Chapter 23 - Redox And Electrode Potentials

Question 1

Define redox

Answer 1

A reaction in which both reduction and oxidation take place.

Question 2

Define oxidation

Answer 2

The loss of electrons or an increase in oxidation number.

Question 3

Define reduction

Answer 3

The gain of electrons or a decrease in oxidation number.

Question 4

What is the Manganate half reaction?

Answer 4

MnO(4)- + 8H+ + 5e- -> Mn2+ + 4H(2)O

Question 5

What is the iodine sodium thiosulphate reaction?

Answer 5

2S(2)O(3)2- + I(2) -> S(4)O(6)2- + 2I-

Question 6

Why is there no clear colour change at the end point of a Vanadium and Potassium Manganate titration?

Answer 6

KMnO(4) is purple/pink

V2+ is violet

Question 7

What property does the voltmeter have in a cell?

Answer 7

High resistance

Question 8

Which way does the current flow in the cell?

Answer 8

Negative electrode to the positive electrode

Question 9

How do the ions flow between cells?

Answer 9

Salt bridge

Question 10

What is the salt bridge generally made of?

Answer 10

Potassium nitrate

Ammonium nitrate

Question 11

What is the electrode made of in a standard hydrogen cell?

Answer 11

Platinum

Question 12

What is in the half cell of Fe3+ and Fe2+?

Answer 12

Inert Pt electrode

Equimolar Fe2+ and Fe3+ (1 mol dm^-3)

Question 13

When two half cells are connected, which is oxidised and which is reduced?

Answer 13

More reactive metal is oxidised

Less reactive metal is reduced

Question 14

Define standard electrode potential (E°)

Answer 14

The emf 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 101kPa.

Question 15

What is the electrode potential of a hydrogen half cell?

Answer 15

0.00V

Question 16

What is the electrode potential of the cell?

Answer 16

Electrode potential(E°) of the more positive terminal minus the electrode potential(E°) of the more negative terminal

Question 17

How do we know if a cell is feasible?

Answer 17

If the electrode potential of the cell is positive, the reaction is feasible.

Question 18

Which electrode goes on the right when asked about a labelled diagram?

Answer 18

• The half-cell with the more +ve E° (electrode potential)

- The metal ions that are more easily reduced

Question 19

Which electrode goes on the left when asked about a labelled diagram?

Answer 19

• The half-cell with the more -ve E°

- The metal ions that are more easily oxidised

Question 20

What does the more positive value for E° mean for the position of the equilibrium?

Answer 20

• Equilibrium lies further to the RIGHT than the other half-cell
• So the metal ions are more easily REDUCED than the other half-cell.
• So the metal is an OXIDISING agent

Question 21

What does the more negative value for E° mean for the position of the equilibrium?

Answer 21

• Equilibrium lies further to the LEFT than the other half-cell
• So the metal ions are more easily OXIDISED than the other half-cell.
• So the metal is an REDUCING agent

Question 22

Does it go Bigger or smaller, Metal ion –> Metal Solid
E.G. The copper
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu

Answer 22

-gets Bigger, metal ion to metal solid
-Less concentrated
-Copper is REDUCED
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu

Question 23

Does it go Bigger or smaller, Metal solid –> Metal ion?
E.G. the chromium
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu

Answer 23

-gets Smaller, metal solid to metal ion,
-More Concentrated
-Chromium is OXIDISED
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu

Question 24

State 2 reasons why the actual cell potential may be different from the value calculated?

Answer 24

• Non- standard conditions

- High Ea, so very slow rate of reaction

Question 25

What’s in a Metal/Metal ion half cell?

E.g. Cu2+ | Cu half cell

Answer 25

• Metal rod (Cu)

- dipped into aq solution of metal ion (Cu2+)

Question 26

What’s in a Ion/ ion half-cell?

E.g. Fe3+ | Fe2+

Answer 26

• Contains ions of the same element in different oxidation states ( Equimolar of Fe3+ & Fe2+ (1 moldm-3
• Pt electrode

Question 27

Why does Fe2+ | Fe half cell sometimes form the metal ion Fe3+?

Answer 27

• Fe2+ oxidises in air to form Fe3+ which has a partially filled d-subshell so gives greater stability.
• So Fe3+ is a more likely product

Question 28

What’s in a Standard Hydrogen half cell?

H2 | H+

Answer 28

• inert Pt electrode
• Glass tube with holes in to allow bubbles of H2(g) to escape
• H2(g) coming in through the top at 298K,1 atm
• Acid solution at bottom containing 1.0 moldm-3 of H+(aq)

Question 29

What is the overall cell eqn and both half eqns for the zinc-copper cell?
Zn2+ +2e- –> Zn , -0.76 V
Cu2+ + 2e- –> Cu , +0.34 V

Answer 29

Red: Cu2+ + 2e- –> Cu (MP)
Oxi: Zn –> Zn2+ + 2e- (MN)
Ovll: Zn + Cu2+ –> Zn2+ + Cu

Question 30

State the key difference between a primary cell and a secondary cell.

Answer 30

• A primary cell is only used one and is non-rechargeable

- A secondary cell is rechargeable

Question 31

State the key characteristics of a fuel cell.

Answer 31

• A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage.
• A fuel cell can be run continuously provided that the fuel and oxygen are continually being supplied to the cell.