Redox II

Question 1

What does an electrochemical cell do?

Answer 1

Converts chemical energy into electrical energy using a redox reaction

Question 2

Draw an electrochemical cell set up (zinc sulphate solution and copper sulphate solution):

Question 3

What is each metal that comes in contact with a solution of its ions called?

Answer 3

Half-cell

Question 4

What happens at the negative electrode?

Answer 4

• Oxidation
• Zn (s) –> Zn2+ (aq) + 2e-
• So Zn electrode gets lighter over time

Question 5

What happens at the positive electrode?

Answer 5

• Reduction
• Cu2+ (aq) + 2e- –> Cu(s)
• So Cu electrode gets heavier over time

Question 6

What does the overall equation look like?

Answer 6

Zn(s) + Cu2+ (aq) ⇌ Zn2+ (aq) + Cu(s)

Question 7

Describe what is happening in the digram:

Answer 7

1. The electrons flow in a clockwise direction, from the zinc rod to the copper
2. The salt bridge, completes the circuit by allowing the passage of ions from the copper sulphate solution to the zinc sulphate solution. This salt bridge is usually a strip of filter paper soaked in saturated potassium nitrate

Question 8

What does the standard hydrogen electrode measure?

Answer 8

Standard for half-cell potentials. It has a cell potential of 0.00V, measured under standard conditions

Question 9

What are the standard conditions?

Answer 9

• Solutions of 1.0 mol dm-3 concentration
• 298K
• 100KPa (1 atm)

Question 10

What is SHE used for?

Answer 10

For reference on all half-cell potentials as it has a standard electrode potential of zero

Question 11

What do positive potentials mean?

Answer 11

The substances are more easily reduced and will gain electrons

Question 12

What do negative potentials mean?

Answer 12

The substances are more easily oxidised and will lose electrons to be come more stable

Question 13

Draw an example of a standard hydrogen electrode that consists of HCl, H2 and platinum electrodes and give the equation of the reaction taking place:

Question 14

Why are platinum electrodes used?

Answer 14

They are metallic, so will conduct electricity, but inert, so will not interfere with the reaction

Question 15

How are conventional cells represented?

Answer 15

• The half-cell with the most negative potential goes on the left
• The most oxidised species from each half-cell goes next to the salt bridge
• A salt bridge is shown using a double line
• State symbols are always included

Question 16

How do you calculate cell Emf?

Answer 16

EMF∅ = E∅ cathode (RHS) - E∅ anode (LHS)

Question 17

What does E∅ being negative mean?

Answer 17

• Less likely to be reduced, more likely to be oxidised
• Position of equilibrium is to the left

Question 18

What does E∅ being positive mean?

Answer 18

• More likely to be reduced, less likely to be oxidised
• Position of equilibrium is to the right

Question 19

How do you calculate E∅ cell?

Answer 19

E∅cell = reduced - oxidised

Question 20

How do you know whether a reaction is feasible or not using electrode potentials?

Answer 20

A reaction is feasible if the E∅cell value is positive overall

Question 21

What are the limitations of predictions using E∅cells?

Answer 21

1. They have been worked out using a standard hydrogen electrode and done under standard conditions. However, this may not necessarily be the case when done under lab conditions
2. Sometimes the reaction is feasible but is prevented from happening because the reaction has a very high activation energy, which means that at the temperature being used the reaction will be extremely slow. “kinetically unfeasible”
3. Barrier eg/ Al2O3 layer on Al surface so Al + acid - really slow

Question 22

What does a hydrogen fuel cell look like?

Question 23

Give the half equations happening at the anode and the cathode of the hydrogen fuel cells?

Answer 23

Anode: H2 –> 2H+ + 2e-
Cathode: 4H+ + O2 + 4e- –> 2H2O

Question 24

Give the overall equation happening at the anode and the cathode of the hydrogen fuel cells?

Answer 24

2H2 + O2 –> 2H2O

Question 25

Why can this process be describe as green?

Answer 25

Only product is H2O, no CO2/CO/NOx produced
- However electricity to recharge the fuel cell may not be from ‘green’ sources

Question 26

Give some uses for a hydrogen fuel cell

Answer 26

Boats and submarines
Plane, trains, buses

Question 27

Name a suitable metal for making the electrodes from and explain your answer

Answer 27

Platinum - it’s inert so won’t react with H2O/H+

Question 28

Would a hydrogen fuel cell work indefinitely or would it eventually stop working?

Answer 28

No - in theory it should work indefinitely but actually the electrode can be ‘poisoned’ by binding to heavy metals or problems with leaking gases/cracks

Question 29

Are there any safety issues with this system?

Answer 29

• H2 is flammable + explosive if mixed with O2
• Very thick walled cylinders and pipes are needed to store hydrogen which has economic impacts
• The production of hydrogen is a by-product of the crude oil industry, which means it relies on a non-renewable, finite resource