equilibria

Question 1

What is a reversible reaction?

Answer 1

● A reaction in which reactants form 
  products and products reform 
  reactants.
● This occurs until an equilibrium is 
  reached.

Question 2

What is dynamic equilibrium?

Answer 2

The rate of the forward reaction equals
the rate of the reverse reaction, hence
the concentrations of the reactants and
products remains constant.

Question 3

What is Le Chatelier’s principle?

Answer 3

If a system in dynamic equilibrium is
subjected to a change, then the position
of equilibrium will shift to minimise this
change.

Question 4

How does increasing the temperature affect the

position of equilibrium?

Answer 4

For an equilibrium where the forward reaction is
exothermic, increasing the temperature will shift the
equilibrium left (so more endothermic reactions occur) to
take in more heat energy and reduce the temperature.
The reverse is true when the forward reaction is
endothermic.

Question 5

How does decreasing the temperature affect the

position of equilibrium?

Answer 5

For an equilibrium where the forward reaction is exothermic,
decreasing the temperature will shift the position of
equilibrium to the right (so more exothermic reactions occur)
to release more heat energy and increase the temperature.

The reverse is true when the forward reaction is endothermic.

Question 6

How does the concentration of reactants affect the

position of equilibrium?

Answer 6

Increasing the concentration of reactants causes the
position of equilibrium to shift right in order to reduce
the concentration of reactants and form more products.
The reverse occurs if the concentration of reactants is
decreased.

Question 7

How does pressure affect the position of equilibrium?

Answer 7

● Increasing the pressure will cause the position of
equilibrium to shift to the side with the fewest gaseous
molecules in order to increase the pressure.
● The opposite occurs if pressure is decreased.
● If there is an equal number of gaseous molecules on
both sides of the equation, changing the pressure will
have no effect on the position of equilibrium.

Question 8

How does the presence of a catalyst affect the position
of equilibrium and the magnitude of the equilibrium
constant?

Answer 8

● The presence of a catalyst doesn’t affect the position
of equilibrium.
● The magnitude of the equilibrium constant therefore is
unaffected.
● It does however increase the rate of the forward and
reverse reactions so equilibrium is established sooner.

Question 9

How does temperature affect the magnitude of the

equilibrium constant

Answer 9

● If the forward reaction is exothermic, increasing the
temperature shifts the position of equilibrium to the left

so Kc decreases.
● If the forward reaction is endothermic, increasing the

temperature shifts the equilibrium to the right so Kc
increases.
● The reverse is true if temperature is decreased.

Question 10

How does pressure affect the magnitude of the

equilibrium constant?

Answer 10

Kc remains the same:
● Doubling the pressure will double both the partial pressures and
concentrations of the species on both sides of the equation.
● The system is no longer in equilibrium so partial pressures of

reactants and products must change to keep Kc the same.

● New equilibrium position will be reached whereby Kc is restored (the

ratio of the Kc expression is the same as before).

Question 11

How does concentration affect the magnitude of the

equilibrium constant?

Answer 11

● Changing the concentration of a reactant or product
means that the system is no longer in equilibrium.
● The concentrations of the reactants and products

now must change so the ratio and hence Kc is
restored.

● Kc is therefore unaffected by concentration changes.

Question 12

What does the equilibrium constant tell you?

Answer 12

● The position of equilibrium of a 
  reaction. 
● The magnitude indicates whether 
  there are more reactants or products 
  in an equilibrium system.

Question 13

What is Kp?

Answer 13

● The equilibrium constant for reactions
in the gaseous phase.

● Similar to Kc but it uses partial
pressures instead of concentrations.

Question 14

How do you calculate the mole fraction of a gas?

Answer 14

(number of moles of gas)/(total number of moles of all present species

Question 15

How do you calculate the partial

pressure of a gas?

Answer 15

mole fraction x total pressure

Question 16

What is the Haber process? Write an
equation for this reaction and state the
source of the reactants

Answer 16

● It is a process that produces ammonia:

N2(g) + 3H2(g)⇌2NH3(g) ΔH = -92kJmol^-1

● Nitrogen - fractional distillation of air
● Hydrogen - natural gas

Question 17

What conditions are used for the Haber process?

Answer 17

● 400 - 450℃
● 200 atm
● Iron catalyst

Question 18

What temperature would you expect to be used for
the Haber process and why? Why is the temperature
used in industry a compromise?

Answer 18

● The forward reaction is exothermic so a low
temperature would give the greatest yield.
● However, a low temperature gives a slow rate of
reaction and so a higher temperature is normally
used to strike a balance between yield and rate.

Question 19

What pressure would you expect to be
used for the Haber process and why?
Why is the pressure used in industry a
compromise?

Answer 19

● According to Le Chatelier’s principle, a high
pressure would give the greatest yield of ammonia.
● However, high pressures can be dangerous and
expensive to maintain so a lower pressure may be
used.

Question 20

What is the Contact process? Write an
equation for this reaction and state the
source of the reactants

Answer 20

● The process for creating sulfur trioxide:

2SO2(g) + O2(g)⇌ 2SO3(g)
ΔH=-196kJmol^-1

● Sulfur dioxide - heat sulfur in air
● Oxygen - air

Question 21

What conditions are used for the Contact

process?

Answer 21

● 400 - 450℃
● 1 - 2 atm

● V2O5 catalyst

Question 22

What temperature would you expect to
be used for the Contact process and
why? Why is the temperature used in
industry a compromise?

Answer 22

● The forward reaction is exothermic so a low
temperature would give the greatest yield.
● However, a low temperature results in a slow rate
of reaction and so a higher temperature may be
used to strike a balance between yield and rate

Question 23

What pressure would you expect to be used for the
Contact process and why? Why is the pressure used
in industry a compromise?

Answer 23

● According to Le Chatelier’s principle, a high pressure would
give the greatest yield of sulfur trioxide.
● However, even at pressures close to atmospheric pressure,

99.5% of SO2 is converted into SO3 so increasing the
pressure would only see a minute improvement in yield that
wouldn’t be economically worthwhile.

Question 24

What is a Brønsted-Lowry acid?

Answer 24

A proton donor

Question 25

What is a Brønsted-Lowry base?

Answer 25

A proton acceptor

Question 26

What is a conjugate acid-base pair?

Answer 26

A conjugate acid-base pair is two species that

differ from each other by a proton (H+ ion)

Question 27

What is a strong acid?

Answer 27

A strong acid is an acid that completely

dissociates in solution

Question 28

What is a strong base?

Answer 28

A strong base is a base that completely

dissociates in solution

Question 29

What is a weak acid?

Answer 29

A weak acid is an acid that only partially

dissociates in solution

Question 30

What is a weak base?

Answer 30

A weak base is a base that only partially

dissociates in solution