electrochemical cells

Question 1

Explain what happens when current is allowed to flow between two half cells.

Answer 1

→ reactions occur separately at each electrode.
→ voltage falls to zero as reactants are used up
→ most positive electrode undergoes reduction
→ most negative electrode undergoes oxidation

Question 2

State why a platinum electrode is used.

Answer 2

→ unreactive
→ can conduct electricity

Question 3

Explain why a wire is not used as a salt bridge.

Answer 3

the metal wire would set up its own electrode system with the solutions

Question 4

Give a suitable example of salt solution used to make the salt bridge & state why.

Answer 4

• potassium nitrate
  → unreactive with both the electrode & solution

Question 5

State the conditions when measuring the standard electrode potential.

Answer 5

→ 1 mol dm⁻³ solution of ions
→ 298K
→ 100kPa

Question 6

State what is meant by the SHE.

Answer 6

→ standard used for comparison of potentials
→ assigned a potential value of 0 volts

Question 7

State why standard conditions are needed when measuring electrode potentials.

Answer 7

→ position of redox equilibrium changes with condition
→ this affects the electrode potential of the half-cell

Question 8

In terms of equilibrium, explain how changes in temperature, pressure or concentration affect the value of the electrode potential.

Answer 8

→ if equilibrium shifts to the left, oxidation reaction is favoured so electrode potential becomes more negative
→ if equilibrium shifts to the right, reduction reaction is favoured so electrode potential becomes more positive

Question 9

State what the value of an electrode potential tells us about the electrode’s reducing/oxidising ability.

Answer 9

→ more negative = more easily oxidised = great reducing agent
→ more positive = more easily reduced = great oxidising agent

Question 10

State two ways of predicting weather a reaction is feasible or not.

Answer 10

→ combine half-equations to obtain feasible reaction & compare with the one given in question
→ all feasible reactions have a positive Ecell value

Question 11

Give examples of non-rechargeable batteries.

Answer 11

• zinc/carbon batteries
• alkaline batteries

Question 12

State the advantages of non-rechargeable batteries.

Answer 12

• cheap
• portable + easy to replace
• wide range of applications

Question 13

State the disadvantages of non-rechargeable batteries.

Answer 13

→ must be disposed of once reactants are used up (can be toxic)
→ battery casing may be corroded by electrolyte
→ non-sustainable

Question 14

State what lithium ion cells are used for.

Answer 14

used to power phones/cameras

Question 15

State the components of a lithium ion cell.

Answer 15

• lithium cobalt oxide electrode
• graphite electrode
• lithium salt dissolved in solvent (electrolyte)

Question 16

State the two half equations for a lithium ion cell.

Answer 16

Li⁺ + e⁻ ⇌ Li (-3.04V)
Li⁺ + CoO₂ + e⁻ ⇌ Li⁺[CoO₂]⁻ (+0.56V)

Question 17

State the overall equation when a lithium ion cell is discharging.

Answer 17

Li+ + CoO₂ + e- → Li⁺[CoO₂]⁻

Question 18

Briefly explain how rechargeable batteries are charged.

Answer 18

→ current is supplied, forcing e- to flow in the opposite direction

Question 19

State :) and :( for rechargeable batteries.

Answer 19

:)
- longer lifespan
- more efficient
:(
- expensive
- need to be charged regularly

Question 20

Describe how an alkaline hydrogen-oxygen fuel cell works.

Answer 20

1. hydrogen is oxidised by hydroxide ions to water at the negative electrode
2. e- flow to positive electrode, react with oxygen to release hydroxide ions
3. hydroxide ions travel to negative electrode (through semi-permeable membrane) and react with hydrogen (cycle repeats)

Question 21

Give the two half equations and the overall equation for the reactions in an alkaline hydrogen-oxygen fuel cell.

Answer 21

oxidation: 2H₂ +4OH⁻ → 4e⁻ + 4H₂O
reduction: 4e⁻ + 2H₂O +O₂ → 4OH⁻
Overall reaction:
2H₂ + O₂ → 2H₂O

Question 22

State the advantages of alkaline hydrogen-oxygen fuel cells.

Answer 22

→ more efficient than internal combustion engines + batteries
→ does not produce pollutants

Question 23

State the limitations of alkaline hydrogen-oxygen fuel cells.

Answer 23

→ expensive
→ hydrogen is explosive + needs to be stored under pressure
→ use of fossil fuels in the production of hydrogen through electrolysis

Question 24

Explain how a battery works.

Answer 24

→ flow of electrons from one electrode to another
→ through an external circuit where they can do work & transfer energy to appliances

Question 25

Write the conventional representation for an alkaline hydrogen– oxygen fuel cell.

Answer 25

Pt|H₂ (g)|OH⁻ (aq), H₂O (l) || O₂ (g)|H₂O (l), OH⁻ (aq)|Pt