Electrochemical cells

Question 1

2 ways of constructing an electrochemical cell

Answer 1

-metal dipped in its ions
-platinum electrode with 2 aqueous ions

Question 2

why is platinum commonly used?

Why does it need these features

Answer 2

inert

electrically conductive

Question 3

Electrochemical cell set up

Answer 3

2 half cells joined by wire, high resistance voltmeter and salt bridge

Question 4

What is EMF

Answer 4

voltage between two cells

Question 5

purpose of voltmeter?

Answer 5

measure EMF

Question 6

observation during oxidation

Answer 6

electrode being oxidised gets thinner,as more ions are being dissolved insolution

Question 6

type of reaction that occurs in electrochemical cells

Answer 6

redox

Question 7

observation during reduction

Answer 7

electrode being reduced gets thicker,as more ions are receiving electrons

Question 8

what is used as a salt bridge and why?

Answer 8

KNO3

-the tube has unreactive ions that can move between two half cells to carry the flow of electrons
-maintains charge balance
-doesn’t interfere with the reaction

Question 9

Define electrochemical series

Answer 9

A list of half cells and their standard electrode potentials in descending numerical order

Question 10

What are electrode potentials?

Answer 10

measure of how easily the half cell gives up electrons (oxidised)

Question 11

In the electrochemical series, which form are the equations written in?

Answer 11

reduction (gains electrons)

in equilibrium

Question 12

Which side of the equation has reducing agents?

Answer 12

Right hand side

They are more easily oxidised

Question 13

Which side of the equation has oxidising agents?

Answer 13

Left hand side

They are more easily reduced

Question 15

Which half cell undergoes oxidation?

Answer 15

More negative half cell

Question 16

Which half cell undergoes reduction?

Answer 16

More positive half cell

Question 17

What is SHE?

Answer 17

Standard Hydrogen Electrode

Used as a reference to measure standard electrode potentials, because they can’t be measured on their own

Question 18

Standard conditions for electrochemical cells?

Answer 18

298K

1.00 mol dm-3

100kPa

Question 19

Which is the strongest reducing agent?

Answer 19

Most negative half cell

Question 20

Which is the strongest oxidising agent?

Answer 20

Most positive half cell

Question 21

Formula to calculate standard cell potential

Answer 21

E r - E o

Question 22

How to draw cell notation?

Answer 22

Reduced | Oxidised || Oxidised | Reduced

Most negative half cell goes on the left

Question 23

Anticlockwise

Answer 23

Oxidised is on top of reduced

Combine both equations to obtain a feasible reaction

Use the formula

Positive value = feasible

Question 24

What are the two forms of batteries?

Answer 24

Rechargeable and non-rechargeable

Question 25

Compare rechargeable and non-rechargeable batteries

Answer 25

Non-rechargeable: -cheaper Rechargeable: -cheaper in the long-term, as they are reversible -when plugged in a current is supplied

Question 26

Example of a rechargeable battery

Answer 26

Lithium ion batteries -phones, electric cars, etc -one electrode has lithium cobalt oxide (LiCoO2) -other electrode has graphite (C) -electrolyte = lithium salt dissolved in organic solvent (Electrolyte allows ions to flow between electrodes) Li+ + e- ~ Li -3.04 Li+ + CoO2 + e- ~ Li+[CoO2]- +0.56V

Question 27

How do fuel cells work?

Answer 27

-electricity is generated by a continuous external supply of chemicals -e.g. alkaline hydrogen-oxygen fuel cell -platinum electrode is a good conductor but inert -hence it is used as a negative electrode here -OH- ions are carried towards anode **Hydrogen electrode produces electrons Oxygen electrode accepts electrons**

Question 28

Equations for hydrogen-oxygen fuel cell

Answer 28

2H₂(g) + 4 OH- (aq) —> 4H₂O (l) + 4 e- O₂ (g) + 2H₂O (l) + 4e- —> 4 OH- (aq) Combined equation: 2H₂ (g) + O₂ (g) —> 2H₂O (g)

Question 29

Advantages of fuel cells

Answer 29

-more efficient than an internal combustion engine in machines -more energy is converted into kinetic energy -less is wasted as thermal energy -don’t need to be recharged -no CO2 is directly emitted -water is the only byproduct

Question 30

Disadvantages of fuel cells

Answer 30

-H2 is highly flammable -expensive to store and transported -energy is required to make H2 and O2 -fossil fuels are used to pass water through an electrolysis process -fossil fuels contribute to CO2 emissions

Question 31

Why are certain substances not used as a salt bridge?

Answer 31

The ions would react with one of the ions in solution Alters cell e.m.f.

Question 32

What side is SHE always on?

Answer 32

Left hand side

Question 33

Suggest why the current in the external circuit of this cell may fall to zero after the cell has operated for some time (1 mark)

Answer 33

Concentrations become equal

Question 34

Why does a fuel cell not need to be recharged?

Answer 34

Hydrogen is continuously supplied

Question 35

Explain why rechargeable cells are connected to solar cells

Question 36

Suggest why waste disposal centres contain a separate section for cells and batteries

Answer 36

Prevents pollution to the environment due to toxic components