward direction
The higher the pressure the more the reaction
moves in the direction with less gas molecules.
• Increasing the pressure will give more N2O4
• Decreasing pressure gives more NO2 at equilibrium..
Look at the reaction of nitrogen and hydrogen to
form ammonia.
Get more gas molecules in backward direction
3H2(g)
+ N2 (g)

2NH3 (g)
Get less gas molecules in forward direction
Which direction produces less gas molecules.
forward
Which direction do reactions move when compressed?
The side that has less gas molecules
Will
high pressure
give more
orgive
less more
NH3 in
the
• Increasing
the pressure
will
NH
3 equilbrium
mixture?
more .
• Decreasing the pressure give less NH3 at equilibrium.
The effect of changing pressure in a gaseous equilibrium
What would happen if you changed the conditions
by increasing the pressure?
If a dynamic equilibrium is disturbed by changing the
conditions, the position of equilibrium moves to
counteract the change.
What would happen if you changed the conditions
by decreasing the pressure?
What happens if there are the same number of
molecules on both sides of the equilibrium reaction?
Temperature & equilibrium
2NO2

Delta H, Enthalpy change
N2O4
Gets hot going forward (exothermic)
ΔH = -ve
ΔH = -ve
Gets cold going backward (endothermic) ΔH = +ve
The rule is – whatever you do to the equilibrium,
the system will change to oppose it
Heating will give more NO2 in the equilibrium mixture
Cooling would give more N2O4 in the equilibrium mixture..
The reaction of nitrogen and hydrogen to form
ammonia (NH3) is exothermic.
How will temperature affect the composition of
the equilibrium mixture?
Gets cold going backward (endothermic)

2NH3 ΔH = -ve
Gets hot going forward (exothermic)
3H2
+ N2
Which direction is endothermic?
backward
Which direction does equilibrium move when heated?
backward
Will heating give more or less NH3 in the equilbrium
mixture?
less
The effect of temperature on the position of an equilibrium
What type of reaction occurs in the forward direction?
What would happen if you changed the conditions by
increasing the temperature?
What would happen if you changed the conditions by
increasing the temperature?
Decreasing the temperature has the opposite effect.
If a dynamic equilibrium is disturbed by changing the
conditions, the position of equilibrium moves to
counteract the change.
Summary
Increasing the temperature of a system in
dynamic equilibrium favours the endothermic
reaction. The system counteracts the change you
have made by absorbing the extra heat.
Decreasing the temperature of a system in
dynamic equilibrium favours the exothermic
reaction. The system counteracts the change you
have made by producing more heat.
Effect of a catalyst on the position of equilibrium
Adding a catalyst makes absolutely no
difference to the position of equilibrium, and
Le Chatelier's Principle doesn't apply to them.
This is because a catalyst speeds up the forward
and back reaction to the same extent.
Because adding a catalyst doesn't affect the
relative rates of the two reactions, it can't affect
the position of equilibrium.
The Haber Process
3H2(g) + N2 (g)
 2NH3 (g)
H= -92kJ/mol
1. Is the forward reaction exothermic or
exothermic
endothermic?
2. Will heating the mixture give an equilibrium
less
mixture with more or less ammonia?
3. Are there more gas molecules of reactant or
product?
reactant
4. Will raising the pressure give an equilibrium
more
mixture with more or less ammonia?
The Haber Compromise
3H2(g) + N2 (g)  2NH3 (g)
H=-92kJ/mol
1. The aim of the chemical industry is not to make
chemicals. It is to make money!
2. If we use low temperatures it takes ages to reach
equilibrium. It’s better to get a 40% yield in 2 minutes
than an 80% yield in 2 hours!
3. If we use very high pressures the cost of the equipment
used increases drastically and there are also safety
issues. Better 90% conversion at 200atm than 95%
conversion at 600 atm.
4. Unchanged reactants can always be recycled.
The Haber Process
The Haber process
Homework – produce a detailed summary of the
Haber process, clearly explaining the reasons for the
conditions used.
METHANE HYDRATE ICE
A Possible Mechanism for Ice Age and
Global Warming Cycles*
Recent discoveries about the existence of a vast band of
Methane Hydrate Ice along the world's continental
Slopes, at approx. 500 meters depth, have revolutionized
the theories of the Ice Age and Global Warming Cycles.
The accumulation of Methane Ice leads to Ice Ages and
the rapid melting and effervescence of this ice and gas
leads to and equally rapid Global Warming.
* http://www.utopiasprings.com/methane.htm
Stored methane in equilibrium
methane hydrate (s)
methane (g) + water (l)
∆H +ve
Why is it bad news if Earth’s temperature rises?
Position of equilibrium will move to right to oppose the change,
methane gas is released, methane gas is a greenhouse gas, and
would contribute to global warming
What would happen if the pressure was increased?
Methane is a gas and so position of equilibrium will move to left to
oppose the change.
Iodine(I) chloride & iodine (III) chloride
ICI (l) + Cl2 (g)
ICl 3 (s)
∆H = +ve
Why is it bad news if the Earth’s temperature rises?
Position of equilibrium will move to right to oppose the change,
releasing more methane, and methane is a greenhouse gas.
What would happen if the pressure was increased?
Methane is a gas and so position of equilibrium will move to left to
oppose the change.
The contact process
What would be the effect of:
•Increasing the temperature?
•Increasing the pressure?
The contact process
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