3.1.11 Electrode Potentials + Electrochemical Cells

Question 1

Explain the term standard electrode potential

Answer 1

The measure of potential of a reaction that occurs at the electrode when all the substances involved in the reaction are in their standard states that is solutions are at 1moldm^-3 concentrations, gases at 100kPa and solids and liquids are in pure form with all at 298K

Question 2

Describe how standard electrode potentials are measured

Answer 2

Connect together 2 different electrodes and measure the potential difference using a salt bridge

Question 3

Describe how standard electrode potentials can be used to predict the direction of a redox reaction

Answer 3

Electrons flow from more negative to more positive

Question 4

Describe how standard electrode potentials can be used to predict the direction of a redox reaction

Answer 4

Electrons flow from more negative to more positive

Question 5

Describe the differences between non-rechargeable, rechargeable, and fuel cells

Answer 5

Non-rechargeable- Electrons flow in one direction
Rechargeable-
Reverse reaction during charging
Fuel cell- produces water as byproduct

Question 6

Electrode reactions at hydrogen-oxygen fuel cell

Answer 6

Fuel cell- hydrogen enters at negative electrode and form with OH^- ions to Electrons water and electrons
H2 (g) + 2OH^- (aq) —> 2H2O (l) + 2e^-
Electrons flow to positive electrode where oxygen enters. Oxygen accepts electrons from other electrode and reacts with water to release OH^- ions
O2 (g) + 2H2O (l) + 4e^—-> 4OH^-which pass through semi-permeable membrane to that electrode
Overall effect is hydrogen and oxygen reacting to from water

Question 7

Benefits and risks of non-rechargeable cells

Answer 7

Benefits-
Cheap
Portable source of electrical energy
Risks-
Don’t last long - chemicals used up until one is used up and emf = OV
Bad for environment- can’t be recharged and must be disposed after use

Question 8

Benefits and risks of rechargeable batteries

Answer 8

Benefits-
Last long time
Less waste
Cheaper in long run
Lower environmental impact
Portable source of electrical energy
Risks-
Some waste

Question 9

Benefits and risks of fuel cells

Answer 9

Betnefits-
Produce water as byproduct
Don’t need recharging
Portable source of electrical energy
Efficient
Risks-
Need constant supply of fuels
Hydrogen is flammable and explosive
Hydrogen usually made of fossil fuels
Expensive

Question 10

Why is platinum used as an electrode

Answer 10

Inert
Conducts electricity

Question 11

Function of porous separator in rechargeable cell

Answer 11

Acts as salt bridge

Question 12

Reactions at positive and negative electrodes of lithium cell

Answer 12

P- Li+ + CoO + e– → Li+[CoO ]–

N- Li → Li+ + e–

Question 13

Explain the function of the salt bridge

Answer 13

Ions in ionic substance in the salt bridge move through the salt bridge
To maintain charge balance/complete circuit

Question 14

Suggest the major advantage of using the fuel cell

Answer 14

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

Question 15

Non-rechargeable battery examples

Answer 15

Zinc-carbon
Alkaline

Question 16

Reacharable battery example

Answer 16

Lead-acid
Lithium
Nickel-cadmium

Question 17

What is the simplest salt bridge made of

Answer 17

Filter paper soaked in saturated solution of KNO3

Question 18

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

Answer 18

Equilibrium is set up between the solid metal and the aqueous metal ions

Question 19

How would an Aluminium/Copper cell be represented

Answer 19

Al(s) | Al^3+ (aq) ||Cu^2+ (aq) | Cu (s)

Question 20

What happens at the left and right hand electrode

Answer 20

L- oxidation (more negative E°)
R- reduction (more positive E°)

Question 21

Draw the standard hydrogen electrode

Question 22

Standard hydrogen electrode conditions

Answer 22

298 K
100 kPa
1.00 moldm^-3 [H^+]

Question 23

What factors change E° values

Answer 23

Ion concentration
Temperature

Question 24

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

Answer 24

Equilibrium moves left to oppose the change of removing ions
More electrons released
E° more negative
E.m.f of cell increases

Question 25

When would you use a platinum electrode

Answer 25

When both oxidised and reduced metals are in aqueous solution

Question 26

2 reactions that take place in zinc/carbon cells

Answer 26

Zn —> Zn^2+ + 2e^-
2NH4^+ + 2e^- —> 2NH3 + H2 (carbon electrode)

Question 27

Reactions that occur in lead/acid battery

Answer 27

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

Question 28

What reactions occur in nickel/cadmium cells

Answer 28

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

Question 29

Where are lithium-ion cells used

Answer 29

Mobile phones
Laptops

Question 30

What is a fuel cell

Answer 30

A cell that is used to generate electrical current
Does not require electrical charging

Question 31

Reactions that take place in alkaline hydrogen fuel cell

Answer 31

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

Question 32

Why is it better to use a fuel cell that to burn H2 in air even though same reaction occurs

Answer 32

In combustion SO2/SO3 + NO2/NOx are produced by high genes and S and N in air
Bad for environment
Doesn’t happen in fuel cells

Question 33

Why might E.m.f of cell change + how to keep constant

Answer 33

Conc of ions change-reactants used up
Constant supply of reagents