Chapter 6.2 Change In The Conditions Of Equilibrium

Question 1

fixed mass of marble is reacted with dilute hydrochloric acid at a constant temperature. Explain why the rate of the reaction is increased if the lumps of marble are reduced in size. (2)

Answer 1

-increased surface area,
- more collisions

Question 2

Give two reasons why, in general, industry prefers to operate processes at pressures lower than 30 MPa. (2)

Answer 2

-cost of producing high pressure,
-cost of plant to resist high pressure

Question 3

If the chromium-based catalyst was replaced with a more efficient catalyst but other conditions were kept the same, deduce what would happen to the equilibrium yield of methanol. Explain your answer (2)

Answer 3

-no change,
-catalyst has no effect on equilibrium position

Question 4

State and explain the effect of an increase in pressure, and the effect of an increase in temperature, on the yield of nitrogen monoxide in the above equilibrium.

N2 + O2 = 2NO

Effect of an increase in pressure on the yield
Explanation (3)

Answer 4

-no change,
-equal number of moles on either side,
-both sides are affected equally

Question 5

State and explain the effect of an increase in pressure, and the effect of an increase in temperature, on the yield of nitrogen monoxide in the above equilibrium.

N2 + O2 = 2NO. H= +120

Effect of an increase in temperature on the yield,
Explanation (3)

Answer 5

-increases,
-equilibrium opposes the change and lowers the temperature,
-forward reaction is endothermic

Question 6

State le chateliers principle (1)

Answer 6

An equilibrium opposes change

Question 7

At equilibrium, a high yield of hydrogen is favoured by high temperature. In a typical industrial process, the operating temperature is usually less than 1200 K. Suggest two reasons why temperatures higher than this are not used. (2)

Answer 7

-cost of high temperature,
-cost of plant to resist High temperature is too high

Question 8

By reference to rates and concentrations, explain the meaning of the term dynamic equilibrium. (2)

Answer 8

-rate of forward reaction = rate of backward reaction,
-concentration remains constant

Question 9

Suggest two reasons why the operation of this process at a pressure much higher than 5 MPa would be very expensive (2)

Answer 9

-power required to provide high pressure,
-strong equipment

Question 10

If a catalyst were not used in this process, the operating temperature would have to be greater than 700 K. Suggest why an increased temperature would be required (1)

Answer 10

To speed up the reaction

Question 11

CO(g) + H2O(g) CO2(g) + H2(g) ∆H = -42 kJ mol–1

Explain why this reaction is described as a homogeneous reaction

Answer 11

-all reagents are in the same state

Question 12

State what is meant by the term dynamic equilibrium (3)

Answer 12

-rate for forward reaction = rate of backward reaction at the same rate
-concentration of reactants and products are constant

Question 13

State and explain the effect, if any, of a catalyst on the position of this equilibrium. (3)

Answer 13

-none,
-rate of both forward and backward reactions increased by same amount

Question 14

Le Chatelier’s principle suggests that a high pressure will produce a low yield of hydrogen in this first stage.

Explain, in terms of the behaviour of particles, why a high operating pressure is used in industry (2)

Answer 14

-the reaction gets to equilibrium faster,
-high pressure leads to more particles in a given volume and more collisions in a given time

Question 15

iState the effect, if any, of a catalyst on the time taken to reach equilibrium. (1)

Answer 15

Speeds up

Question 16

State the effect, if any, of a catalyst on the position of an equilibrium. (1)

Answer 16

None

Question 17

Methanol can be formed on an industrial scale from carbon dioxide and hydrogen by a reversible reaction as shown below.

CO2(g) + 3H2(g) = CH3OH(g) + H2O(g)

The reaction can be carried out in the presence of a chromium-based catalyst at a temperature of 700 K and a pressure of 30 MPa. Under these conditions, equilibrium is reached when 2% of the carbon dioxide has been converted.

