3.1.6 - Equilibria

Question 1

What does the term dynamic mean in dynamic equilibrium?

Answer 1

Both forward and backward reactions are occurring simultaneously.

Question 2

What are two features of dynamic equilibrium?

Answer 2

1. Forward and backward reactions occurring at equal rates.
2. Concentrations of both reactants and products remain constant.

Question 3

What does Le Chatelier’s principle state?

Answer 3

If an external condition is changed, equilibrium will shift to oppose the change.

Question 4

What is the effect of increasing temperature on equilibrium?

Answer 4

Equilibrium shifts in the endothermic direction, trying to reduce temperature by absorbing heat.

Question 5

What is the effect of decreasing temperature on equilibrium?

Answer 5

Equilibrium shifts in the exothermic direction, trying to increase temperature by giving out heat.

Question 6

What is the effect of increasing pressure on equilibrium?

Answer 6

Equilibrium shifts to the side with fewer moles to oppose the change and thereby reducing pressure.

Question 7

What is the effect of decreasing pressure on equilibrium?

Answer 7

Equilibrium shifts to the side with more moles to oppose the change and thereby increase pressure.

Question 8

What are the drawbacks of using high pressures in industry?

Answer 8

1. High electrical energy costs for pumping gases to maintain high pressure.
2. Equipment is expensive.

Question 9

What is the effect of increasing concentration on equilibrium?

Answer 9

Equilibrium shifts to oppose this change, moving in the direction to remove and decrease the concentration of the substance added.

Question 10

What is the effect of decreasing concentration on equilibrium?

Answer 10

Equilibrium shifts to oppose this change, moving in the direction to remove and increase the concentration of the substance removed.

Question 11

What is the effect of catalysts on equilibrium?

Answer 11

No effect on position of equilibrium, speeds up the rate at which equilibrium is achieved.

Question 12

Define the term ‘catalyst’.

Answer 12

A substance that speeds up the rate of reaction without being used up.

Question 13

What is the only external condition that affects the value of Kc?

Answer 13

Temperature.

Question 14

How does temperature affect the value of Kc?

Answer 14

Temperature will cause a shift in the position of equilibrium. If equilibrium shifts to the right, Kc will increase as there are more products. If equilibrium shifts to the left, Kc will decrease as there are less products.

Question 15

What does Kc only change with?

Answer 15

Temperature.

Question 16

How does a catalyst increase the rate of reaction?

Answer 16

A catalyst increases the rate of both the forward and reverse reactions equally.

Question 17

Define dynamic equilibrium in the context of reversible reactions.

Answer 17

Dynamic equilibrium occurs when the forward and backward reactions proceed at equal rates, resulting in constant concentrations of reactants and products.

Question 18

What does the term ‘position of equilibrium’ refer to?

Answer 18

It refers to the composition of the equilibrium mixture in terms of which side (reactants or products) is favored in a reversible reaction.

Question 19

State Le Chatelier’s principle and its significance.

Answer 19

Le Chatelier’s principle states that if an external condition (like temperature or pressure) is changed, the equilibrium will shift to oppose that change. This principle helps predict how changes affect reaction conditions.

Question 20

How does an increase in temperature affect the equilibrium of an exothermic reaction?

Answer 20

For an exothermic reaction, increasing the temperature shifts the equilibrium towards the reactants (left side), decreasing the yield of products as the system absorbs extra heat to counteract the temperature increase.

Question 21

Explain the impact of increasing pressure on a reaction equilibrium involving gases.

Answer 21

Increasing pressure shifts the equilibrium towards the side with fewer moles of gas, as the system attempts to decrease the pressure. This can increase the yield of the product if the product side has fewer moles.

Question 22

What is the effect of a catalyst on the position of equilibrium?

Answer 22

A catalyst does not affect the position of equilibrium but speeds up the rate at which equilibrium is reached by increasing the rates of both the forward and reverse reactions equally.

Question 23

Why is understanding equilibrium important for industrial processes like the Haber and Contact processes?

Answer 23

Equilibrium concepts guide the optimisation of conditions to achieve desirable reaction yields and rates, balancing factors such as temperature and pressure to minimise costs and maximize production efficiency.

Question 24

Describe how to calculate the equilibrium constant (Kc) for a reaction.

Answer 24

The equilibrium constant Kc is calculated using the expression [C]^p[D]^q / [A]^m[B]^n where the letters represent concentrations at equilibrium and the exponents their stoichiometric coefficients. The units of Kc depend on the balance of moles of gases on each side of the reaction.

Question 25

What happens to the position of equilibrium if the concentration of a reactant is increased?

Answer 25

Increasing the concentration of a reactant shifts the equilibrium towards the products to reduce the impact of the change, potentially increasing the yield of the products.

Question 26

What effect would increasing the temperature have on the yield of ammonia in the Haber process?

Answer 26

Increasing the temperature shifts the equilibrium to the left in this exothermic reaction, resulting in a lower yield of ammonia as the system absorbs heat to counteract the increase in temperature.

Question 27

How does increasing pressure affect the yield of methanol in the reaction CO(g) + 2H2(g) ⇌ CH3OH(g)?

Answer 27

Increasing pressure shifts the equilibrium towards the right, towards the side with fewer moles of gas (methanol side), thus increasing the yield of methanol.

Question 28

Discuss how the principles of equilibrium are applied to the industrial synthesis of ammonia and methanol regarding temperature and pressure.

