Topic 14 Standard Electrode (redox) Potentials

Question 1

The standard hydrogen electrode

Answer 1

The electrode consists of:
- Hydrogen gas at a pressure of 100kPa.
- Bubbling over a piece of platinum foil dipped into hydrochloric acid.
- Hydrogen ion concentration of 1moldm^-3 at a temperature of 298K.
—> The surface of the foil is covered in porous platinum- large surface area and allows an equilibrium between hydrogen ions in solution and hydrogen gas to be established quickly.

Question 2

Why do we need an unreactive salt bridge?

Answer 2

• It is needed to complete the electrical circuit.
• Allows movement of ions.

Question 3

Standard conditions

Answer 3

• 100kPa
• 298K
• 1 moldm^-3 (conc. of ions)
  The position of an equilibrium can be changed by altering conditions.

Question 4

What do E°cell values tell us?

Answer 4

• The more negative the E° value, the further to the left the equilibrium (ie. the more readily the metal ions loses electrons).
• The more positive (/less negative) the E° value, the further to the right the equilibrium lies (ie. the less readily the metal loses electrons to form ions).

Question 5

Electromotive force (emf)

Answer 5

• The potential difference = the electromotive force (emf).
• Symbol: E°cell
• Sometimes called the standard cell potential.

Question 6

The reference point when measuring standard electrode potentials

Answer 6

The reference point is the potential of the standard hydrogen electrode, which is set at zero.

Question 7

Standard electrode potential definition

Answer 7

The standard electrode potential of a half-cell is the emf of a cell containing the half-cell connected to the standard hydrogen electrode. Standard conditions apply.

Question 8

The electrochemical series: in order of their E° values

Answer 8

• In order of their standard electrode (redox) potentials.
• The most negative E° values are placed at the top of the series, and most positive at the bottom.

Question 9

The electrochemical series: reducing agents

Answer 9

• The most powerful reducing agent: lithium (its redox system has the most negative E° value).
• The least powerful reducing agent: fluoride ion F- (its redox system has the least negative (most positive) E° value).

Question 10

The electrochemical series: oxidising agents

Answer 10

• The most powerful oxidising agent is fluorine.
• The least powerful oxidising agent is the lithium ion, Li+.

Question 11

Electrochemical cells

Answer 11

Question 12

Thermodynamic feasibility

Answer 12

• Can be predicted using standard electrode potentials.
• Although the standard electrode potentials indicate that a reaction is thermodynamically feasible, it may not take place for two reasons:
  -the reactant may not be kinetically stable because the activation energy for the reaction is very large.
  -the reaction may not be taking place under standard conditions.

Question 13

Relationship between total entropy and E°

Answer 13

Question 14

What type of species goes on the outside (furthest from the salt bridge) in standard cell notation?

Answer 14

The most reduced species.

Question 15

What symbol is used to represent a salt bridge in standard notation?

Answer 15

II

Question 16

What does | indicate in standard cell notation?

Answer 16

Phase boundary (change of state).

Question 17

Standard cell notation

Answer 17

Zn | Zn^2+ || Cu^2+ | Cu
reduced | oxidised || oxidised | reduced

Question 18

What happens at the left-hand electrode?

Answer 18

Oxidation occurs.
The left-hand electrode is the half cell with the most negative E° value.

Question 19

What happens at the right hand electrode?

Answer 19

• Reduction occurs.
• The right-hand electrode is the half cell with the most positive E° value.

Question 20

Which side of the cell has the most negative E° value and what happens to the metal with the most negative E° value?

Answer 20

The left-hand electrode is most negative.
The metal will be oxidised.

Question 21

What does it mean for the E° value to be more negative?

Answer 21

It means it is a better reducing agent (ie. easier to oxidise).

Question 22

What does it mean for the E° value to be more positive?

Answer 22

It means it is a better oxidising agent (ie. easier to reduce)

Question 23

What factors will change E° values?

Answer 23

• Concentration of ions
• Temperature

Question 24

What happens if you reduce the concentration of the ions in the left-hand half cell?

Answer 24

• Equilibrium moves to the left to oppose the change of removing ions.
• This releases more electrons.
• The E° of the left hand cell becomes more negative, so the emf of the cell increases.

Question 25

How to calculate the emf of a cell from E° values?

Answer 25

E°cell = E°right - E°left

Question 26

How would you predict if a reaction would occur?

Answer 26

1. Take the 2 half equations
2. Identify the species that is being reduced (this is effectively the right hand electrode)
3. Calculate its E° value minus the value of the species that is being oxidised (effectively the left hand cell)
4. If E°cell > 0, a reaction will occur

Question 27

What is a fuel cell?

Answer 27

A cell that is used to generate electric current: it does nat require electrical charging.

Question 28

cell diagram (conventional)

Answer 28

pt (s) | R O || O R | pt (s)
- Plantum electrode- pt
- Positive electrode on right
- Left side oxidised
- Right side reduced
- Ecell positive if positive on right
- Hydrogen electrode always on left