Chemical equilibria: reversible reactions, dynamic equilibrium

Explain the term dynamic equilibrium and a reversible reaction

The phrase dynamic equilibrium means that the molecules or ions of reactants and products are continuously reacting. Reactants are continuously being changed to products and products are continuously being changed back to reactants.

A reaction in which the products can react to re-form the original reactants is called a reversible reaction.

The reaction `2SO_2(g) + O_2(g) ⇌ 2SO_3(g)`

reaches dynamic equilibrium in a closed vessel. The forward reaction is exothermic. The reaction is catalysed by V₂O₅.

What will happen to the position of equilibrium when:

i. some sulfur trioxide, SO₃, is removed from the vessel?

ii. the pressure in the vessel is lowered?

iii. more V₂O₅ is added?

iv. The temperature of the vessel is increased?

i. It will shift to the right

ii. It will shift to the right

iii. The position of equilibrium is not affected because the catalyst does not affect the position

iv. It will shift to the left

a. State Le Chatelier’s principle.

b. Use Le Chatelier’s principle to explain what will happen to the position of equilibrium in the reaction

`H_2(g) + CO_2(g) ⇌ H_2O(g) + CO(g)`

when the concentration of hydrogen is increased

a. Le Chatelier’s principle states that if one or more factors that affect an equilibrium is changed, the position of equilibrium shifts in the direction that reduces (opposes) the change.

b. When the concentration of hydrogen is increased, the equilibrium of reaction will shift to the right due to more hydrogen being reacted with CO₂ to generate more CO and H₂O until the value of Kc keeps constant

Hydrogen, iodine and hydrogen iodide are in equilibrium in a sealed tube at constant temperature. 

The equation for the reaction is:

`H _2 + I_2 ⇌ 2HI(g)" " ΔHr = –96 kJ mol^(–1)`

The partial pressures of each gas are shown in the table below.

a. Explain the meaning of the term partial pressure

b. Calculate the total pressure of the three gases in this mixture.

c. Write an equilibrium expression for this reaction in terms of partial pressures.

d. Calculate a value for K p for this reaction, including the units.

a. The pressure exerted by any one gas in the mixture is called its partial pressure.

b. The total pressure of the three gases in this mixture = `2.330 xx 10^6 + 0.925 xx 10^6 + 10.200 xx 10^6 = 13.455 xx 10^6`

c. `K_p = (pHI)^2/ (pH_2 + pI_2)`

d. `K_p = (10.200 xx 10^6)^2/((2.330 xx 10^6) xx (0.925 xx 10^6)) = 48.3`

The equilibrium between three substances, A, B and C is shown below.

A(g) + B(g) ⇌ C(g)

Initially there were 0.1 mol of A and 0.2 mol of B in the reaction mixture. A and B reacted together to produce an equilibrium mixture containing 0.04 mol of C. The total volume of the mixture was 2.00 dm³.

a. Calculate the number of moles of A and B at equilibrium

b. Calculate the concentrations of A, B and C at equilibrium

c.

i. Write the equilibrium expression for Kc.

ii. Calculate the value of K c and give the units.

`N_2O_4 ⇌ 2NO_2 " " ΔHr = +58 kJ mol^(–1)`

a. Describe three characteristic features of chemical equilibrium.

b. When 1 mol of N₂O₄ gas is allowed to come to equilibrium with NO₂ gas under standard conditions, only 20% of the N₂O₄ is converted to NO₂.

`N_2O_4 ⇌ 2NO_2 " " ΔHr = +58 kJ mol^(–1)`

i. Give the equilibrium expression for this reaction.

ii. Calculate the value of K c for the reaction. Assume that the volume of the reaction mixture is 1 dm³.

a. Three characteristic features of chemical equilibrium could be

(1) reactants and products at constant concentration;

(2) closed system;

(3) the backward and forward reactions are both occurring at the same time

b.

