Chapter Four · Secondly: Electrolytic cells: practice questions

When a contact occurs between aluminum and copper, which metal is corroded firstly according to the local galvanic cells principle?

• ACopper, because it is less active than aluminum
• BAluminum, because it is the most active metal
• CBoth corrode at the exact same rate
• DNeither corrodes due to natural passivation

A steady electric current of 5 A is passed through an electrolytic cell for 2 minutes. Which of the following represents the quantity of electricity passed through the cell?

• A10 C
• B60 C
• C36000 C
• D600 C

Faraday's constant (96500 C) represents which of the following quantities?

• AThe charge carried by a single electron.
• BThe number of particles contained in one mole of a substance.
• CThe quantity of electricity carried by one mole of electrons.
• DThe mass of one mole of any metal deposited at the cathode.

In the context of protecting iron ships from corrosion, why is magnesium preferred over tin?

• AMagnesium is less active than iron, preventing galvanic cell formation.
• BTin acts as an anodic protector, which is less effective than magnesium.
• CMagnesium forms an organic layer that insulates the iron from salty water.
• DMagnesium acts as an anode and sacrifices itself to protect the iron chassis.

The same quantity of electricity is passed through three cells containing solutions of silver, copper and aluminium salts connected in series. According to Faraday's second law, the masses of the three metals deposited are in the ratio of their:

• Aatomic numbers.
• Bdensities.
• Cmolar masses.
• Dchemical equivalent masses (molar mass divided by valency).

Aqueous copper (II) sulphate solution is electrolysed using two copper electrodes. Which of the following correctly describes the concentration of the copper (II) sulphate solution during the process?

• AIt decreases because Cu2+ ions are only removed at the cathode.
• BIt falls to zero because all the copper is deposited at the cathode.
• CIt remains almost constant because copper dissolves at the anode as fast as it is deposited at the cathode.
• DIt increases because copper dissolves at both electrodes.

A current of 9.65 A is passed through acidified water for 1000 seconds, evolving hydrogen at the cathode (2H+ + 2e- -> H2). Which of the following represents the volume of hydrogen gas evolved measured at STP? (molar gas volume = 22.4 L/mol, F = 96500 C)

• A11.2 L
• B0.56 L
• C1.12 L
• D2.24 L

In an electrolytic cell containing copper chloride (CuCl2) solution, what specific half-cell reactions occur at the electrodes and what are their corresponding potential values?

• AOxidation at anode: Cu2+ + 2e- -> Cu° (potential = +0.34 V); Reduction at cathode: 2Cl- -> Cl2 + 2e- (potential = -1.36 V)
• BOxidation at anode: Cu° -> Cu2+ + 2e- (potential = -0.34 V); Reduction at cathode: Cl2 + 2e- -> 2Cl- (potential = +1.36 V)
• COxidation at anode: 2Cl- -> Cl2(g) + 2e- (oxidation potential = -1.36 V); Reduction at cathode: Cu2+ + 2e- -> Cu°(s) (reduction potential = +0.34 V)
• DOxidation at anode: 2Cl- -> Cl2(g) + 2e- (potential = +1.36 V); Reduction at cathode: Cu° -> Cu2+ + 2e- (potential = -0.34 V)

Why is copper of 99% purity subjected to an electrolytic purification process when intended for manufacturing electrical wires?

• ABecause 99% copper contains dissolved gases that lower the melting point of the wires
• BBecause impurities increase the reduction potential of copper beyond the standard cell potential
• CBecause impurities like iron, zinc, silver, and gold decrease its ability to conduct electricity and reduce its quality
• DBecause 99% copper is too dense to be drawn into fine wires without breaking

Copper is deposited from copper(II) sulphate solution using two different constant currents I1 and I2 (with I2 greater than I1); the mass of copper deposited is plotted against time for each, giving two straight lines (a) and (b) through the origin. Which of the following is correct?

• AThe steeper line (a) with the greater slope corresponds to the larger current I2
• BThe steeper line (a) with the greater slope corresponds to the smaller current I1
• CThe two lines do not pass through the origin because time starts later
• DBoth lines have the same slope because copper is the same metal