How does the rate of the forward reaction compare with that of the backward reaction when 2% of the carbon dioxide has been converted? (1)

Answer 17

Same

Question 18

Methanol can be formed on an industrial scale from carbon dioxide and hydrogen by a reversible reaction as shown below.

CO2(g) + 3H2(g) = CH3OH(g) + H2O(g)

The reaction can be carried out in the presence of a chromium-based catalyst at a temperature of 700 K and a pressure of 30 MPa. Under these conditions, equilibrium is reached when 2% of the carbon dioxide has been converted

If the pressure was reduced but the temperature was kept the same, deduce what would happen to the equilibrium yield of methanol. Explain your answer (3)

Answer 18

-decreases,
-more molecules on the left hand side,
-equilibrium moved to increase the pressure

Question 19

the equilibrium yield of methanol is low. Suggest what is done with the unreacted carbon dioxide and hydrogen (1)

Answer 19

Recycled

Question 20

Which has the greater effect on the initial rate of a first-order reaction:

either increasing the temperature of the reaction mixture from 300 K to 309 K (a 3% increase)

or increasing the concentration of the reagent from 1.00 M to 1.03 M (also a 3% increase)?

Briefly explain your answer (3)

Answer 20

-temperature,
-for small change in temperature more molecules have energy greater than the activation energy

Question 21

At high temperatures, nitrogen is oxidised by oxygen to form nitrogen monoxide in a reversible reaction as shown in the equation below.

N2(g) + O2(g) = 2NO(g) ∆H= +180 kJ mol–1

State and explain the effect of an increase in pressure, and the effect of an increase in temperature, on the yield of nitrogen monoxide in the above equilibrium.

Effect of an increase in pressure on the yield (3)

Answer 21

-no change,
-equal number of gaseous moles on each side,
-both sides effected equally,

Question 22

At high temperatures, nitrogen is oxidised by oxygen to form nitrogen monoxide in a reversible reaction as shown in the equation below.

N2(g) + O2(g) = 2NO(g) ∆H= +180 kJ mol–1

State and explain the effect of an increase in pressure, and the effect of an increase in
temperature, on the yield of nitrogen monoxide in the above equilibrium

Effect of an increase in temperature on the yield (3)

Answer 22

-increases,
-equilibrium moves to lower the temperature/oppose the change,
-endothermic reaction favoured /forward reaction is endothermic

Question 23

Sulphur dioxide and oxygen were mixed in a 2:1 mol ratio and sealed in a flask with a catalyst. The following equilibrium was established at temperature T1

2SO2(g) + O2(g) = 2SO3(g) ∆H = –196 kJ mol–1

In a further experiment, the amounts of sulphur dioxide and oxygen used, the catalyst and the temperature, T1, were all unchanged, but a flask of smaller volume was used.

Deduce the effect of this change on the yield of sulphur trioxide and on the value of Kp. (2)

Answer 23

-increases,
-none

Question 24

Hydrogen is produced on an industrial scale from methane as shown by the equation below

CH4(g) + H2O(g) = CO(g) + 3H2(g) ∆H = +205 kJ mol

The following changes are made to this reaction at equilibrium. In each case, predict what would happen to the yield of hydrogen from a given amount of methane. Use Le Chatelier’s principle to explain your answer.

(i) The overall pressure is increased

Effect on yield of hydrogen:
Explanation: (3)

Answer 24

-decreases,
-pressure lowered,
-by favoring fewer moles of gas,

Question 25

CH4(g) + H2O(g) = CO(g) + 3H2(g) ∆H = +205 kJ mol–1

The following changes are made to this reaction at equilibrium. In each case, predict what would happen to the yield of hydrogen from a given amount of methane. Use Le Chatelier’s principle to explain your answer.

