3.1 Redox And Standard Electrode Potential

Question 1

Oxidation in terms of electron transfer

Answer 1

Loss of electrons

Question 2

Reduction in terms of electron transfer

Answer 2

Gain of electrons

Question 3

Steps for separating a redox equation into two half equations

Answer 3

1. Separate parts associated with each element
2. If applicable - if oxygen is in a half equation then all water and hydrogen ions mist be associated with that half equation
3. If applicable- simplify
4. Balance the charges using electrons

Question 4

How can a redox half equation be replicated as an experiment in the lab

Answer 4

In an electrochemical half cell

Question 5

Metal/metal ion half cell

Answer 5

Electrode made of the metal dipped into a 1 moldm-3 solution of the metal ions

Question 6

Solution half cell

Answer 6

• an element in an aqueous ion is oxidised or reduced to form different aqueous ions
• consists of a solution made up of both forms of concentration 1 moldm-3 and an electrode made of platinum and coated with fine platinum powder (platinum black)

Question 7

Gas half cell

Answer 7

• reduced or oxidised species is a gas and the other species is an aqueous ion
• made up on 1 moldm-3 solution of the aqueous ions associated with the gas, platinum black electrode dipped into solution and a glass cover over the platinum electrode that surrounds the electrode in a steady stream of the gas at 1 atm

Question 8

What is a cell diagram

Answer 8

A way of representing the electrochemical cell by showing the different species involved in the redox reactions in both half cells

Question 9

Rules for setting up cell diagrams

Answer 9

1. Metal electrodes are placed far left and far right of the cell and shod include any platinum electrodes used
2. A double vertical line separates the 2 half cells - shows the salt bridge
3. If there is a change in state between the 2 species in a half cell then a single vertical line is shown as a phase boundary
4. If there is no change between states it is shown as a comma
5. The reduced form of each half cell is either the conducting metal or the species next to the platinum electrode
6. The oxidised form of each half cell is closest to the salt bridge

Question 10

What does a cell diagram mirror

Answer 10

How the cell should be set up

Question 11

What is standard electrode potential (E•)

Answer 11

The potential difference when any half cell is connected to the standard hydrogen electrode under standard conditions

Question 12

How can a cell potential of an electrochemical cell be measured

Answer 12

With a high-resistance voltmeter

Question 13

Can the individual potential of a half-cell be accurately measured alone

Answer 13

No

Question 14

What is needed to obtain the individual reduction potential of a half-cell

Answer 14

Standard electrode potential

Question 15

What is the electrode potential of the standard hydrogen electrode

Answer 15

0 volts

Question 16

How can standard electrode potential be measured

Answer 16

By pairing it with the standard hydrogen electrode and measuring the cell potential

Question 17

What is the oxidation potential of an electrode

Answer 17

The negative of its reduction potential

Question 18

The more positive the standard electrode potential the more likely the system is to…

Answer 18

Gain electrons (be reduced)

Question 19

If 2 half cells are connected the electrons flow from..

Answer 19

The more negative to more positive half cell

Question 20

Equation that shows how the reading on the high-resistance voltmeter is given

Answer 20

Standard electrode potential = E•(more positive) - E•(less positive)

Question 21

Standard hydrogen electrode- how is it set up

Answer 21

Hydrogen gas at a pressure of 1 atm bubbles over an inert platinum electrode dipped in 1 moldm-3 H+(aq) at a temperature of 298K

Question 22

Which terminal and side of the circuit is the standard hydrogen electrode connected to

Answer 22

Positive terminal
Left hand side of the circuit

Question 23

If the voltmeter gives a positive reading during standard hydrogen electrode it means

Answer 23

The hydrogen gas is oxidised to H+ ions
Electrons flow from left to right

Question 24

If a voltmeter gives a negative reading during standard hydrogen electrode it means

Answer 24

The H+ ions are reduced to hydrogen gas
Electrons flow from right to left

Question 25

What is EMF

Answer 25

The maximum potential difference which is present between two electrodes

Question 26

Rules to follow to calculate cell EMF using the equation E•cell=E•(more positive)-E•(more negative)

Answer 26

• most negative half-cell is on the left hand side of the cell and connected to the positive terminal of the voltmeter
• the left hand side electrode always undergoes oxidation and so is the negative electrode as the electrons are released at this electrode

Question 27

State the function of the salt bridge

Answer 27

To complete the circuit by allowing ions to move whilst also preventing the solutions from mixing

Question 28

What must the EMF be in order for the redox reaction to be feasible

Answer 28

Positive after using the equation
E•(cell) = E•(reduction)-E•(oxidation)

Question 29

How are fuel cells similar to electrochemical cells

Answer 29

They convert chemical energy into electrical energy through a redox reaction

Question 30

How are fuel cells different to electrochemical cells

Answer 30

They require a continuously flowing source of fuel as well as a continuously flowing source of an oxidising agent

Question 31

Fuel cells can produce electricity continuously as long as what 2 things are supplied?

Answer 31

Fuel
Oxygen

Question 32

Are fuel cells efficient?

Answer 32

Yes, very
Convert between 50-70% of the chemical energy in the fuel into electrical energy
The remaining energy is lost as heat

Question 33

Why is hydrogen fuel cells being trialed as an alternative for powering vehicles?

Answer 33

It releases energy more efficiently and is much lighter than traditional batteries

Question 34

What happens during a hydrogen fuel cell

Answer 34

The fuel (Hydrogen gas) is passed over the platinum electrode and so the platinum acts as a catalyst
At the anode hydrogen gas is oxidised to H+ ions
The H+ ions then pass through a semi-permeable membrane and reach the cathode where they accept electrons and react with oxygen to form water

Question 35

Overall reaction of hydrogen fuel cell

Answer 35

O2 + 2H2 –> 2H2O

Question 36

Reaction that occurs at the anode of hydrogen fuel cell

Answer 36

H2 –> 2H+ + 2e-

Question 37

Reaction that occurs at cathode of hydrogen fuel cell

Answer 37

O2 + 4H+ + 4e- –> 2H2O

Question 38

Advantages of hydrogen fuel cell

Answer 38

• water is the only waste product - no carbon dioxide or greenhouse gases produced
• far less energy is lost as heat - more efficient
• hydrogen gas can be produced by the electrolysis of water - a renewable energy source

Question 39

Disadvantages of hydrogen fuel cells

Answer 39

• hydrogen gas is highly flammable - difficult and expensive fuel to store
• most common source of hydrogen is reformation of methane from fossil fuels - crude oil is still used and so carbon dioxide still formed when hydrogen is used just it is produced at an earlier stage
• electrolysis to produce hydrogen requires a lot of energy - this will cause the production of carbon dioxide unless electricity is produced from a renewable source