5.2.3 (f-h) - Electrode Potentials

Question 1

What is an electrochemical cell>

Answer 1

• Called a voltaic cell that converts chemical energy to electrical energy

Question 2

What is a half cell?

Answer 2

• Contains chemical species present in a redox half equation
• An electrochemical/voltaic cell is made by combining 2 half cell to allow electron flow
  
  • The half cells must be kept apart

2 Types

• Metal/metal ion half cell
• Ion/ion half cell

Question 3

What is in a metal/metal ion half cell?

Answer 3

• An electrode of the metal
• A solution containing the ions
• At the boundary between the ions and the rod, the phase boundary, there is an equilibrium

Cu²⁺_(aq) + 2e^- ⇔ Cu_(s)

By convention, reduction is written forward and oxidation backwards

Question 4

What is in an ion/ion half cell?

Answer 4

• Contains ions of the same element in different oxidation states
• E.g. Fe³⁺_(aq) + e^- ⇔ Fe²⁺_(aq)
• The half-cell has no metal to transfer electrons so an inert metal electrode of Platinum is used

Question 5

In a cell with two metal/metal ion half cells:

What is the negative electrode?

What is the positive electrode?

Answer 5

• Negative electrode has the more reactive metal that loses electrons and is oxidised
• Positive electrode has the less reactive metal that gains electrons and is reduced

Question 6

Define standard electrode potential

Answer 6

The e.m.f of a half cell compared with a standard hydrogen half cell at 298K, [solution] = 1M, and gas pressure 100kPa

Question 7

What is a stadard hydrogen half cell?

Answer 7

• Has standard electrode potential value of 0V

2H⁺_(aq) + 2e^- → H_2(g)

Question 8

How is an electrochemical cell set up, and how does it measure standard electrode potential?

Answer 8

• Connect the half cell to be tested with a standard hydrogen electrode
  
  • Connected by a wire - allowing e^- flow
  • Connected by a salt bridge
    
    • Porous material soaked in an unreactive electrolyte such as potassium nitrate to allow ion flow
• The voltmeter gives the reading for electrode potential

Question 9

What does a more negative electrode potential value indicate?

Answer 9

• Greater tendancy to lose electrons and undergo oxidation
• Lower tendancy to gain electrons and be reduced
• Metals tend to have negative standard electrode potentials
  
  • The more negative the electrode potential, the more reactive the metal is in losing electrons

Question 10

What does a more positive electrode potential value indicate?

Answer 10

• Greater tendance to gain electrons and be reduced
• Lower tendancy to lose electrons and be oxidised
• Non-metals have a more positive electrode potential
• The more positive the electrode potential of a non-metal, the more reactive it is in gaining electrons

Question 11

How do you calculate standard cell potential?

Answer 11

• Meausre the cell potential of any 2 half cells

Standard cell potential

E_cell = E(+ve electrode) - E(negative electrode)