1.11 Electrode Potentials & Cells

Question 1

What happens when a rod of metal is in a solution of its own ions?

Answer 1

An equilibrium is set up between the solid metal and the aqueous metal ions

Question 2

Why are salt bridges necessary in electrochemical cells?

Answer 2

To complete the circuit without reacting with electrodes

Question 3

How do salt bridges balance charge?

Answer 3

Free movement of ions

Question 4

What symbol is used to represent a salt bridge in a cell representation?

Answer 4

||

Question 5

What type of species is written next to the salt bridge in cell representation?

Answer 5

Most oxidised species
(Highest charge)

Question 6

What does | indicate in cell representation?

Answer 6

Phase boundary

Question 7

What happens at the left hand electrode of a cell?

Answer 7

Oxidation
Half cell with most negative E° value

Question 8

What happens at the right hand electrode of a cell?

Answer 8

Reduction
Half cell with most positive E° value

Question 9

What conditions is a standard hydrogen electrode used in?

Answer 9

Temperature = 298K
Pressure = 100kPa
[H⁺] = 1.00 mol dm⁻³

Question 10

What is the standard hydrogen electrode used for?

Answer 10

Comparing other cells against
E° of SHE defined as 0

Question 11

Why are standard electrodes other than SHE used?

Answer 11

Cheaper - platinum is expensive
Easier
Quicker to use
Provide just as good a reference

Question 12

What is meant in terms of oxidising/reducing power of more negative E° values?

Answer 12

Better reducing agent - oxidised

Question 13

What is meant in terms of oxidising/reducing power of more positive E° values?

Answer 13

Better oxidising agent - reduced

Question 14

What factors change E° values?

Answer 14

Concentration of ions
Temperature

Question 15

What happens if the concentration of ions in the left half cell is reduced?

Answer 15

Equilibrium moves left to oppose change of removing ions
More electrons released so left E° more negative
emf of cell increases

Question 16

How is emf of a cell calculated from E° values?

Answer 16

E°(cell) = E°(right) - E°(left)

Question 17

When are platinum electrodes used?

Answer 17

When neither oxidised and reduced forms of a metal aren’t in a solid state

Question 18

Why is platinum chosen to add as an electrode?

Answer 18

Inert so doesn’t take part in electrochemistry
Good conductor to complete circuit

Question 19

How can it be predicted whether or not a reaction would occur in a cell?

Answer 19

Take 2 half equations
Find species being reduced
Calcualte its E° value minus E° value of oxidised species
If E°overall > 0, reaction occurs

Question 20

What is the commercial cell known as the Daniell cell made from?

Answer 20

Zinc/Copper

Question 21

What are zinc/carbon cells commonly known as?

Answer 21

Diposable batteries

Question 22

What are the two reactions that take place in zinc/carbon cells?

Answer 22

Zn oxidised Zn²⁺
NH₄⁺ reduced to NH₃ at carbon electrode

Question 23

How can cells be recharged?

Answer 23

Reactions are reversible
They are reversed by running a higher voltage through the cell than the cells E°

Question 24

What reactions occur at the electrodes of nickel/cadmium batteries?

Answer 24

Cd(OH)₂ (s) + 2e⁻ -> Cd (s) + 2OH⁻
NiO(OH) (s) + H₂O + e⁻ -> Ni(OH)₂ (s) + OH⁻

Question 25

What reactions occur at the electrodes of lead/acid batteries?

Answer 25

Pb + SO₄²⁻ -> PbSO₄ (s) + 2e⁻
PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ -> PbSO₄ + H₂O

Question 26

Where are lithium ion cells used?

Answer 26

Mobile phones
Laptops