Answer 28

In both processes, conditions are optimized to balance yield and cost. For ammonia, moderate temperatures and high pressures are used to increase yield without excessive costs. For methanol, similarly, high pressures and moderate temperatures favor the forward reaction, increasing yield while managing operational costs.

Question 29

Explain why catalysts are used in industrial reactions like the Haber process despite not affecting the position of equilibrium.

Answer 29

Catalysts are used to speed up the rate at which equilibrium is reached without altering the position of equilibrium, thus reducing the time to achieve maximum yield and enhancing overall production efficiency.

Question 30

How does Le Chatelier’s principle guide the choice of conditions in terms of pressure in the synthesis of ammonia?

Answer 30

According to Le Chatelier’s principle, increasing pressure favors the formation of ammonia, which has fewer gas moles compared to reactants. This is exploited industrially to increase yield, though it’s balanced against the higher costs of maintaining high pressures.

Question 31

How is the equilibrium constant Kc calculated in a reaction where the number of moles on the reactant and product sides are different?

Answer 31

Kc is calculated using the concentrations of reactants and products raised to the power of their stoichiometric coefficients in the balanced equation. The unit of Kc reflects the difference in moles between reactants and products.

Question 32

How can you prepare an equilibrium mixture?

Answer 32

1. Use burettes to prepare a mixture in a boiling tube of carboxylic acid, alcohol, and dilute sulfuric acid.
2. Swirl and bung the tube. Leave the mixture to reach equilibrium for one week.

Question 33

Explain how you can titrate an equilibrium mixture.

Answer 33

1. Rinse a 250 cm3 volumetric flask with distilled water. Use a funnel to transfer the contents of the boiling tube into the flask. Rinse the boiling tube with water and add the washings to the volumetric flask.
2. Use distilled water to make up the solution in the volumetric flask to exactly 250 cm3. Stopper the flask, then invert and shake the contents thoroughly.
3. Use the pipette to transfer 25.0 cm3 of the diluted equilibrium mixture to a 250 cm3 conical flask.
4. Add 3 or 4 drops of phenolphthalein indicator to the conical flask.
5. Set up the burette with sodium hydroxide solution.
6. Add the sodium hydroxide solution from the burette until the mixture in the conical flask just turns pink. Record this burette reading in your table.
7. Repeat the titration until you obtain a minimum of two concordant titres.

Question 34

Why are high temperatures and low pressures used in the industrial preparation of hydrogen from methane and steam?

Answer 34

1. High temperatures favor the endothermic reaction CH4(g) + H2O(g) → CO(g) + 3H2(g), increasing the yield of hydrogen.
2. Low pressures are used because the reaction produces more moles of gas, and reducing pressure shifts the equilibrium towards the products, maximizing hydrogen production while minimizing costs.

Question 35

What is the effect of a catalyst on the rate of the reverse reaction in a reversible process?

Answer 35

A catalyst increases the rate of the reverse reaction, as well as the forward reaction, but it does not affect the position of equilibrium or the equilibrium constant.

Question 36

Describe the effect of decreasing pressure on the equilibrium yield of SO3 in the reaction 2SO2(g) + O2(g) → 2SO3(g).

Answer 36

Decreasing pressure decreases the yield of SO3 because the reaction has fewer moles of gas on the product side. Lowering the pressure shifts the equilibrium towards the reactants to increase the pressure, thereby reducing the yield of SO3.

Question 37

Calculate the equilibrium constant Kp for the reaction 2SO2(g) + O2(g) → 2SO3(g) given the partial pressures.

Answer 37

Kp = ([SO2]^2[O2] / [SO3]^2), with units of kPa−1 if pressures are given in kPa. Calculate using the provided partial pressures.

Question 38

What is the effect on the value of Kp if the pressure of the equilibrium mixture is increased at constant temperature?

Answer 38

The value of Kp remains the same because it only depends on temperature and not on changes in pressure.

Question 39

Predict the effect of increasing pressure on the equilibrium yield of hydrogen in the reaction C2H5OH(g) + H2O(g) → 2CO(g) + 4H2(g) and explain.

Answer 39

Increasing pressure decreases the yield of hydrogen because the reaction produces a higher number of moles of gas. To minimise pressure, the equilibrium shifts towards the reactants, thus reducing hydrogen production. Kc is unaffected by pressure changes.

Question 40

Describe the effect of adding more carbon monoxide to the equilibrium mixture of CO(g) + 2H2(g) → CH3OH(g) on the partial pressure of methanol and the value of Kp.

Answer 40

Adding more carbon monoxide increases the partial pressure of methanol by shifting the equilibrium towards more products. The value of Kp remains unchanged as it is independent of changes in the amounts of reactants or products.

Question 41

Explain the effect of a catalyst on the yield of methanol and the value of Kp in the reaction CO(g) + 2H2(g) → CH3OH(g).

Answer 41

A catalyst does not affect the yield of methanol nor the value of Kp, as it speeds up both the forward and reverse reactions equally, allowing the system to reach equilibrium faster but not altering the equilibrium position or constant.

Question 42

Predict and explain the effect of pressure increase on the equilibrium yield of ammonia in the reaction N2(g) + 3H2(g) → 2NH3(g).

Answer 42

Increasing the pressure favors the production of ammonia because the reaction results in a decrease in the number of gas molecules (from 4 moles of reactants to 2 moles of products). Thus, increasing pressure shifts the equilibrium towards the products (ammonia) to reduce the overall volume and pressure, thereby increasing the yield.