9. Redox Processes

Question 1

What is a redox reaction?

Answer 1

The simultaneous reduction and oxidation.

Question 2

What is an oxidation number?

Answer 2

A theoretical description of the charge that an atom would have if it were to be bonded ionically.

Question 3

What does an increase in oxidation number mean?

Answer 3

Oxidation has occurred.

Question 4

List the 8 rules of oxidation numbers.

Answer 4

1. Uncombined elements have an O.N of 0.
2. The O.N on a monatomic ion= the charge on an ion.
3. Group 1= +1
4. Group 2= +2
5. Fluorine= -1
6. Hydrogen= +1
7. Oxygen= -2
8. Sum of O.N= total charge on the compound.

Question 5

What is disproportionation?

Answer 5

The simultaneous oxidation and reduction of an element in a single species.

Question 6

Give an example of a disproportionation reaction.

Answer 6

the decomposition of hydrogen peroxide.

Question 7

What is a primary cell also known as?

Answer 7

A voltaic cell

Question 8

Summarise how primary cells work.

Answer 8

They use irreversible chemical reactions to generate an electrical current.

Question 9

What is a half-cell?

Answer 9

A metal in contact with an aqueous solution of its own ions.

Question 10

What does a primary voltaic cell look like?

Answer 10

Two different half cells connected by an external wire and a salt bridge.

Question 11

What is the purpose of a salt bridge in a primary cell?

Answer 11

Allows the free movement of ions.

Question 12

What are salt bridge made of?

Answer 12

A filter paper/ glass tube with a saturated solution of KNO3.

Question 13

What does the voltage produced by a primary cell depend on?

Answer 13

The relative difference between the two metals in the activity series.

Question 14

Where is the half cell undergoing reduction placed?

Answer 14

On the right

Question 15

In a primary voltaic cell, what is the negative electrode called?

Answer 15

Anode.

Question 16

In a primary voltaic cell, what is the positive electrode called?

Answer 16

Cathode.

Question 17

What process occurs at the anode?

Answer 17

Oxidation

Question 18

What process occurs at the cathode?

Answer 18

Reduction

Question 19

Give a summary of a secondary cell.

Answer 19

A rechargeable cell that involves redox reactions that can be reversed using electrical energy.

Question 20

What is a battery?

Answer 20

An electrochemical cell in which chemical energy is converted into electrical energy.

Question 21

Give an example of a primary cell.

Answer 21

Zinc - copper

Question 22

Why does an electrochemical cell have internal resistance?

Answer 22

It takes time for the ions to diffuse.

Question 23

What direction is the electron flow in a primary voltaic cell?

Answer 23

Anode –> Cathode

Question 24

What are the three advantages of fuel cells that use alcohol biofuels?

Answer 24

• Growing renewable crops to make biofuel absorbs CO2
• Preserves finite fossil fuels.
• H2 production could use a renewable source.

Question 25

What are the three disadvantages of fuel cells that use alcohol biofuels?

Answer 25

- Fuel cell releases CO2. - Land is taken which could be used for food. - CO2 is released from the agriculture refinement.

Question 26

Summarise fuel cells.

Answer 26

Require a constant supply of fuel, which undergoes a redox reaction.

Question 27

What can be used as fuel in a fuel cell?

Answer 27

hydrogen and methanol

Question 28

Give the half equation at the anode for a methanol fuel cell.

Answer 28

CH3OH + H2O --> CO2 + 6H+ + 6e-

Question 29

Give the half equation at the cathode for a methanol fuel cell.

Answer 29

4H+ + O2 + 4e- --> 2H2O

Question 30

What is the overall reaction for a methanol fuel cell?

Answer 30

2CH3OH + 3O2 --> 2CO2 + 4H2O

Question 31

Give the equation for the anode in a hydrogen fuel cell.

Answer 31

H2 --> 2H+ + 2e-

Question 32

Give the equation for the cathode in a hydrogen fuel cell.

Answer 32

4H+ + 4e- + O2 --> 2H2O

Question 33

What is the overall equation for a hydrogen fuel cell?

Answer 33

2H2 + O2 --> 2H2O

Question 34

What carries the charge across the bridge in a fuel cell?

Answer 34

Protons

Question 35

What are the 6 advantages of a hydrogen fuel cell?

Answer 35

- Only waste product is H2O - H2 leak has limited impact - Quieter than conventional engines - Expelled steam can be used to generate energy. - Moderate operating range of 150-200C (lower operating cost) - Jobs created when funding is invested.

Question 36

What are the 3 disadvantages of a hydrogen fuel cell?

Answer 36

- Pressurised hydrogen is flammable - H2 produced either by electrolysis or burning fossil fuels. - No refill stations and no vehicles get converted.

Question 37

How does charging a battery work?

Answer 37

The external current reverses the chemical reactions at each half cell, raising the cell's potential difference until it can be discharged again.

Question 38

How are lead-acid cells used?

Answer 38

In cars to provide the initial power to start the motor.

Question 39

Give the equation at the anode for a lead-acid cell.

Answer 39

Pb (s) + SO4^2- (aq) --> PbSO4 (s) + 2e-

Question 40

Give the equation at the cathode for a lead-acid cell.

Answer 40

PbO2 (s) + SO4^2- (aq) + 4H+ + 2e- --> PbSO4 (s) + 2H2O (l)

Question 41

What is the overall equation for a lead-acid cell?

Answer 41

Pb (s) + PbO2 (s) + 2SO4^2- (aq) + 4H+ --> 2PbSO4 (s) + 2H2O (l)

Question 42

What temperature does molten electrolysis require?

Answer 42

1000C