Chapter Three : Converting chemical energy to electrical energy

Question 1

Redox reactions definition

Answer 1

Reactions that involve the transfer of one or more electrons between chemical species

Question 2

Reduction definition

Answer 2

A decrease in oxidation number; a gain of electrons

Question 3

Oxidation definition

Answer 3

An increase in the oxidation number; a loss of electrons

Question 4

Oxidising agents definition

Answer 4

Electron acceptors

Question 5

Reducing agents

Answer 5

Electron donors

Question 6

Galvanic cells

Answer 6

Consists of

• 2 half cells with an anode and cathode
• a conducting wire (external circuit)
• a salt bridge (internal circuit)

Question 7

Cathode

Answer 7

Positive electrode
Reduction occurs here
Strongest oxidising agent

Question 8

Anode

Answer 8

Negative electrode
Oxidation occurs here
Strongest reducing agent

Question 9

Three types of half cells

Answer 9

Metal ion-metal half cell
The solution half cell
The gas-non-metal half-cell

Question 10

The metal ion-metal half cell

Answer 10

Contains a metallic electrode
Ion solution (ion of metal)

Question 11

The solution half cell

Answer 11

Inert electrode

Solution with 2 ions

Question 12

The gas-non-metal ion half cell

Answer 12

Inert electrode
Gas bubbled into solution
Ions of the gas in solution

Question 13

Standard electrode potential definition

Answer 13

Potential to gain an electron

Question 14

Electronic potential definintion

Answer 14

The ability of the cell to produce an electric current

Question 15

The reduction potential definition

Answer 15

Is a measure of the tendency of an oxidising agent to accept electrons

Question 16

Standard cell potential difference definition

Answer 16

The measured cell potential difference under standard conditions

Question 17

Cell potential difference equation

Answer 17

E cell = E oxidising agent - E reducing agent

Question 18

The unpredictability of rate of reaction

Answer 18

Although reactants may be spontaneous they may nt happen quickly
If conditions are too different from the standard it could change the order of the electrochemical series

Question 19

Primary cells definition

Answer 19

A cell in which the cell reaction is not reversible

Question 20

Secondary cells definition

Answer 20

Cells in which the cell reaction is reversible

Question 21

Examples of non rechargeable cell

Answer 21

Dry cell
Button cell
Alkaline cell

Question 22

Examples of rechargeable cell

Answer 22

Lithium ion

Lead-acid

Question 23

Examples of fuel cells

Answer 23

Hydrogen-oxygen

Methane-oxygen

Question 24

Factors affecting selection of cells

Answer 24

Initial and operating cost
Size and shape
Mass
Memory effect
Voltage provided
Discharge curve
Current
Shelf life
Ease of disposal
Environmental issues

Question 25

Fuel cells

Answer 25

An electrochemical device that converts chemical energy into electrical energy

Question 26

Design of fuel cell

Answer 26

``` Anode and cathode are in the same cell Porous electrodes Reactants are often gases Oxygen is always at cathode H+ means cell is acidic OH- means cell is alkaline ```

Question 27

Acidic fuel cell reactants

Answer 27

H+, H2O, H2, O2

Question 28

Alkaline fuel cell reactants

Answer 28

OH-, H2O, O2, H2

Question 29

Anode and cathode change

Answer 29

Cathode increases in mass as cell functions | Anode remains unchanged as cell functions

Question 30

Why is a fuel cell compared to power station?

Answer 30

Multistep process in the factory | Gas has to combust, trun the tubine etc.

Question 31

Advantages of fuel cells

Answer 31

High energy conversion efficiency Low chemical pollution Fuel flexibility

Question 32

Disadvantages of fuel cells

Answer 32

Manufacturing process and materials are expensive Need a reliable and continuous supply of fuel Distribution and storage of hydrogen

Question 33

Purpose of porous electrodes

Answer 33

Maximise surface area available for the electrode reaction, increasing rate of reaction and current output. doesnt increase cell p or voltage

Question 34

How to increase the rate of reaction in a fuel cell

Answer 34

Making electrodes porous | Using electrodes that also catalyse the reaction

Question 35

What happens when galvanic cells are in the same cell?

Answer 35

The identical reactions still occur but the electrons travel directly rather than in an external circuit Energy is released as heat rather than being converted to electrical energy