Equilibria Exam qs

Question 1

Hydrogen can be prepared on an industrial scale using the reversible reaction
between methane and steam.
CH4(g) + H2O(g) ⇌ CO(g) + 3 H2(g) ΔH = +206 kJ mol−1
The reaction is done at a temperature of 800 °C and a low pressure of 300 kPa
in the presence of a nickel catalyst.
Explain, in terms of equilibrium yield and cost, why these conditions are used.

Answer 1

Stage 1: Temperature
1a. The reaction is endothermic (so equilibrium shifts to RHS to reduce the
temperature)
1b. So, higher temperature increases the yield 1c. High temperatures are costly
(so compromise temperature used)
Stage 2: Pressure
2a. More moles of gas on the right hand side, (so equilibrium shifts to RHS to
increase the yield)
2b. So, lower pressure increases the yield
2c. A low pressure means a low cost
Stage 3: Catalyst
3a. Catalyst has no effect on yield
3b. Adding a catalyst allows a lower temperature to be used
3c. So, this lowers the cost

Question 2

This question is about the equilibrium
2 SO2(g) + O2(g) ⇌ 2 SO3(g)
(a) State and explain the effect, if any, of a decrease in overall pressure on the equilibrium yield of SO3

Answer 2

M1 decreases yield

M2 So equilibrium shifts to side with more moles/molecules or
more moles/molecules on LHS

M3 So equilibrium shifts (to left side) to oppose decrease in
pressure OR to increase pressure

Question 3

The pressure of the equilibrium mixture was increased by reducing the
volume of the container at constant temperature.
Predict the effect of increasing the pressure on the equilibrium yield of
hydrogen. Explain your answer.
Predict the effect of increasing the pressure on the value of Kc

Answer 3

yield would decrease
equilibrium (position) moves left / shifts left / in direction of reverse
reaction
to oppose increase in pressure / to reduce pressure
fewer moles/molecules of gas on left hand side / fewer
moles/molecules of gaseous reactants
no effect on Kc

Question 4

State the effect, if any, of the addition of a catalyst on the value of Kp for
this equilibrium.
Explain your answer.

Answer 4

No effect
M2 Increases rate of forward and backward reaction equally/by the same
amount OR catalyst does not affect position of equilibrium

Question 5

Some more carbon monoxide is added to the mixture in part (b). The new
mixture is allowed to reach equilibrium at temperature T.
State the effect, if any, on the partial pressure of methanol and on the
value of Kp

Answer 5

Increases

M2 No effect

Question 6

State and explain the effect of using a catalyst on the yield of methanol in
this equilibrium

Answer 6

no effect
increases rate of forward and reverse reactions equally