The concentration of steam in the reaction mixture is increased.
Effect on yield of hydrogen:
Explanation: (3)

Answer 25

-increases,
-steam reduced,
-by shifting to right

Question 26

CO(g) + 2H2(g) = CH3OH(g) ∆H = –91 kJ mol–1

The process operates at a pressure of 5 MPa and a temperature of 700 K in the presence of a copper-containing catalyst. This reaction can reach dynamic equilibrium

Explain why a high yield of methanol is favoured by high pressure. (2)

Answer 26

-fewer moles of gas on right hand side,
-methanol favored by reducing pressure

Question 27

CO(g) + 2H2(g) = CH3OH(g) ∆H = –91 kJ mol–1

The process operates at a pressure of 5 MPa and a temperature of 700 K in the presence of a copper-containing catalyst. This reaction can reach dynamic equilibrium.

State the effect of an increase in temperature on the equilibrium yield of methanol and explain your answer (3)

Answer 27

-decreases,
-reaction is exothermic ,
-system tries to lower temperature, endothermic side is favored

Question 28

When carbon monoxide reacts with steam at 670 K, the following homogeneous dynamic equilibrium is established.

CO(g) + H2O(g) = CO2(g) + H2(g) ∆H = -42 kJ mol

State the effect, if any, of the following changes on the concentration of hydrogen in the equilibrium mixture. In each case, explain your answer.
(i) An increase in the concentration of steam (3)

Answer 28

-increases,
-equilibrium moved to the right,
-system acts oppose to the change

Question 29

When carbon monoxide reacts with steam at 670 K, the following homogeneous dynamic equilibrium is established.

CO(g) + H2O(g) = CO2(g) + H2(g) ∆H = -42 kJ mol

State the effect, if any, of the following changes on the concentration of hydrogen in the equilibrium mixture. In each case, explain your answer.
A decrease in the temperature (3)

Answer 29

-increases,
-equilibrium moved to the right,
-system acts oppose to the change

Question 30

The Stage 1 reaction is carried out by passing ammonia and air over a platinum gauze at a temperature of 900°C and a pressure of 700kPa.
What are the advantages of using this temperature and this pressure? (2)

Answer 30

-to increase rate of reaction,
-to increase rate of reaction

Question 31

Hydrogen used in the Haber Process is produced in the following dynamic equilibrium reaction.

CH4(g) + H2O(g) = CO(g) + 3H2(g)

State how an increase in pressure will affect the equilibrium yield of hydrogen. Explain your answer. (3)

Answer 31

-yield: decreases,
-reaction favors fewer molecules,
-equilibrium shifts to the left

Question 32

Ammonia is produced in the Haber Process according to the following equation.

N2(g) + 3H2(g) 2NH3(g) ∆Hf = –92 kJ mol–1

Typical operating conditions are 450 °C and 20 MPa (200 bar)

Explain why 450 °C is a compromise temperature. (3)

Answer 32

-reaction is exothermic,
-rate of reaction higher at high temperature,
-low temperature favors the reaction

Question 33

Ammonia is produced in the Haber Process according to the following equation.

N2(g) + 3H2(g) = 2NH3(g) ∆Hf = –92 kJ mol–1

Typical operating conditions are 450 °C and 20 MPa (200 bar)

Explain why 20 MPa is a compromise pressure. (3)

Answer 33

-Higher pressure gives a higher yield,
-4 moles of gaseous reactant form 2 moles of gaseous product,
-Higher pressure equipment is expensive to produce

Question 34

Cl2(g) + H2O(l) = H+(aq) + Cl–(aq) + HClO(aq)

Give the formula of the species responsible for the pale green colour in the solution of chlorine in water.

Use Le Chatelier’s principle to explain why the green colour disappears when sodium hydroxide solution is added to this solution (3)

Answer 34

-Cl2 provides a pale green color,
-NaOH reacts with the acid,
-Equilibrium shifts to the right

Question 35

Give two features of a reaction at equilibrium (2)

Answer 35

-concentration of reactants and products remain constant
-rate of forward reaction = rate of backward reaction