Topic 14: Redox II (incomplete)

Question 1

Standard Electrode Potential

also called electromotive force

Answer 1

The driving force which allows electrons to travel from the oxidation half-cell to the reduction half-cell, under standard conditions.

Question 2

What are standard conditions

Answer 2

Standard: 298K, 1M, 100kPa

Question 3

Why is a reference electrode necessary?

Answer 3

The standard hydrogen electrode is used as the reference electrode for finding the standard emf of a half-cell. This is because you cannot measure the emf of a standalone cell but you can measure it with respect to a standard, allowing comparison.

Question 4

What are the features of a standard hydrogen electrode?

Answer 4

• Pt electrode
• H2 gas
• 1 M HCl

Question 5

Half cell: Metal + metal ion

Answer 5

Solid metal electrode submerged in 1M solution of metal ions

Question 6

Half cell: Non-metals with their ions in solution

Answer 6

Gas in test tube overturned
Pt electrode
1 M solution of non metal ions

Question 7

Half cell: Ions of same element in different oxidation states

Answer 7

Pt electrode
1 M of ion 1
1 M of ion 2

Question 8

Total emf?

Answer 8

emf = E(ox)+E(red)

This formula assumes you changed the sign of the oxidation reaction

WITHOUT SIGN CHANGE
emf = E(red) - E(ox)

Question 9

What does a more negative emf(red) mean? a

Answer 9

species on LHS is weaker oxidising agent and the species on the right is a stronger reducing agent (i.e prefers being oxidised)

Question 10

What is the cell diagram for Cu/Cu2+ and H2/H+

Answer 10

Cu | Cu2+ || H+ | H2 | Pt

Question 11

What is the cell diagram for Cu+/Cu2+ and Cl2/Cl-

Answer 11

Pt |Cu+ , Cu2+ || Cl2 | Cl- | Pt

Question 12

What does a positive E(cell) indicate about the reaction?

Answer 12

The reaction is feasible/ spontaneous ( ΔG < 0)

Question 13

What does a positive E(cell) indicate about ΔS(tot)

Answer 13

ΔS (tot) > 0

Since ΔG = -T ΔS, if ΔG < 0 then ΔS(tot) > 0

Question 14

What is the relationship between E(cell) and ΔS(tot)

Answer 14

proportional

Question 15

What is the relationship between E(cell) and lnK

K = equilibrium constant

Answer 15

Proportional

Question 16

What happens if different conditions are used when measuring emf?

Answer 16

Changes in concentration, pressure, and temperature all change the value of E [the tendency of electrons to flow] because they affect the position of equilibrium of the half-reactions.

Question 17

What is an electrochemical series

Answer 17

A list which describes the arrangement of elements in the order of their increasing electrode potentials ( with reference to standard H electrode )

Question 18

Explain the limitations of predictions made using standard electrode potentials

Answer 18

• Thermodynamic feasibilty can be deduced but does not provide information about the rate of reaction
• Need to measure le Chatelier’s effect on concentrations using non-standard conditions
• Reaction kinetics also affect prediction (slow rate = no noticeable reaction, high Ea)

Question 19

How can standard electrode potentials be used to determine if disproportionation reactions are feasible

Answer 19

The E(cell) which is more negative will be the oxidation reaction. Add together E(ox) and E(red). If E(tot) > 0 then the disportionation reaction is feasible.

Question 20

What is a fuel cell?

Answer 20

An electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode.

Question 21

How does the voltage in a fuel cell occur?

Answer 21

As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell.

Question 22

What is the half reaction at the anode in a hydrogen-oxygen fuel cell?

Answer 22

Alkaline: H2(g) + 2OH- -> 2H2O + 2e-
Acidic: H2(g) -> 2H+ + 2e-

Question 23

What is the half reaction at the cathode in a hydrogen-oxygen fuel cell?

Answer 23

Alkaline: O2(g) + 2H2O + 4e- -> 4OH-
Acidic: O2(g) + 4H+ + 4e- -> 2H2O

Question 24

What fuels can be used to supply H in fuel cells?

Answer 24

Methanol and other hydrogen-rich fuels

Question 25

what happens at the anode?

Answer 25

oxidation

Question 26

what happens at the cathode

Answer 26

reduction