ES copq: Electrolysis; equilibrium

Question 1

What is formed during the electrolysis of molten lead bromide and at which electrodes?

Answer 1

• Molten lead metal (cathode)
• Bromine gas (anode)

Question 2

At which electrode do the following occur?

• Reduction
• Oxidation

Answer 2

• Reduction: negative electrode (cathode)
• Oxidation: positive electrode (anode)

Vowels + consonants match

Question 3

What are the rules for the reaction at the cathode in the electrolysis of aqueous solutions?

Answer 3

• Metals in group 1/2, Al and acids give hydrogen gas
• Other metals are plated on cathode

Question 4

What are the rules for the reaction at the anode in the electrolysis of aqueous solutions?

Answer 4

• Halides give (corresponding) halogen
• Sulfates/nitrates give oxygen + H⁺ ions
• Hydroxides give oxygen + water
• When anode + aqueous metal cations are the same element, atoms in anode are oxidised, giving aqueous ions

Question 5

Give half equations what occurs at each electrode during the electrolysis of copper sulfate solution using copper electrodes. Include half-equations.

Answer 5

​Anode: oxidation of copper atoms in electrode

Cu_(S) →Cu²⁺_(aq) + 2e^-

Cathode: reduction of aqueous copper ions

Cu²⁺_(aq) + 2e^- → Cu_(s)

Concentration of Cu²⁺ constant

Question 6

Write the equation for the reduction of water at the cathode.

Answer 6

• Dissociation of water: H₂O(l) → OH^-(aq) + H⁺(aq)*
• Half-equation: H⁺ + e^- → 1/2H₂*
• Overall:* H₂O_(l) + e^- → 1/2H_{2 (g)} + OH^-_(aq)

Question 7

Write the equation for the oxidation of water at the anode.

Answer 7

• Dissociation of water: H₂O(l)* → OH^- + H⁺
• Half-equation: OH^- → 2e^- + H⁺ + 1/2O₂*
• Overall:* H₂O_(l) → 2e^- + 2H⁺_(aq) + 1/2O_{2 (g)}

Question 8

Write the half-equation at the anode for the electrolysis of aqueous sodium hydroxide.

Answer 8

4OH^-_(aq) → O_{2 (g)} + 2H₂O_(l) + 4e^-

Question 9

Explain what is meant by “dynamic equilibrium”.

Answer 9

• Closed system
• Forward + reverse reactions occur at equal rates
• Concentrations of products + reactants are constant (but not equal)

Question 10

How do you calculate K_c (equilibrium constant)?

Answer 10

product of conc. of products

product of conc. of reactants

Raise each conc to power of moles in equation

Question 11

K_c > 1

What does this tell you about the equilibrium position and relative concentrations of products + reactants?

Answer 11

• Product of conc. of products > that of reactants
• Equilibrium lies to right
• Higher concentration of products than reactants (both still constant)

Question 12

K_c is dependent on what?

Answer 12

Temperature

Question 13

What is the effect on the equilibrium position of increasing the concentration of a reactant in an equilibrium system?

Answer 13

• Concentration term on denominator of K_c expression becomes larger
• K_c is a constant, so numerator must also become larger
• So equilibrium position shifts to right
• More reactants react to form products

Question 14

How do changes in concentration or total pressure affect the magnitude of the equilibrium constant, assuming a constant temperature?

Answer 14

No effect

(Equilibrium position (composition of mixture) changes in order to restore K_c)

Question 15

How does increasing the pressure of an equilibrium system affect rate and yield?

Answer 15

• Rate increased, since higher frequency of successful collisions
• Increases yield if forward reaction produces fewer molecules; decreases yield if it produces more

• It’s overall rate that increases; pressure has no effect on relative rates (since equilibrium position shifts to restore K_c)*
• The yield of a reaction is essentially the concentration of its products*

Question 16

How does increasing the temperature of an equilibrium system affect rate and yield?

Answer 16

• Increases rate, since a greater frequency of collisions have E ≥ E_a
• Increases yield if forward reaction is endothermic; decreases yield if exothermic

• It’s overall rate that increases; temperature has no effect on relative rates (since equilibrium position shifts to restore K_c)*
• The yield of a reaction is essentially the concentration of its products​*

Question 17

CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅ (l) + H₂O(l)

This system is allowed to reach equilbirium. Describe the result when the concentration of ethanol is increased.

Answer 17

• System opposes change to decrease [ethanol]
• Rate of forward reaction increases
• More ethanol reacts with ethanoic acid; more ethyl ethanoate + water produced
• Equilibrium position moves to right until K_c restored

Question 18

Nitrogen dioxide (brown) exists in equilibrium with its dimer, dinitrogen tetraoxide (colourless).

2NO₂ ⇌ N₂O₄ ΔH = negative

A sealed container of the equilibrium mixture is placed in iced water. Explain what is observed.

Answer 18

• Temperature decreases
• System counteracts change by increasing rate of exothermic forward reaction
• Equilibrium position moves to right to restore K_c
• More NO₂ converted into N₂O₄
• Mixture becomes paler as more N₂H₄ formed