23 Redox

Question 1

What is an oxidising agent?

Answer 1

A reagent that takes electrons from the species being oxidised.

Question 2

What is a reducing agent?

Answer 2

A reagent that adds electrons to the species being reduced.

Question 3

What is the equation for the manganate titration with iron?

Answer 3

MnO4(2-) + 5Fe (2+) + 8H+ -> Mn(2+) + 5Fe (3+) + 4H2O

Question 4

What is the equation for the iodine/thiosulfate titration?

Answer 4

2S2O3(2-) + I2 -> S4O6 (2-) + 2I (-)

Question 5

How do you identify the endpoint for the iodine/sodium thiosulfate titration?

Answer 5

Add starch, and the blue-black colour disappears

Question 6

What is a voltaic cell?

Answer 6

A voltaic cell is a type of electrochemical cell that converts chemical energy into electrical energy.

Question 7

How can a voltaic cell be made?

Answer 7

A voltaic cell can be made by connecting together two different half-cells.

Question 8

What does a metal/metal ion half cell consist of?

Answer 8

Metal rod (solid) dipped into solution of metal ions (aqueous)

Question 9

What does an ion/ion half cell consist of?

Answer 9

• It contains ions of the same element in different oxidation states.
• Inert metal electrode made out of platinum is used.

Question 10

What is the negative electrode in the cell?

Answer 10

The electrode with the more reactive metal loses electrons and is oxidised. This is the negative electrode.

Question 11

What is the positive electrode in the cell?

Answer 11

The electrode with the less reactive metal gains electrons and is reduced. This is the positive electrode.

Question 12

What is the standard electrode potential?

Answer 12

The e.m.f of a half-cell compared with a standard hydrogen half-cell, measured at 298K with solution concentrations of 1 mol dm-3 and a gas pressure of 100kPa.

Question 13

What is measured as a standard electrode potential?

Answer 13

The tendency to gain electrons and be reduced.

Question 14

How do you measure a standard electrode potential?

Answer 14

• Half-cell connected to a standard hydrogen electrode.
• Two ELECTRODES connected by a WIRE to allow controlled flow of electrons.
• Two SOLUTIONS connected with a SALT BRIDGE to allow flow of ions.

Question 15

What does a salt bridge typically contain?

Answer 15

A salt bridge typically contains a concentrated solution of an electrolyte that doesn’t react with either solution. E.g a strip of filter paper soaked in aqueous potassium nitrate

Question 16

The more negative the electrode potential value…

Answer 16

The greater the tendency to lose electrons and be oxidised.

Question 17

The more positive the electrode potential value…

Answer 17

The greater the tendency to gain electrons and be reduced.

Question 18

The more negative the electrode potential value…

Answer 18

The greater the reactivity of a metal in losing electrons.

Question 19

The more positive the electrode potential value…

Answer 19

The greater the reactivity of a non-metal gaining electrons.

Question 20

How do you calculate a standard cell potential?

Answer 20

(Positive electrode) - (Negative electrode)

Question 21

What are limitations of predictions using E values?

Answer 21

1) Electrode potentials may indicate the thermodynamic feasibility of a reaction, but give no indication of the rate of reaction.
2) If concentration of a solution is not 1 mol dm-3, then the value of the electrode potential will be different from standard value, and so affects overall cell potential.
3) Actual conditions used for reaction may be different from the standard conditions used to record E values.

Question 22

What is a primary cell?

Answer 22

• Primary cells are non-rechargeable and designed to be used once only.
• Reactions cannot be reversed so eventually chemicals used up.

Question 23

What are secondary cells?

Answer 23

• Secondary cells are rechargeable as the chemicals in the cell are regenerated.
  E.g lead-acid batteries used in car batteries

Question 24

What does a fuel cell use?

Answer 24

• A fuel cell uses the energy from the reaction of fuel with oxygen to create a voltage.
• Fuel cells can operate continuously as long as fuel and oxygen are supplied into the cell.
• They don’t need to be recharged.

Question 25

What is the electrolyte in a hydrogen fuel cell?

Answer 25

Acid or alkali electrolyte.