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Slide 1

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Chapter 9
Equilibria…read more

Slide 2

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Dynamic equilibrium
· The state of balance in a reversible
process when neither the forward change
nor the backward change is complete;
both changes are still going on at equal
rates so that they cancel each other out
and there is no overall change
· Equilibrium can only be reached in a
closed system. The system does not
have to be sealed
· Equilibrium can be approached from
either direction and the final equilibrium
position will be the same
· Equilibrium is a dynamic process. It is
reached when the rates of two opposing
processes which are going on all the time
are the same
· Equilibrium has been reached when the
macroscopic properties of the system do
not change with time. These are
properties like density, concentration,
colour, and pressure; properties that do
not depend on the total quantity of matter…read more

Slide 3

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Changing the conditions
of an equilibrium reaction
· If the proportion of products in the
equilibrium mixture is increased we
say that the equilibrium has moved
to the right, or in the forward
· If the proportion of reactants in the
equilibrium mixture is increased we
say that the equilibrium is moved to
the left, or in the backward
· We can often move the equilibrium
position to the left or right by
varying conditions like temperature,
the concentration of species
involved or the pressure (only in
reactions involving gases)…read more

Slide 4

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Le Châtelier's principle
· The principle states that when the conditions of a system at
equilibrium change, the position of equilibrium shifts in the
direction that tends to counteract the change.
Changing concentrations
· If we increase the concentration of one of the reactants Le
Châtelier's principle says that the equilibrium will shift in the
direction that tends to reduce the concentration of this
Changing the overall pressure
· Pressure changes only affect reactions involving gases.
Changing the overall pressure will only change the position of
equilibrium of a gaseous reaction if there are a different
number of moles on either side of the equation. Increasing the
pressure of a gas means that there are more molecules of it in
a given volume; it is equivalent to increasing the concentration
of a solution. Increasing the pressure of a mixture of gases
increases the concentration of all the reactants and products
by the same amount, not just one of them. If we increase the
pressure on this system Le Châtelier's principle tells us that
the position of equilibrium will move to decrease the pressure.
If there are the same number of moles of gas on both sides of
the equation then pressure has no effect on the equilibrium
position. The rate at which equilibrium is reached will be
speeded up by increasing the pressure, as there will be more
collisions in a given time.
Changing the temperature
· Reversible reactions that are exothermic in one direction are
endothermic in the other. The size of the enthalpy is the same
but the sign changes.
· Have no effect on the position of equilibrium so they do not
alter the composition of the equilibrium mixture. They work by
producing an alternative route for the reaction, which has a
lower activation energy of the reaction. This affects the
forwards and backwards reactions equally. Catalysts do
however allow equilibrium to be reached more quickly and
therefore are important to industry.…read more

Slide 5

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Equilibrium reactions in industry
Ammonia, NH3
· World production over 140 million tonnes each year, around
80% is used to make fertilisers like ammonium nitrate,
ammonium sulfate and urea. The rest is used to make
synthetic fibres, dyes, explosives and plastics.
· Nitrogen and hydrogen react together by a reversible reaction
which, at equilibrium, forms a mixture of nitrogen hydrogen
and ammonia. The % of ammonia obtained at equilibrium
depends on temperature and pressure. Low temperature and
high pressure produce a yield of almost 100% however a high
temp and low pressure produce almost none.
· Almost all ammonia is produced by the Haber process which
was developed in the early 20th century by Fritz Haber and
Carl Bosch and allowed Germany to make explosives and
fertilisers, prolonging WW1
· The raw materials for the Haber process are air (nitrogen),
water and natural gas (methane). This reaction produces
carbon monoxide and hydrogen. The nitrogen and hydrogen
are fed into a converter in the ration 1:3 and passed over an
iron catalyst. Most plants run at 200 atm and 670K (this is a
lower pressure and higher temperature than would give the
maximum conversion)
· Nitrogen and hydrogen flow continuously over the catalyst so
the gases do not spend long enough in contact with the
catalyst to reach equilibrium; there is about 15% conversion to
ammonia. The ammonia is cooled so that it becomes liquid
and is piped off. Any nitrogen and hydrogen that is not
converted into ammonia is fed back into the reactor. The
catalyst is iron in pea-sized lumps (increase surface area) and
lasts about 5 years before it becomes `poisoned' by impurities
in the gas stream and has to be replaced.
· The largest use of ammonia is in making nylon.…read more

Slide 6

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Ethanol, C2H5OH
· Ethanol is the alcohol in alcoholic drinks and has
been produced by mankind for thousands of years
by the fermentation from sugars, such as glucose,
using the enzymes in yeast as a catalyst.
· It also has many industrial uses such as making
cosmetics, drugs, detergents and inks and as a
motor fuel. UK production is around 330000
tonnes per year. The main source of ethanol for
industrial use is from crude oil. This is obtained by
fractional distillation, then cracking.
­ Ethanol is made by the hydration (adding of water)
to ethene
­ The reaction is reversible
­ Sped up by using phosphoric acid absorbed on
silica as a catalyst
­ The reactants and products are all gaseous at the
temperature used
­ Applying Le Châtelier's principle to this equilibrium
predicts that the maximum yield will be produced with a
high pressure (force the equilibrium to move to the right
to the side with fewer molecules), a low temperature
(force the equilibrium to move to the right to give out
heat), excess steam (force the equilibrium to the right to
reduce the steam concentration). HOWEVER low temp
will reduce the rate of reaction, high pressure tends to
cause the ethene to polymerise to poly(ethene), high
pressure increases costs of building the plant and the
energy costs of running it and too much steam dilutes the
­ In practice conditions of about 570K and 6500 KPa are
used which give a conversion of about 5%, but the
unreacted ethene is separated and recycled.…read more

Slide 7

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