5.2.3 (i)

Question 1

Is the reaction between cadmium and magnesium nitrate feasible given the following:

Cd²⁺_(aq) + 2e^- ⇔ Cd_(s) E^Ø= -0.40V

Mg²⁺_(aq) + 2e^- ⇔ Mg_(s) E^Ø= -2.37V

Answer 1

• For there to be a reaction between Cd_(s) and Mg(NO₃)₂, the reactants must be Cd_(s) + Mg²⁺ based on the ionic equation
• The half equation for cadmium, is less negative than for magnesium, therefore the cadmium half equation favours the forward reaction and the magnesium favours the reverse
  
  • ​In other words, Cadmium favours reduction, Magnesium favours oxidation

Cd²⁺_(aq) + 2e^- → Cd_(s)

Mg_(s) → Mg²⁺_(aq) + 2e^-

Overall redox equation: Mg_(s) + Cd²⁺_(aq) → Mg²⁺_(aq) + Cd_(s)

Therefore the reaction is not feasible

Question 2

What can be used to reduce iron(II) to iron but not aluminium ions to aluminium?

Fe²⁺ + 2e^- ⇔ Fe E^Ø= -0.44V

Al³⁺ + 3e^- ⇔ Al E^Ø= -1.66V

Zn²⁺ + 2e^- ⇔ Zn E^Ø= -0.76V

Answer 2

• To reduce Fe²⁺, the iron half equation must go forwards and to ensure aluminium is not reduced, the aluminium half equation must be in reverse
• So the iron half equation needs an oxidising agent with a lower electrode potential, but not too low where it will reduce aluminium

-1.66 > -0.76 > -0.44

Al > Zn > Fe

The zinc half equation, has a more negative electrode potential than the iron one, so the iron half equation will go forwards, but the aluminium half equation has a more negative electrode than the zinc half equation, so it will favour the reverse direction and be oxidised, not reduced

Question 3

What are the limitations of predictions from electrode potentials?

Answer 3

• E^Ø values are with standard conditions, actual conditions may differ, changing the electrode potential
• E_a may be so high, a reaction that is feasible based on e values, may not take place
  
  • The electrode potential values may indicate thermodynamic stability but there is no indication of reaction rate or kinetics

Question 4

What are the properties of primary storage cells?

Answer 4

• Non rechargable, to be used once
• Electrochemical reactions produce electricity (from oxidation and reduction at electrodes)
• Reactions cannot be reversed, chemicals will be used up

Whereas in secondary storage cells, they are rechargable as the overall cell reaction is reversible so the chemicals can be regenerated

Question 5

What are the differences between fuel and storage cells?

Pros and cons of storage cells

Answer 5

• Storage cells don’t need a constantly supply of oxygen and fuel as they have the species

Pros and Cons of Storage Cells

• Storage cells have a high power density and are cheap
• Cons: They have disposal issues due to toxic compounds
  
  • They can be flammable, especially lithium ion batteries

Question 6

What are the properties of fuel cells?

Answer 6

• Uses energy from reaction between fuel w/ O₂ creating a voltage
  
  • Fuel is externally supplied and oxygen comes from the air
  • Fuel cells don’t need to be recharged
  • H_2(g) is the most common fuel

Question 7

Give the:

Diagram and redox equations for

Acid and alkali hydrogen/oxygen fuel cells

Answer 7

See notes and diagram below