Electrode Potentials

Question 1

Voltaic cell def

Answer 1

Type of electrochemical cell which converts chemical energy to electrical energy

Question 2

What are electode potentials used for

Answer 2

Electrode potentials compare the ease of which a metal gives up its electrons to form positive hydrated ions

Question 3

Electrode potentials of Mg and Cu.
Explain what happens
Give half equations

Answer 3

Mg loses 2e- to become Mg2+
Mg2+ attracted to negative strip
Picks up e- again to become Mg(s)
More reactive so equilibrium lies to LHS

Mg2+ + 2e- = Mg(s)

Cu less reactive than Mg
Less readily forms ions
So Cu2+ more likely to pick up e-
Equilibrium lies to RHS
Cu2+ + 2e- = Cu(s)

Question 4

Half cell def

Answer 4

Contains the chemical species present in a redox half equation

Question 5

Metal/metal ion half cell def

Answer 5

Consists of a metal rod dipped in a solution of aqueous ions

Question 6

What is the convention for writing half cell equations

Answer 6

Reduction reaction is the forward reaction

Electrons and cations are written on the left hand side

Question 7

What type of reactions are half cell reaction

Answer 7

Reversible

So they form a DYNAMIC EQUILIBRIUM

Question 8

Ion/ion half cell def

Answer 8

Contains ions of the same element in different oxidation states

Question 9

Why is platinum used as an electrode?

Answer 9

It is inert (unreactive)
Means there is no transfer of electrons between the electrode, and the two metals added to solution
Electron transfer therefore only occurs between two added metals

Question 10

Standard electrode potential def

Answer 10

E (standard state symbol)

The electron motive force of a half cell compared with a standard hydrogen half cell, under standard conditions:
Measured at 298K
With solution conc. of 1moldm-3
With a gas pressure of 100kPa

Question 11

Electron motive force def

Answer 11

The tendency for a substance to lose or gain electrons

Question 12

How to measure the electrode potential of a substance

Answer 12

Connect the half cell to a hydrogen half cell
Electrode potential values are relative to electrode potential of a hydrogen half cell
E^o of hydrogen half cell = 0 Volts

Question 13

What is a salt bridge used for?

Give example of a salt bridge

Answer 13

Allows ions to flow
But contains a solution that doesnt react with the half cell solutions
E.g. filter paper soaked in KNO3

Question 14

Positive and negative electrodes in a cell

Answer 14

In an operating cell, the electrode with the more reactive element/metal loses electrons and is (more) negative
More likely to be oxidised

The less reactive element/metal gains electrons, so is (less) negative
So the element is more likely to be reduced

Question 15

Equtaion to calculate cell potentials

Answer 15

E^o(cell) = E^o(+ve electrode) - E^o(-ve electrode)

Question 16

General mechanism behind electrode potentials

Answer 16

More negative the E^o value
More equilibrium lies to left
More readily electrons are lost
Therefore stronger reducing agent

Less negative E^o value
More equilibrium lies to RHS
More readily electrons are gained
Thereofre stringer oxidising agent

Question 17

What is e.m.f.

Answer 17

The cell potential, E^o(cell)

Question 18

Primary cell info

Answer 18

Non-rechargeable batteries
Alkaline based
- made of zinc and manganese oxide, and KOH electrolyte

Question 19

Secondary cell info

Answer 19

Rechargeable cell/battery
Lead-acid batteries - used in cars
NiCd (Nickel Cadmium) - used in radios, torches etc.
Lithium-ion and lithium-ion polymer cells used in modern appliances

Question 20

Fuel cell info

Answer 20

Uses energy from a reaction of a fuel with oxygen to create a voltage
Hydrogen fuel cells most common as produces no CO2 byproduct

Question 21

Advantages of primary cells

Answer 21

Good for low voltage
No need to charge
Come in various sizes
Cheaper (than secondary)
Last longer for the charge

Question 22

Disadvantages of primary cells

Answer 22

Can only be used once - non-rechargeable - large amount of waste
Chemicals are used up
Harder to recycle - End up in landfill - less environmentally friendly (than secondary cells)
Low current only used on low current devices

Question 23

Advantages of secondary cells

Answer 23

Rechargeable - chemical reaction can be reversed during charging
Chemical regenerated
More cost-efficient over time
Lithium-ion polymers are lightweight

Question 24

Disadvantages of secondary cells

Answer 24

Poor charge retention - over time the voltage reduces
Higher initial costs
Can take a long time to recharge
Unstable at high temperatures 
Difficult to recycle

Question 25

Advantages of fuel cells

Answer 25

High efficiency
Do not have to be recharged, as long as H2 and O2 supplied can operate continuously
No CO2 produced (pollutants) for hydrogen fuel cells
Removes reliance on fossil fuels

Question 26

Disadvantages of fuel cells

Answer 26

Hydrogen is a gas, so difficult to store
Less durable - not as long lasting - limited life cycle due to H2 adsorber/absorber
Expensive 
No hydrogen fuelling stations
Difficult to make batteries

Question 27

Two different hydrogen fuel cells

Answer 27

Alkali fuel cell

Acid fuel cell

Question 28

Anodes and cathodes in electrochemical cells

Answer 28

Anode is negative

Cathode is positive

Question 29

Electrochemical cell diagrams for alkali and acid hydrogen fuel cells

Answer 29

Both acid and alkali fuel cells have same electrode potentials

Question 30

When is a reaction with electrode potentials feasible

Answer 30

When overall electrode potential is greater than 0

Question 31

What substance is usually used as a salt bridge

Answer 31

KNO3 - potassium nitrate

Question 32

What is the pH of a standard hydrogen half cell

Answer 32

0

Question 33

Fuel cells are a type of electrochemical cell being developed as a potential source of energy
in the future.
• State one important difference between a fuel cell and a conventional electrochemical cell.
• Write the equation for the overall reaction that takes place in a hydrogen fuel cell.
• State two ways that hydrogen might be stored as a fuel for cars.
• Suggest why some people consider that the use of hydrogen as a fuel for cars consumes more energy than using fossil fuels such as petrol and diesel.
(5 Marks)

Answer 33