A2 - Electrochemistry

Question 1

What is a half cell?

Answer 1

half an electrochemical cell, made up of an electrode (platinum or using the metal itself) in a solution containing aqueous ions

Question 1

What makes up an electrochemical cell?

Answer 1

• 2 half cells
• salt bridge
• voltmeter
• wire

Question 2

What does the salt bridge contain?

Answer 2

KNO3/NaNO3

Question 3

What is the role of the salt bridge?

Answer 3

allows the transfer of ions/charge

Question 4

What is the electrode potential of a cell?

Answer 4

a value that tells us how easily the half cell is to be oxidised

Question 5

What will the most negative half cell undergo?

Answer 5

oxidation

Question 6

What will the most positive half cell undergo?

Answer 6

reduction

Question 7

What is the SHE?

Answer 7

Standard Hydrogen Electrode
used as a reference for electrode potential values

Question 8

What is the set up of the SHE?

Answer 8

Pt electrode
1mol dm3 H+ ions in solution
298K
100kPa

Question 9

What are the standard conditions for electrochemical cells?

Answer 9

100kPa
298K
1 mol dm-3 ions

Question 10

What is the function of the porous separator in a chemical cell (battery)?

Answer 10

salt bridge - allows transfer of ions

Question 11

What happens to the chemical reaction when a cell is recharged?

Answer 11

it is reversed

Question 12

Why are some chemical cells non-rechargeable?

Answer 12

• reaction cannot be reversed
• reaction produces a gas

Question 13

Why does the metal case of some chemical cells break down over time?

Answer 13

the metallic case is a reactant and gets used up

Question 14

What happens to cell emf over time?

Answer 14

decreases as reactants get used up

Question 15

Which half cell is on the left?

Answer 15

the half cell with the most negative electrode potential

Question 16

What is the equation showing the positive electrode in a lithium ion cell?

Answer 16

Li+(aq) + CoO2(s) + e- –> Li+CoO2- (aq)

Question 17

What is the equation showing the negative electrode in a lithium ion cell?

Answer 17

Li(s) –> Li+(aq) + e-

Question 18

What is the standard cell representation for a lithium ion cell?

Answer 18

Fe / Li+ / Li // Li+ / CoO2 / Li+CoO2- / Fe

Question 19

What is the discharge equation for a lithium ion cell?

Answer 19

Li (s) + CoO2 (s) –> Li+CoO2- (aq)

Question 20

What metals make up a Daniell cell?

Answer 20

Zn and Cu

Question 21

How is cell emf calculated from electrode potentials?

Answer 21

(right) - (left)

Question 22

What is an electrochemical series?

Answer 22

list of electrode potentials in numerical order

Question 23

What is one property a salt bridge MUST have?

Answer 23

must not react with the electrolyte/ions in solution

Question 24

Why would a salt bridge containing a carbonate ion not be suitable for use (when H+ is present)?

Answer 24

carbonate ions will react with acid

Question 25

What are 2 advantages of using Hydrogen Fuel Cells compared to using hydrogen in combustion?

Answer 25

HFCs:
- less energy wasted as thermal energy
- reaction happens at low temperatures

Question 26

What are the half equations for a hydrogen-oxygen fuel cell operating in alkaline conditions? Use these half equations to explain how an electric current can be generated

Answer 26

O2 + 2H2O + 4e- ⎯→ 4OH-
H2 + 2OH- ⎯→ 2H2O + 2e-
Oxygen electrode gains electrons (is reduced), hydrogen electrode loses electrons (is oxidised)

Question 27

Why does a hydrogen fuel cell does not need to be recharged?

Answer 27

emf is constant (given that supply of reactants is constant)

Question 28

What is one major hazard associated with the use of a hydrogen–oxygen fuel cell in a vehicle?

Answer 28

Hydrogen is flammable/explosive

Question 29

What would happen to the cell emf if the surface area of the electrodes was increased?

Answer 29

no change/nothing

Question 30

What are the advantages of using hydrogen fuel cells?

Answer 30

• constant emf as long as reactant supply is constant
• works at room temperature so low thermal energy loss (compared to burning hydrogen)
• produces H2O as by-product, no greenhouse emissions or other pollutants such as NOx, SO2 (be specific)

Question 31

What are the disadvantages of using hydrogen fuel cells?

Answer 31

• little hydrogen infrastructure in place (petrol stations)
• production of H2 currently energy intensive (produces greenhouse gases in power stations)
• storage of hydrogen is difficult and dangerous (explosive gas)

Question 32

What substances and conditions are needed in a SHE?

Answer 32

• H2(g) AND 100kPa
• 1 mol dm−3 AND HCl/HNO3/H+
• Pt electrode AND temperature of 298 K (25oC)