20.1 - 20.3 Electrode potentials and electrochemical cells

Question 1

Define standard electrode potential

Answer 1

The potential difference, in volts, of a substance in an aqueous solution of its ions relative to the standard hydrogen electrode. Under standard conditions; 298k, 100kPa and 1 mol dm-3 solutions

Question 2

What physical changes can be noted if electrons are allowed to flow between two half cells?

Answer 2

One electrode dissolves:
- Increases the conc of the equivilant ion in solution

Build up on the other electrode:
- Electrons combine with the ion to form more solid
- Decreases the conc of the equivilant ion in solution

Question 3

What is a salt bridge and why is it used instead of a wire?

Answer 3

Filter paper soaked in a solution of salt (usually saturated potassium nitrate)

It is used rather than a piece of wire to avoid introducing further metal ions to the circuit and altering the potential

Question 4

Define standard hydrogen electrode

Answer 4

The standard reference electrode which other half cells are connected to in order to measure e.m.f

The e.m.f of the standard hydrogen electrode is 0.00V

Place on the left

Question 5

Describe the standard hydrogen electrode

Answer 5

• Hydrogen gas is bubbled into a solution of H+(aq) ions
• H doesn’t conduct so electrical contact is made via a piece of unreactive platinum metal
• The platinum is coated with finely divided platinum to increase the surface area and allow any reaction to proceed rapidly
• The electrode is used under standard conditions

Question 6

How to calculate e.m.f?

Answer 6

Enote (RHS) - Enote (LHS)

Question 7

What is necessary for a reaction to be feasible?

Answer 7

The e.m.f value must be positive

Question 8

Describe the correlation between no. of electrons and Enote value

Answer 8

Higher no e- = Lower Enote value

Question 9

Describe how to write the standard hydrogen electrode

Answer 9

Pt(s)/H2(g)/H+(aq)// …

Question 10

Describe non-rechargeable cells

Answer 10

• Chemicals are used up overtime and emf drops
• Once one or more of the chemicals have been completely used up, the cell is flat and the emf is 0V
• These cells cannot be recharged and have to be disposed of after single use

Question 11

Describe zinc-carbon non-rechargeable cells

Answer 11

• Cheaper but has a shorter life
• Useful for short intermittent current
• Zinc walls of the battery get used up during its life so as they become thinner, the battery is prone to leakage
• e.g doorbell

Question 12

Describe non-rechargeable alkaline cells

Answer 12

• More expensive but has a longer life
• Useful for continuous high current
• e.g torches, radios

Question 13

Describe rechargeable cells

Answer 13

• Reactions are reversible
• Can be reversed by applying an external current to regenerate the chemicals

Question 14

Describe Li ion rechargeable cells

Answer 14

• Used in phones, tablets, cameras, laptops e.t.c

Li⁺(aq) + CoO₂(s) + e⁻ ⇌ LiCoO₂(s)
Li(s) ⇌ Li⁺(aq) + e⁻

Question 15

Describe fuel cells

Answer 15

• Have continuous supply of the chemicals into the cell
• Neither run out or need recharging
• e.g hydrogen-oxygen fuel cell

Question 16

Define battery

Answer 16

A number of cells connected together

Question 17

Describe a nickel cadmium rechargeable cell

Answer 17

• Expensive
• Can be recharged up to 500 times

Question 18

Describe a lead-acid rechargeable battery

Answer 18

• Made of 6 cells
• Used in cars
• When engine is running, battery is recharged by the cars generator

Question 19

Give the half-equations in an alkaline hydrogen-oxygen fuel cell

Answer 19

H₂(g) + 2OH⁻(aq) ⇌ 2H₂O(l) + 2e⁻
O₂(g) + 2H₂O(l) + 4e⁻ ⇌ 4OH⁻(aq)

Question 20

Give the advantages of non-rechargeable cells

Answer 20

• Cheaper
• Small/portable

Question 21

Give the disadvantages of non-rechargeable cells

Answer 21

• Cost more long term
• Short life span
• More waste produced than with rechargeables
• Landfill issues
• Leakage

Question 22

Give the advantages of a rechargeable cell

Answer 22

• Less cost long term
• Environmentally friendly
• Long life-span
• Portable

Question 23

Give the disadvantages of a rechargeable cell

Answer 23

• More expensive
• Limited capacity
• Need to be recharged
• Landfill waste

Question 24

Give the advantages of a fuel cell

Answer 24

• Renewable
• Energy efficient
• Minimal carbon residue
• Minimal sound pollution
• Environmentally friendly (only byproduct is water)

Question 25

Give the disadvantages of a fuel cell

Answer 25

• Lack of infrastructure to distribute hydrogen
• Hydrogen is expensive to produce and not widely available
• Explosive
• Platinum is expensive

Question 26

Define electrochemical series

Answer 26

List of electrode potentials in numerical order

Question 27

Why might the actual emf of a cell be different from the emf calculated?

Answer 27

Non-standard conditions

Question 28

What alteration do you make to a cell after measuring its emf to allow the cell to go to completion?

Answer 28

Remove the voltmeter