This question is about the following reaction:

9.20 g of ethanol are mixed with 12.00 g of ethanoic acid in an inert solvent. The total volume of solution is 250 cm³. The mixture is left to equilibrate for several days. At equilibrium 70% of the reactants are converted to products.

a. What is the concentration of each reactant at the start?

b. What is the concentration of each reactant at equilibrium?

c. What is the concentration of each product at equilibrium?

d. i. Write the equilibrium expression for this reaction.

ii. Calculate the value of K_c for the reaction.

iii. Explain why there are no units for K_c for this reaction.

e. What will happen to the numerical value of K_c if 100 cm³ of water is added to the equilibrium mixture?

f. What will happen to the yield of ethyl ethanoate if 100 cm³ of water is added to the equilibrium mixture? Explain your answer.

a. The volume of solution = 250 cm³ = 0.25 dm³

The number of moles of ethanol = `9.20 / 46` = 0.2 mol

The number of moles of ethanoic acid = `12.00 / 60` = 0.2 mol

The concentration of ethanoic acid = the concentration of ethanol = `0.2 / 0.25` = 0.8 mol dm^-3

b. At equilibrium, the number of moles of ethanoic acid = the number of moles of ethanol = `0.2 - 0.2xx0.7` = 0.06 mol

The concentration of ethanoic acid = the concentration of ethanol = `0.06/0.25` = 0.24 mol dm^-3 

c. The number of moles of ethyl ethanoate = The number of moles of water =`0.2 xx 0.7` = 0.14 mol

The concentration of ethyl ethanoate = The concentration of water = `0.14 / 0.25` = 0.56 mol dm^-3

d.

i. `K_c = ([CH_3COOC_2H_5][H_2O])/([CH_3COOH][C_2H_5OH])`

ii.`K_c = ((0.56)xx(0.56))/((0.24)xx(0.24)) = 5.44`

iii. Because all units are cancelled each other

e. The value of K_c does not change if a catalyst is added, concentration of reactants or products change or pressure of the system changes.

f. The equilibrium will shift to the left if more water is added leading the yield of ethyl ethanoate to become lesser

Dinitrogen tetraoxide, N₂O₄, and nitrogen dioxide exist in equilibrium. This is represented by the reaction below: 2NO₂ (g) ⇌ N₂O₄ (g)      ∆H = -57 kJ mol^-1

Which conditions give the greatest percentage of NO₂ at equilibrium?

A. High pressure and high temperature

B. Low pressure and high temperature

C. High pressure and low temperature

D. Low pressure and low temperature

The answer is B.

The enthalpy change of this reaction is exothermic as ∆H is negative. Thus, backward reaction is endothermic. 

In accordance with Le Chatelier’s principle, if one or more factors that affect an equilibrium is changed, the position of equilibrium shifts in the direction that reduces (opposes) the change.

A is incorrect because high pressure will shift the equilibrium to the right

C is incorrect because high pressure will shift the equilibrium to the right

D is incorrect because low temperature will shift the equilibrium to the right

Sulphuric acid is manufactured via a series of reactions in the Contact process. The reaction for the main reaction is shown below.

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)       ∆H= -197 kJ mol^-1

Which statement about the reaction is correct? 

A. Iron is used as a catalyst.

B. Increased temperature gives a higher yield of SO₂.

C. Increased temperature gives a higher yield of SO₃.

D. Increased pressure gives a higher yield of SO₂.

The answer is B.

The enthalpy change of this reaction is exothermic as ∆H is negative. Thus, backward reaction is endothermic. 

In accordance with Le Chatelier’s principle, if one or more factors that affect an equilibrium is changed, the position of equilibrium shifts in the direction that reduces (opposes) the change.

A is incorrect because iron is used in the Haber process, not the Contact process

C is incorrect because increasing the temperature will shift the equilibrium to the endothermic

D is incorrect because increasing pressure will shift the equilibrium to the side with less moles of a gas and shift to the right

Ethanol is manufactured by reacting steam with ethene.

C₂H₄ (g) + H₂O (g) ⇌ C₂H₅OH (g)    ∆H = -45 kJ mol^-1

What would increase the equilibrium yield of ethanol in this process?

1. Adding a catalyst.

2. Increasing the pressure.

3. Decreasing the temperature.

A. 1 and 2

B. 1 and 3

C. 2 and 3

D. 1, 2 and 3

The answer is C.

In accordance with Le Chatelier’s principle, high pressure would shift the equilibrium to the side of reaction with fewer moles of gas => Shift to the right-hand side of reaction => increasing the yield of ethanol

The enthalpy change of the forward reaction is exothermic because ∆H is negative => Lowering temperature will shift to the right-hand side of reaction 

A, B and D are incorrect because adding a catalyst will increase the rate of reaction not effect on the position of the equilibrium.