Topic 10/11 Equilibrium 1/2

Question 1

Define the term ‘dynamic equilibrium’.

Answer 1

The rate of the forward reaction is equal to the rate of the reverse reaction.
(Hence, the concentrations of reactants and products do not change.)

Question 2

Give an essential condition for an equilibrium mixture.

Answer 2

● Equilibrium occurs in a closed system where reactants and products cannot escape.
OR
● Macroscopic properties do not change with time.

Question 3

State Le Chatelier’s principle.

Answer 3

If a system at equilibrium is altered, the position of equilibrium moves in the direction that reduces the effect of the initial change.

Question 4

What effect would increasing the temperature have on the position of equilibrium?
CH 4(g) + H 2 O (g) ⇌ CO (g) + 3H 2(g)
[ΔH°= +210 kJ mol-1]

Answer 4

The equilibrium position shifts to
the right.
(This is because the forward reaction is endothermic
shown by the +ve ∆H value.)

Question 5

What effect would increasing the pressure have on the position of equilibrium?
CH 4(g) + H 2 O (g) ⇌ CO (g) + 3H 2(g)
[ΔH°= +210 kJ mol-1]

Answer 5

The equilibrium position shifts to the left.
(This is because the rhs of the equation has more moles of gas than the LHS. The side with fewer moles is favoured.)

Question 6

Suggest and explain why an industrial chemist may use a high pressure for the production of hydrogen from:
CH 4(g) + H 2 O (g) ⇌ CO (g) + 3H 2(g)
[ΔH°= +210 kJ mol-1]

Answer 6

1. The high pressure increases the collision frequency, increasing the rate of reaction.
2. This is a compromise pressure between an economically viable rate of reaction and a slightly lower yield of hydrogen from the equilibrium reaction

Question 7

What effect does a catalyst have on the position of
equilibrium?

Answer 7

No effect.
(This is because a catalyst affects rate of forward and reverse reactions equally rsulting in no overall effect.)

Question 8

What condition affects the value of K c ?

Answer 8

Temperature only

Question 9

For the reaction below, deduce an expression for K c .
2[A] + 3[B] + [C] ⇌ [D]+ 4[E]

Answer 9

’ [D][E] ^ 4
K c = ———————-
[A] ^ 2 [B] ^ 3 [C]

Question 10

Deduce units for the value of K c when:
‘ [D][E] ^ 4
K c = ———————-
[A] ^ 2 [B] ^ 3 [C]

Answer 10

mol ^ -1 dm ^ 3

Question 11

What type of system is K c relevant for?

Answer 11

Homogeneous systems in equilibrium.

Question 12

What does K c being greater of lesser than 1 suggest for the position of equilibrium?

Answer 12

Greater than 1 = shifted to the right
Lesser than 1 = shifted to the left

Question 13

What effect does decreasing the temperature in an endothermic reaction have on K c ?

Answer 13

K c decreases
(The endothermic reaction isn’t favoured so equilibrium shifts
to the left.)

Question 14

What effect does increasing the temperature in an endothermic reaction have on K c ?

Answer 14

K c increases
(The endothermic reaction is favoured so equilibrium shifts to
the right.)

Question 15

What effect does decreasing the temperature in an exothermic reaction have on K c ?

Answer 15

K c increases
(The exothermic reaction is favoured so equilibrium shifts to
the right.)

Question 16

What effect does increasing the temperature in an exothermic reaction have on K c ?

Answer 16

K c decreases
(The exothermic reaction isn’t favoured so equilibrium shifts
to the left.)

Question 17

What is partial pressure?

Answer 17

Each gas’s contribution to the total pressure

Question 18

How would you calculate the partial pressure of a gas?

Answer 18

Partial pressure p = mole fraction x total pressure

Question 19

A reaction is represented by aA (g) + bB (g) ⇌ cC (g) + dD (g), what
Kp for the system?

Answer 19

’ pC^c pD^d
Kp = ———————–
pA^a pB^b

Question 20

How do you calculate the units for Kp?

Answer 20

Write out the units for the partial pressures/ concentrations in the same arrangement as the Kp/Kc equation and cancel out/multiply together.
For Kp usually in Pa, kPa, atm etc. DO NOT CHANGE UNITS
For Kc units for concentration is mol dm -3

Question 21

What is the effect of increasing temperature on Kp for an endothermic reaction?

Answer 21

Equilibrium shifts to the right, so partial pressures of products increase, so Kp increases

Question 22

What is the effect of increasing temperature on Kp/Kc for an endothermic reaction?

Answer 22

Equilibrium shifts to the left, so partial pressures/concentration of reactants increase, so Kp decreases

Question 23

What is the effect of increasing the overall pressure on Kp for this reaction?

Answer 23

Pressure does not affect Kp as, if moles of gas are not the
same on each side), either top or bottom of Kp expression will
have a total pressure term that does not cancel.

Question 24

What will be the kinetic effect of increasing the temperature and pressure for any reaction?

Answer 24

Increasing both will increase the rate of reaction as:
Temperature - many more particles have energy greater than or equal to the activation energy → more successful collisions per second
Pressure - more particles in the same volume → more successful collisions per second.

Question 25

What is the effect of changing concentration or pressure or by the addition of a catalyst on the value of equilibrium constant?

Answer 25

There is no effect