DM 3 - Redox

Question 1

What is a half-cell?

Answer 1

A half-cell contains the chemical species present in a redox half equation

Question 2

What is a metal/metal ion half-cell?

Answer 2

• A half-cell where a metal rod is dipped in a solution containing an ion of the same metal
• An equilibrium will be set up at the boundary where the metal is in contact with its ions

Question 3

What is an ion/ion half-cell?

Answer 3

• An ion/ion half-cell contains a solution of ions of the same element but of different oxidation states
• The electron is usually graphite or platinum

Question 4

What is a standard electrode potential?

Answer 4

The EMF/voltmeter reading when a half-cell of interest is connected to a hydrogen half-cell via a salt bridge, with all solutions having a concentration of 1 moldm-3, under standard conditions (101 or 100 kPa and 298K)

Question 5

What is a hydrogen half-cell?

Answer 5

• A half-cell containing hydrogen gas and a solution containing hydrogen ions
• An inert platinum electrode is used to allow electrons into and out of the half-cell
• 2H+(aq) + 3e- <=> H2(g)

Question 6

How do you use standard electrode potentials to calculate the standard cell potential, E0cell?

Answer 6

E0cell = E0(positive electrode) - E0(negative electrode)

Question 7

What is an electrochemical cell?

Answer 7

• Two different half-cells are connected by a salt bridge, with their electrodes connected to a voltmeter (measures EMF/cell potential)
• This allows the flow of electrons
• It generates electrical energy from chemical redox reactions

Question 8

What is a salt bridge?

Answer 8

• A salt bridge allows the transfer of ions
• It is typically a concentrated solution of an electrolyte i.e. KNO3(aq) that doesn’t react with either half cell solution

Question 9

Why may a graphite or platinum electrode be used?

Answer 9

They are very unreactive i.e. will not react with the half-cell solutions and will not affect the voltmeter readings. Usually used in ion/ion half-cells

Question 10

How do you predict which half-cell is being oxidised and which one is being reduced?

Answer 10

• The more negative the E0 value, the greater the tendency for the system to be oxidised
• The more positive the E0 value, the greater the tendency for the system to be reduced
• Hence the half-cell with the more positive E0 is oxidised and the half-cell with the more negative E0 is reduced

Question 11

How do you predict the feasibility of a redox reaction?

Answer 11

• The reaction is feasible if the oxidising agent has a lower standard cell potential than the reducing agent
• The greater the difference in E0 value, the more likely the reaction to occur

Question 12

Why may a reaction not occur?

Answer 12

• Non-standard conditions (298 K, 101 kPa, 1 moldm-3 solutions)
• Ambient energy of the system is lower than the activation energy

Question 13

What is rusting?

Answer 13

• Rust in an iron oxide, (usually red) formed from the redox reaction of iron and oxygen in the presence of water/moisture
• Iron is oxidised, oxygen (dissolved in water) is reduced. The products of this redox reaction then react to form a hydrated iron oxide

Question 14

How do you prevent rusting?

Answer 14

• To protect iron from rust, the surface needs to be separated from air and water, i.e. by keeping the metal in oil
• Coatings can be applied to iron
• Alloys (e.g. stainless steel) can be made to prevent rusting