electrochemistry [P2] PAPER 1

Question 1

A salt bridge was used in a cell to measure electrode potential.

Explain the function of the salt bridge.

Answer 1

the ions in the ionic substance in the salt bridge move through the salt bridge to maintain the charge balance and complete the circuit

Question 2

What does standard electrode potential, E°, refer to?

Answer 2

• temperature of 298K
• pressure of 100kPa (if gases involved in the half equations)
• 1.00 mol/dm3 solution (of ions involved in the half equations)
• zero current

Question 3

what happens at the cathode?

Answer 3

reduction

Question 4

what happens at the anode?

Answer 4

oxidation

Question 5

which species is at the cathode?

Answer 5

one with the more positive E°

Question 6

the more positive electrode ___ electrons

Answer 6

gains

Question 7

what happens to the reducing and oxidising power as you go down the electrochemical series?

Answer 7

reducing power increases

oxidising power decreases

Question 8

How to determine which half equation is at what electrode?

Answer 8

The higher E° = reduction = cathode

(more positive = GAINS electrons)

Question 9

how to calculate the EMF of a cell?

Answer 9

right E°- left E°

Question 10

cell representation?

Answer 10

INCLUDE STATE SYMBOLS
RO||OR

left hand cell || right hand cell
R = reduction
O = oxidation
|| = salt bridge
| = in between RO / OR if there is a change in state
if conductor needed - Pt (s) - goes on the ends

Question 11

There are two ways to use hydrogen as a fuel for cars. One way is in a fuel cell to power an electric motor, the other is as a fuel in an internal combustion engine.

Suggest the major advantage of using the fuel cell.

Answer 11

a fuel cell converts more of the available energy from combustion of hydrogen into kinetic energy of the car

Question 12

Aluminium is used as an electrode in an experiment to measure the emf of a cell. Suggest why the aluminium is rubbed with sandpaper prior to use.

Answer 12

to remove the oxide layer on the aluminium

Question 13

Explain how a salt bridge provides an electrical connection between two electrodes in an electrochemical cell.

Answer 13

has mobile ions / ions can move through it

Question 14

Suggest why the recharging of a lithium cell may lead to release of carbon dioxide into the atmosphere.

Answer 14

Electricity for recharging the cell may come from power stations burning fossil fuels

Question 15

Rechargeable lithium cell electrode reactions?

Answer 15

Question 16

The electrode reactions in an alkaline hydrogen–oxygen fuel cell?

Answer 16

negative electrode:
H2 + 2OH– → 2H2O + 2e–

positive electrode:
O2 + H2O + 4e– → 4OH–

overall equation:
2H2 + O2 → 2H2O

Question 17

Explain how the electrode reactions in an alkaline hydrogen–oxygen fuel cell can be used to generate an electric current.

Answer 17

• Hydrogen releases electrons
• Oxygen accepts electrons

Question 18

Explain why a hydrogen fuel cell doesn’t need to be recharged.

Answer 18

Hydrogen is supplied continuously

Question 19

To provide energy for a vehicle, hydrogen can be used either in a fuel cell or in an internal combustion engine.

Suggest the main advantage of using hydrogen in a fuel cell rather than in an internal combustion engine.

Answer 19

In the fuel cell, a greater proportion of the energy available from the hydrogen-oxygen reaction is converted into useful energy

Question 20

Identify one major hazard associated with the use of a hydrogen-oxygen fuel cell in a vehicle.

Answer 20

• hydrogen is flammable
•hydrogen is explosive
• H+ is corrosive
• OH– is corrosive

Question 21

Solar cells generate an electric current from sunlight. These cells are often used to provide electrical energy for illuminated road signs.

Explain why rechargeable cells are connected to these solar cells.

Answer 21

• solar cells don’t provide electrical energy all the time
• rechargeable cells can store electrical energy for use when the solar cells aren’t working

Question 22

Suggest one reason why many waste disposal centres contain a separate section for cells and batteries.

Answer 22

prevent pollution of the environment by toxic substances

Question 23

State the meaning of the term electrochemical series.

Answer 23

list of electrode potentials in numerical order

Question 24

Answer 24

• • E° (Li+ / Li) < E° (H2O / H2 ,OH-)
• Lithium would react with the electrolyte/water

Question 25

An EMF value for a commercial lithium-iodine cell is 2.80 V.
Suggest why this value is different from a value calculated.

Answer 25

non-standard conditions used

Question 26

what material would you use as an electrode in a cell involving only aqueous solutions and why?

Answer 26

• platinum
• conducts electricity
• inert

Question 27

State one essential requirement of the soluble ionic compound used to make the salt bridge in a cell.

Answer 27

must not react with the electrolyte

Question 28

Standard electrode potentials are measured by comparison with the standard hydrogen electrode.

State the substances and conditions needed in a standard hydrogen electrode.

Answer 28

• H2 (g) at 100kPa
• 1.0 mol/dm3 HCl
• platinum electrode
• temperature of 298K

Question 29

Answer 29

Question 30

Answer 30

• E° (H+ / H2) < E° (Cu2+ / Cu), therefore H+ cannot oxidise Cu to Cu2+
• E° (Cu2+ / Cu) < E° (NO3– / NO), therefore NO3– can oxidise Cu to Cu2+

Question 31

State the change that needs to be made to the apparatus in Figure 1 to allow the cell reaction to go to completion.

Answer 31

replace voltmeter with lamp/wire/ammeter

Question 32

Describe a standard hydrogen electrode.

Answer 32

• hydrogen gas
• 1.0m moldm-3 HCl
• at 298K and 100kPa
• Pt electrode

Question 33

State why the electrode potential for the standard hydrogen electrode is equal to 0.00V.

Answer 33

by definition

Question 34

Give one reason, other than cost, why the platinum electrodes are made by coating a porous ceramic material with platinum rather than by using platinum rods.

Answer 34

Increases the surface area, so the reaction is faster

Question 35

Suggest why the e.m.f. of a hydrogen–oxygen fuel cell, operating in acidic conditions, is exactly the same as that of an alkaline fuel cell.

Answer 35

overall reaction is the same

Question 36

Other than its lack of pollution, state briefly the main advantage of a fuel cell over a re-chargeable cell such as the nickel–cadmium cell when used to provide power for an electric motor that propels a vehicle.

Answer 36

hydrogen and oxygen are supplied continuously

Question 37

Hydrogen–oxygen fuel cells are sometimes regarded as a source of energy that is carbon neutral. Give one reason why this may not be true.

Answer 37

Hydrogen may need to be made using an energy source that is not ‘carbon neutral’

Question 38

Name the standard reference electrode against which all other electrode potentials are measured.

Answer 38

standard hydrogen electrode

Question 39

What is the function of the platinum electrode in an electrochemical cell?

Answer 39

to allow the transfer of electrons

Question 40

why may a cell not be recharged?

Answer 40

the cell reaction cannot be reversed

Question 41

In the external circuit of this cell, the electrons flow through the ammeter from right to left.

Suggest why the electrons move in this direction.

Answer 41

• Cu2+ ions in the left-hand electrode are more concentrated
• So, Cu → Cu2+ + 2e– happens at right-hand electrode

Question 42

Write the conventional representation for an alkaline hydrogen–oxygen fuel cell.

Answer 42

Question 43

State what must be done to maintain the EMF of a fuel cell when in use.

Answer 43

constantly add reactant