ES copq: Electrolysis; equilibrium Flashcards

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1
Q

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

A
  • Molten lead metal (cathode)
  • Bromine gas (anode)
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2
Q

At which electrode do the following occur?

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

Vowels + consonants match

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3
Q

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

A
  • Metals in group 1/2, Al and acids give hydrogen gas
  • Other metals are plated on cathode
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4
Q

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

A
  • 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
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5
Q

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

A

​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

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6
Q

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

A
  • Dissociation of water: H2O(l) → OH-(aq) + H+(aq)*
  • Half-equation: H+ + e- → 1/2H2*
  • Overall:* H2O(l) + e- 1/2H2 (g) + OH-(aq)
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7
Q

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

A
  • Dissociation of water: H2O(l)* → OH- + H+
  • Half-equation: OH- → 2e- + H+ + 1/2O2*
  • Overall:* H2O(l) → 2e- + 2H+(aq) + 1/2O2 (g)
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8
Q

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

A

4OH-(aq) → O2 (g) + 2H2O(l) + 4e-

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9
Q

Explain what is meant by “dynamic equilibrium”.

A
  • Closed system
  • Forward + reverse reactions occur at equal rates
  • Concentrations of products + reactants are constant (but not equal)
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10
Q

How do you calculate Kc (equilibrium constant)?

A

product of conc. of products

product of conc. of reactants

Raise each conc to power of moles in equation

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11
Q

Kc > 1

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

A
  • Product of conc. of products > that of reactants
  • Equilibrium lies to right
  • Higher concentration of products than reactants (both still constant)
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12
Q

Kc is dependent on what?

A

Temperature

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13
Q

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

A
  • Concentration term on denominator of Kc expression becomes larger
  • Kc is a constant, so numerator must also become larger
  • So equilibrium position shifts to right
  • More reactants react to form products
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14
Q

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

A

No effect

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

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15
Q

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

A
  • 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 Kc)*
  • The yield of a reaction is essentially the concentration of its products*
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16
Q

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

A
  • Increases rate, since a greater frequency of collisions have E ≥ Ea
  • 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 Kc)*
  • The yield of a reaction is essentially the concentration of its products​*
17
Q

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.

A
  • 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 Kc restored
18
Q

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.

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