20. Electrode potentials and electrochemical cells

Question 1

What occurs when you place two different metals in a salt solution and connect them together

Answer 1

An electric current flows so that electrons pass from the more reactive metal to the less reactive

Question 2

Give the equation for the half cell of zinc

Answer 2

Zn(s) —> Zn2+ (aq) + 2e-

Question 3

What is oxidation

Answer 3

Loss of electrons

Question 4

What is reduction

Answer 4

Gain of electrons

Question 5

What is an oxidising agent

Answer 5

An electron acceptor

Question 6

What is an reducing agent

Answer 6

An electron donator

Question 7

Give an example of an ionic solution used for a salt bridge

Answer 7

KNO3

Question 8

What is a salt bridge

Answer 8

In its simplest form, a piece of paper socked in an ionic solution which allows the movement ions between half cells

Question 9

Why is a salt bridge used rather than a wire

Answer 9

Wire only allows the transfer of electrons whereas a salt bridge transfers ions

Question 10

What does a more negative potential difference mean

Answer 10

• a more negative electrode loses electrons more readily
• it is more easily oxidised
• it is a better reducing agent

Question 11

What can be used to measure the potential difference of a half cell

Answer 11

SHE - standard hydrogen electrode

Question 12

What is in a SHE electrode

Answer 12

Hydrogen gas is bubbled into a solution of H+ (aq) ions (HCl) with a Pt electrode

Question 13

What are the standard conditions in cells

Answer 13

100kPa
1 mol dm-3
298K

Question 14

What is the potential of a SHE

Answer 14

Defined as 0

Question 15

What is emf

Answer 15

• electromotive force E

- the maximum potential difference between two half cells

Question 16

Which side does a SHE always go

Answer 16

Left

Question 17

What does EMF=

Answer 17

EMF= E(R)- E(L)

Question 18

The species with the highest what is written next to the salt bridge

Answer 18

oxidation state

Question 19

What is an oxidation state

Answer 19

Represents how many electrons a species has lost

Question 20

Why is the reaction Cu2+ (aq) + Zn (s) —> Cu (s) + Zn2+ (aq) feasible given:
Zn2+ (aq) + 2e- Zn(s) E= -0.76V
Cu2+ (aq) + 2e- Cu(s) E= +0.34

Answer 20

Zn2+/Zn E < Cu2+/Cu therefore electrons are flowing from the more negative to the more positive (Zn giving Cu2+ electrons)
EMF= +ve therefore feasible as electrons moving from left to right (-ve to +ve)

Question 21

In a redox system that only involves metal ions but not metal e.g. Fe3+/Fe2+ what is used as an electrode

Answer 21

Platinum electrode

Question 22

What is a battery

Answer 22

A number of cells connected together

Question 23

Name 2 non-rechargeable cells

Answer 23

• zinc/copper

- zinc/carbon

Question 24

What does zinc/carbon cell consist of

Answer 24

• carbon electrode coated in manganese (IV) oxide
• zinc canister
• paste of ammonium chloride (NH4Cl)
• water

Question 25

Why is the carbon coated in manganese (IV) oxide (zinc/carbon cell)

Answer 25

H2 gas produced in the redox reaction is oxidised to water by the manganese (IV) oxide to prevent build up of pressure

Question 26

Why is there water in the zinc/carbon cell

Answer 26

Ammonia gas dissolves in it to prevent build up of pressure

Question 27

Why are zinc/carbon cells prone to leakage

Answer 27

- as zinc is used up the walls become thin and prone to leakage - NH4Cl is corrosive and acidic - old batteries should therefore be removed from equipment

Question 28

Name a rechargeable battery

Answer 28

Lead-acid battery

Question 29

How are rechargeable batteries recharged

Answer 29

By reversing the cell reactions by applying external voltage greater than the voltage of the cell to drive the electrons in the opposite directions

Question 30

What are lead acid batteries used in

Answer 30

Used to operate the starter motors of cars.

Question 31

What do acid lead batteries consist of

Answer 31

Six 2V cells connected in series (giving 12V)

Question 32

What do acid lead cells consist of

Answer 32

- two plates dipped into a solution of sulfuric acid - positive plate is made of lead coated with lead (IV) oxide - the negative plate is made of lead

Question 33

Name 3 portable batteries

Answer 33

Nickel/cadmium Lithium ion Alkaline hydrogen-oxygen fuel cell

Question 34

Why is the nickel/cadmium battery replacing the traditional zinc/carbon

Answer 34

They are rechargeable so more cost effective

Question 35

What does the lithium ion cell consist of

Answer 35

- positive electrode is made of lithium cobalt oxide LiCoO2 - negative electrode is carbon - these are layered with a sandwich of solid electrolyte in between

Question 36

What are the reactions that take place in the lithium ion cell

Answer 36

negative electrode: Li+ +e- ---> Li | positive electrode: Li+(CoO2)- ----> Li+ +CoO2 + e-

Question 37

What are the reactions that take place in an alkaline hydrogen-oxygen fuel cell

Answer 37

``` negative electrode: 2H2 (g) + 4OH- (aq) ---->4H2O (l) +4e- positive electrode: O2 (g) + 2H2O (l) + 4e- --->4OH- (aq) overall: 2H2 (g) + O2 (g) ---->2H2O (l) ```

Question 38

What happens in an alkaline hydrogen-oxygen fuel cell

Answer 38

-hydrogen enters at the negative electrode: 2H2 (g) + 4OH- (aq) ---->4H2O (l) +4e- -this releases electrons which flows through the circuit to the other electrode -this reaction then takes place at the positive electrode O2 (g) + 2H2O (l) + 4e- --->4OH- (aq) -this accepts electrons from the other electrode and releases OH- ions

Question 39

What are the electrodes made of in an alkaline hydrogen-oxygen fuel cell

Answer 39

porous platinum based material

Question 40

Why are alkaline hydrogen-oxygen fuel cells used in spaceships

Answer 40

The only bi-product is water which can be used by the astronauts

Question 41

Why are alkaline hydrogen-oxygen fuel cells not as green as they appear

Answer 41

Most hydrogen is made from crude-oil, the non renewable

Question 42

Why is using an alkaline hydrogen-oxygen fuel cell in cars not safe/ good

Answer 42

It requires the storage of hydrogen gas which can explode/ is extremely flammable. It is also hard to store hydrogen as it takes up alot of space but dangerous to pressurise.