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

What is the simplest salt bridge?

Answer 3

Fliter paper soaked in saturated solution of potassium nitrate

Question 4

How do salt bridges balance charge?

Answer 4

Allow free movement of ions

Question 5

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

Answer 5

||

Question 6

What type of species is written furthest from the salt bridge in cell representation?

Answer 6

Most reduced species
(Lowest charge)

Question 7

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

Answer 7

Most oxidised species
(Highest charge)

Question 8

What does | indicate in cell representation?

Answer 8

Phase boundary

Question 9

What happens at the left hand electrode of a cell?

Answer 9

Oxidation
Half cell with most negative E° value

Question 10

What happens at the right hand electrode of a cell?

Answer 10

Reduction
Half cell with most positive E° value

Question 11

What conditions is a standard hydrogen electrode used in?

Answer 11

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

Question 12

What is the standard hydrogen electrode used for?

Answer 12

Comparing other cells against
E° of SHE defined as 0

Question 13

Why are standard electrodes other than SHE used?

Answer 13

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

Question 14

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

Answer 14

Better reducing agent - oxidised

Question 15

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

Answer 15

Better oxidising agent - reduced

Question 16

What factors change E° values?

Answer 16

Concentration of ions
Temperature

Question 17

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

Answer 17

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

Question 18

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

Answer 18

E°(cell) = E°(right) - E°(left)

Question 19

When are platinum electrodes used?

Answer 19

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

Question 20

Why is platinum chosen to add as an electrode?

Answer 20

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

Question 21

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

Answer 21

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 22

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

Answer 22

Zinc/Copper

Question 23

What are zinc/carbon cells commonly known as?

Answer 23

Diposable batteries

Question 24

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

Answer 24

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

Question 25

How can cells be recharged?

Answer 25

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

Question 26

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

Answer 26

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

Question 27

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

Answer 27

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

Question 28

Where are lithium ion cells used?

Answer 28

Mobile phones Laptops