1.11 Electrode Potentials and Cells

Question 1

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

Answer 1

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

Question 2

Write a half-equation for zinc (s) to zinc (II)

Answer 2

Zn(s) ⇌ Zn2^+ (aq) + 2e-

Question 3

Write a half-equation for copper (II) to copper (III)

Answer 3

Cu2+(aq) ⇌ Cu3+ (aq) + e-

Question 4

What is the simplest salt bridge made of?

Answer 4

Filter paper soaked in saturated solution of KNO3 (potassium nitrate)

Question 5

Why are salt bridges necessary?

Answer 5

Complete the circuit, but avoid further metal/ion potentials as does not perform electrochemistry. Allow ion movement to balance the change. Do not react with electrodes

Question 6

What symbol is used to represent a salt bridge in standard notation?

Answer 6

||

Question 7

What type of species goes on the outside (furthest from the salt bridge) in standard cell notation

Answer 7

The most reduced species

Question 8

What does | indicate?

Answer 8

Phase boundary (solid/liquid/gas)

Question 9

How would an Aluminium/copper cell be represented?

Answer 9

Al(s) | Al3+ (aq) || Cu2+ (aq) | Cu(s)

Question 10

What happens at the left hand electrode?

Answer 10

Left hand electrode is where oxidation occurs. Left hand electrode is the half cell with the most negative E^0 value

Question 11

What happens at the right-hand electrode?

Answer 11

Right hand electrode is where reduction occurs. Right hand electrode is the half cell with the most positive E^0 value

Question 12

Which side of the cell has the most negative E0 value? What happens to the metal with the most negative E^0 value?

Answer 12

Oxidation- left hand electrode

Question 13

Draw the standard hydrogen electrode

Question 14

What conditions is the standard hydrogen electrode used in?

Answer 14

Temp = 298K
Pressure = 100kPa
[H+] = 1.00 mol dm^-3

Question 15

Why might you use other standard electrodes occasionally?

Answer 15

They are cheaper/easier/quicker to use and can provide just as good a reference

Platinum is expensive

Question 16

If an E^0 value is more negative, what does it mean in terms of oxidising/reducing power?

Answer 16

Better reducing agent (easier to oxidise)

Question 17

If an E^0 value is more positive, What does it mean in terms of oxidising/reducing power?

Answer 17

Better oxidising agent (easier to reduce)

Question 18

What factor will change E^0 values?

Answer 18

Concentration of ions
Temperature

Question 19

What happens if you reduce the concentration of the ions in the left hand half cell?

Answer 19

Equilibrium moves to the left to oppose the change of removing ions; this releases more electrons, the E^0 of the left hand cell becomes more negative, so the e.m.f. Of the cell increases

Question 20

How do you calculate the emf of a cell from E^0 values

Answer 20

E^0cell = E^0right - E^0left

Question 21

When would you use a platinum electrode?

Answer 21

When both the oxidised and reduced forms of the metal are in aqueous solution

Question 22

Why is platinum chosen

Answer 22

Inert so does not take part in the electrochemistry

Good conductor to complete circuit

Question 23

How would you predict if a reaction would occur?

Answer 23

Take the 2 half equations

Find the species that is which reduced(this is effectively the right hand electrode)

Calculate its E^0 value minus the E^0 value of the species that is being oxidised (effectively the left hand cell)

If E^0 overall > 0, reaction will occur

Question 24

What was the first commercial cell made from (Daniell cell)

Answer 24

Zinc/copper (II)

Question 25

What are zinc/carbon cells more commonly known as?

Answer 25

Disposable batteries

Question 26

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

Answer 26

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

Question 27

What are the reactions that occur in a lead/acid battery (car batteries)?

Answer 27

Pb + SO4 2- —> PbSO4 (S) + 2e- PbO2 + 4H+ + SO4 2- + 2e- —> PbSO4 + 2H2O

Question 28

How are cells recharged (if they are rechargeable)

Answer 28

Reactions are reversible and are reversed by running a higher voltage through the cell than the cells E^0

Question 29

Nickel/cadmium cells are rechargeable AA batteries etc. what reactions occur at the electrodes?

Answer 29

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

Question 30

Where are lithium-ion cells used?

Answer 30

Mobile phones Laptops

Question 31

What reactions occur on discharge in lithium-ion cells?

Answer 31

Li+ + CoO2 + e- —> Li+[CoO2]- Li—> Li+ + e-

Question 32

What is a fuel cell?

Answer 32

A cell that is used to generate electric current; doesn’t require electrical recharging

Question 33

what are the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?

Answer 33

2H2 + 4OH- —> 4H2O + 4e- O2 + 2H2O + 4e- —> 4OH-

Question 34

Draw a diagram of a hydrogen fuel cell

Question 35

Why is it better to use a fuel cell than to burn H2 in air, even though the same overall reaction occurs

Answer 35

In combustion, sulfer containing compounds (SO2, SO3) and nitrogen containing compounds (NO2, NOx) are produced due to the high temperatures and the S and N in the air. These are bad for the environment. This does not occur in fuel cell; the only product is water. More efficient

Question 36

Disadvantages of fuel cells?

Answer 36

Hydrogen is a flammable gas with a low b.p. —> hard and dangerous to store and transport —> expensive to buy Fuel cells have a limited lifetime to use toxic chemicals in their manufacture

Question 37

How do you find the weakest reducing agent from a table of electrode potential date?

Answer 37

Most positive E^ θ value. Then it is the PRODUCT of the reduction equation I.e. imagine equation going from right to left

Question 38

What is the reason that some cells cannot be recharged?

Answer 38

Reaction of the cell is not reversible - a product is produced that either dissipates or cannot be converted back into the reactants

Question 39

Why might the e.m.f of a cell change after a period of time?

Answer 39

Concentration of the ions change - the reagents are used up

Question 40

How can the e.m.f. Of a cell be kept constant

Answer 40

Reagents are supplied constantly, so the concentrations of the ions are constant; E^0 remains constant