1.11 Electrode Potentials And Electrochemical Cells

Question 1

What is the definition of a cell?

Answer 1

Redox reactions that generates electrons

Question 2

Why is a high resistance voltmeter used in measuring EMF?

Answer 2

EMF may be very low so allows it to be measured

Question 3

Why is a saltbridge used in a cell?

Answer 3

Ions free to move and carry charge
To complete the circuit

Question 4

What is the order of a conventional cell notation?

Answer 4

Most positive on RHS
Most oxidised form on inside nearest salt bridge
Most reduced form on outside

Question 5

What are the features of the more reactive metal of a cell?

Answer 5

• More negative e cell value
• Therefore is oxidised
• Equilibrium lies to left

Question 6

What a re the features of the least reactive metal in a cell?

Answer 6

• More positive e cell value
• Therefore reduced
• Equilibrium lies to right

Question 7

What is the effect of changing conditions of a cell?

Answer 7

• If current is allowed rto flow reaction will occur until e cell falls to 0
• Use le chateliers principles to increase again

Question 8

What are the features of a standard hydrogen electrode?

Answer 8

• Pt electrode
• H+ aqueous solution is a conc of 1 moldm^-3
• H2 gas at 100kpa
• 298K
• Sits on the LHS

Question 9

What does it mean if overall E cell = +ve?

Answer 9

Reaction is spontaneous

Question 10

What can cause a reaction which has an E cell of +ve to not be spontaneous?

Answer 10

• High Ea
• Slow rate of reaction
• No visible reaction

Question 11

What is the process for answering questions on why reactions occur?

Answer 11

1. Work out reactants and write on arrows
2. Check that e cell works so that oxidation=more -ve
3. Check e- are equal and combine half equations
4. E potential reduced>E potential oxidised
5. E cell= E RHS- E LHS
   6 So E cell reduced oxidises e oxidised

Question 12

What are the three types of electrochemical cells?

Answer 12

• Primary- None rechargeable cells
• Secondary- Rechargeable cells
• Fuel cells-Require continual input of reactants

Question 13

Why can’t none rechargeable batteries be recharged?

Answer 13

• Negative electrode is often in casing so would get thinner
• Could potentially leak/explode

Question 14

What is the relationship between the recharge and discharge equation in Secondary e cells?

Answer 14

• Same equation just in reverse
• Same e cell just with opposite sign

Question 15

What are the two equations in lithium ion cells?

Answer 15

Li+ + CoO2 +e-n ><Li+[CoO2]-
Li+ +e- >< Li

Question 16

What is used instead of solution in a Lithium ion cell?

Answer 16

Powdered graphite as is very conductive

Question 17

What possible conditions are there in a hydrogen fuel cell?

Answer 17

Acid or Alkaline

Question 18

What is necessary for discharge and recharge to work?

Answer 18

For product to still be attached to electrode

Question 19

What are the equations for alkali conditions in a Hydrogen fuel cell?

Answer 19

2H2O + 2e- >< H2 + 2OH-
O2+2H2O+4e-><4OH-
Overall: 2H2 + O2 —> 2H2O

Question 20

What are the advantages and disadvantages of H2 fuel in a car?

Answer 20

Adv- No CO2,less Nox,SO2,CO, More efficient (more energy for kinetic rather than thermal)
Disadvantages- Expensive: Storing,transporting, electrolysis to make H2, Potentially dangerous if not stored properly

Question 21

How can alkaline fuel cells be made to have a higher e cell?

Answer 21

• Increasing temperature to increase rate of reaction