3.1 - Redox and Standard Electrode Potential

Question 1

Define oxidation in terms of electron transfer

Answer 1

Loss of electrons

Question 2

Define reduction in terms of electron transfer

Answer 2

Gain of electrons

Question 3

What is a redox reaction?

Answer 3

A redox reaction is a reaction in which oxidation and reduction occur on different species simultaneously.

Question 4

What is standard electrode potential?

Answer 4

Standard electrode potential is the potential across the electrodes when a redox system is connected to a hydrogen half-cell under standard conditions.

Question 5

What conditions are required for measuring the standard electrode potential?

Answer 5

• 298 K temperature
• 100 kPa pressure
• 1.00 mol dm-3 concentration of ions

Question 6

Why is a hydrogen half-cell needed as a reference?

Answer 6

The hydrogen half cell is used to allow for easy comparison between the electrode potential of different elements. The standard electrode potential for hydrogen is assumed to be zero volts at any temperature.

Question 7

What is the experimental setup to calculate the standard electrode potential for zinc?

Answer 7

Question 8

Why must metal electrodes be cleaned with sandpaper before creating an electrochemical cell?

Answer 8

To remove any metal oxide that has formed on the surface and improve electrical conductivity.

Question 9

Describe the movement of electrons in an electrochemical cell

Answer 9

Electrons flow through the wire from the positive electrode to the negative electrode.

Question 10

Why is a salt bridge used in an electrochemical cell?

Answer 10

• To maintain the charge balance and complete the circuit.
• Negative electrons are moving from one half cell to another. Without the salt bridge, positive charge would build up in the half cell containing the anode and negative charge would build up in the half cell containing the cathode. This would cause the reaction to stop.

Question 11

Why must an inert salt be used in the salt bridge?

Answer 11

The salt must be inert so that it doesn’t react with the solutions and alter the ion concentrations. If a reactive salt was used, the cell potential would change.

Question 12

What moves across the salt bridge?

Answer 12

Ions

Question 13

For what range of cell potential values is a process feasible?

Answer 13

Cell potential must be greater than 0.

Question 14

How can cell potential be calculated?

Answer 14

Eθcell =
Eθ(positive terminal) - Eθ(negative terminal)

Question 15

Why might theoretical cell potential values be different to values obtained experimentally?

Answer 15

Conditions may be non-standard.

Question 16

Draw a diagram showing a standard Fe2+/Fe3+ cell

Answer 16

Question 17

A cell is made up of the following half cells: Ag+(aq) + e- ⇌ Ag(s) Eθ = +0.80V Cu2+(aq) + 2e- ⇌ Cu(s) Eθ = +0.34V Write the overall cell equation and calculate the standard cell potential

Answer 17

Write the overall cell equation and calculate the standard cell potential
2Ag+(aq) + Cu(s) → Ag(s) + Cu2+(aq) Eθcell = +0.80 - (+0.34) = 0.46V

Question 18

In an electrochemical cell, is the more negative half cell oxidised or reduced?

Answer 18

Oxidised

Question 19

What is a fuel cell?

Answer 19

A cell that continually produces a voltage as long as it is supplied with oxygen and a fuel (like hydrogen).

Question 20

What is the only product of a hydrogen-oxygen fuel cell?

Answer 20

Water

Question 21

How does a hydrogen-oxygen fuel cell work?

Answer 21

Hydrogen and oxygen are pumped through porous electrodes. The electrolyte is often an acid such as phosphoric acid.
Hydrogen and oxygen react, producing energy and water.

Question 22

What are the two half equations taking place in a hydrogen-oxygen fuel cell?

Answer 22

2H2+ 4OH- ⇌ 4H2O + 4e- O2 + 2H2O + 4e- ⇌ 4OH-

Question 23

Write an equation for the overall reaction that takes place in a hydrogen-oxygen fuel cell

Answer 23

2H2 + O2 → 2H2O

Question 24

What are the advantages of using fuel cells?

Answer 24

• No pollution.
• They produce more energy than an
  alternative fuel like petrol.
• The process is continuous as long as the fuel is supplied.

Question 25

What are the disadvantages of using fuel cells?

Answer 25

• Materials used to make them are expensive.
• High pressure tanks are required to store oxygen and fuels like hydrogen.
• Hydrogen is expensive and hard to store.
• Efficiency is affected by temperature.