Reversible Reactions

Question 1

(2) Explain what is meant by a dynamic equilibrium.

Answer 1

Marking point 1 – one from
• forward and back reactions
take place (at the same
time) (1)
• rate of the forward and
back reactions is the same
(1)
Marking point 2 – one from
• no (overall) change in the
{amount/concentration/mas
s/ volume} of each
{substance / reactant /
product} (1)
• no observable change (1)

Question 2

(2) The pressure used is 250 atmospheres.
Explain how the use of a higher pressure would affect the equilibrium yield of
ammonia.

Answer 2

higher pressure favours
forward reaction/equilibrium
shifts to the right (1)
• because decrease in
{volume / number of
molecules}/side with lower
volume (1)
• yield increases (1)

Question 3

(2) Explain how the use of a lower temperature would affect the equilibrium yield
of ammonia.

Answer 3

lower temperature favours
forward reaction/equilibrium
shifts to the right (1)
• because (forward) reaction
is exothermic (1)
• yield increases (1)

Question 4

(1) Even at 450°C, the reaction is very slow.

State what is used in industry to overcome this problem.

Answer 4

catalyst

Question 5

(1) What is the source of the hydrogen used in the Haber process?

Answer 5

natural gas

Question 6

(2) When nitrogen reacts with hydrogen, the amount of ammonia gradually
increases until it becomes constant.
Explain why the amount of ammonia remains constant.

Answer 6

forward and back reactions take
place / reversible / dynamic (1)
at the same rate / equilibrium (1)

Question 7

(6) If nitrogen and hydrogen were reacted at 90 atm pressure and 300 °C, without a
catalyst, some ammonia would be formed eventually.
In the Haber process a pressure of 150 atm and a temperature of 450 °C are used,
in the presence of an iron catalyst.
Explain, with reasons, why the Haber process conditions are better for the
manufacture of ammonia.

Answer 7

Higher pressure:
 higher pressure gives increased yield
 equilibrium shifts to right hand side
 because decrease in number of molecules going from left to
right
 therefore decrease in volume
 favoured by increase in pressure
 reaches equilibrium faster
 because molecules closer together
 so get more frequent collisions
Higher temperature
 higher temperature reaches equilibrium faster
 because molecules move faster
 therefore more frequent collisions
 molecules have more energy
 therefore more collisions have required energy
 but yield will be lower
 because higher temperature favours endothermic reaction
 equilibrium shifts to left hand side
 which is decomposition of ammonia / ammonia reforms
elements
Use of catalyst
 catalyst causes reaction to reach equilibrium faster /
catalyst increases rates (of both forward and back reactions)
 lowers the activation energy (of both forward and back
reactions)
 reaction follows a new pathway
 does not affect yield
 equilibrium position not affected

Question 8

(2) The minimum volumes of nitrogen and hydrogen that must react completely
to form 5000 dm3
of ammonia are calculated.
These volumes are mixed and left, under appropriate conditions, until the
reaction reaches equilibrium.
Explain which gas or gases will be present when equilibrium is reached.

Answer 8

planation linking
 all / three gases present/
nitrogen, hydrogen and
ammonia (1)
 ammonia decomposes/
ammonia turns back to
reactants/ reaction goes
both ways / reversible (1)

Question 9

(2)The Haber process is carried out under a pressure of about 200 atm.
Explain the effect on the equilibrium yield of ammonia, if the process is
carried out at a pressure higher than 200 atm.

Answer 9

increased / higher {yield /
amount of ammonia} (1)
 because fewer (gas)
molecules/ moles on RHS/
4 mole(cule)s on left and
2 on right/ decreased
volume on
RHS/equilibrium shifts to
RHS/ equilibrium shifts in
forward direction (1)

Question 10

(3) Explain the effect on the rate of attainment of equilibrium, if the process is
carried out at a pressure higher than 200 atm.

Answer 10

 rate increased/ time to
reach equilibrium reduced
(1)
 because gas molecules
closer / more concentrated
(1)
 so increased collision rate
/ more collisions in a
given time / more frequent
collisions(1)