Electrochem I

Question 1

What is electrochemistry?

Answer 1

The study of the interconversion of electrical and chemical energy via the movement of electrons from one chemical species to another

Question 2

What is galvanic cell?

Answer 2

Redox reaction which is spontaneous and the chemical energy is converted to electrical energy

Question 3

What is electrolytic cell?

Answer 3

Redox reaction which is non-spontaneous and electrical energy is converted to chemical energy

Question 4

What occurs at the anode?

Answer 4

Oxidation (an - ox)

Question 5

What is the sign of anode of galvanic cell and why?

Answer 5

negative
Anode, oxidation, e- are produced and build up, resulting in negative electrode

Question 6

What occurs at the cathode?

Answer 6

Reduction (red - cat)

Question 7

What is the sign of cathode of galvanic cell and why?

Answer 7

positive
Cathode, reduction, gains electrons thus to receive electrons, cathode must be positive

Question 8

What does salt bridge consist of?

Answer 8

• inverted U-tube consisting of a salt solution, eg. NaNO3 that does not react with ions in either half-cell
• ends of the salt bridge stuffed with porous plugs to prevent direct mixing of two solutions but allow the passage of ions through them

Question 9

What are the functions of a salt bridge?

Answer 9

• Prevents the direct mixing of two solutions but allows the passage of ions between the two half-cells to complete the electrical circuit
• Helps to maintain the electrical neutrality of each half-cell

Question 10

What is emf or cell potential?

Answer 10

potential difference between two electrodes in a galvanic cell that provides a driving force that pushes electrons through an external circuit

Question 11

What is electrode potential?

Answer 11

• When metal is placed in a solution of its ions, some metal atoms lose electrons to form hydrated ions and electrons will be left behind on the metal
• Over time, there is build-up of negative charge on the metal surface which will attract positive metal ions in the solution which may recombine to form metal ions
• At equilibrium, there will be a constant negative charge on the metal and a constant number of positive metal ions present in the solution around the metal
• Absolute electrode potential is the potential difference due to the separation of charge between metal strip and metal ions in the solution

Question 12

By convention, what is the forward reaction of the half equation of electrode potential?

Answer 12

reduction

Question 13

How to write electrode potential or reduction potential?

Answer 13

E(M n+(aq)/M (s))

Question 14

What does a positive electrode potential mean?

Answer 14

Position of equilibrium lies more to the right, reduction favoured

Question 15

What does a negative electrode potential mean?

Answer 15

Position of equilibrium lies more to the left, oxidation favoured

Question 16

What would affect the value of electrode potential?

Answer 16

Factors that affect the position of equilibrium

Question 17

What are the factors affecting electrode potential?

Answer 17

1. Nature of metal
2. Concentration of M n+ ions
3. Temperature
4. Pressure of gaseous species

Question 18

How does the nature of metal affect electrode potential?

Answer 18

When a metal is highly electropositive, the metal atoms have a higher tendency to lose electrons and become positive ions, the position of equilibrium lies more to the left and the electrode potential therefore becomes more negative

Question 19

How does the concentration of M n+ ions affect electrode potential?

Answer 19

If the concentration of the hydrated metal ions is decreased in the equilibrium system, the position of the equilibrium will shift to the left to replenish some of metal ions removed, thus electrode potential becomes more negative

Question 20

How does temperature affect electrode potential?

Answer 20

If the temperature increases, there is an increase in tendency of metal to dissolve in the solution to form hydrated ions, the position of equilibrium shifts to the left and electrode potential becomes more negative

Question 21

How does pressure of gaseous species affect electrode potential?

Answer 21

When the pressure of gas increased, the position of equilibrium shifts to the right to reduce the number of gaseous molecules and hence the electrode potential becomes more positive

Question 22

What is the conditions for standard electrode potential?

Answer 22

• 298 K and 1 bar
• All solutions have a concentration of 1 mol dm-3
• Any gas involved has a pressure of 1 bar
• Platinum is used as electrode when half-cell does not include a metal
• Voltmeter has high resistance so negligible current flows and maximum potential difference can be measured

Question 23

Can the absolute electrode potential of a single half-cell be measured?

Answer 23

No, only the difference in potentials of two electrodes in a complete circuit is measurable

Question 24

What is standard electrode potential of an electrode?

Answer 24

Relative potential of this electrode under standard conditions compared with the standard hydrogen electrode whose electrode potential is assigned as 0.00V

Question 25

What does standard hydrogen electrode consist of?

Answer 25

H2 (g) at 1 bar bubbling over platinum electrode coated with finely divided platinum which is dipped into 1 mol dm-3 H+ (aq) at 298K

Question 26

What is the three most common types of half-cells used to measure standard electrode potential?

Answer 26

- metal in contact with its ions in aqueous solution - non-metal in contact with its ion in aqueous solution - ions of the same element in different oxidation states

Question 27

Is electrode potential dependent on the number of electrons being transferred?

Answer 27

It is independent

Question 28

Are species on the left or right of the equation oxidising agents?

Answer 28

Oxidising agents itself is always reduced, therefore it is on the left side

Question 29

Are species on the left or right of the equation reducing agents?

Answer 29

Reducing agents itself is always oxidised, therefore it is on the right side

Question 30

Does a stronger oxidising agent have a more positive or more negative standard electrode potential?

Answer 30

Stronger oxidising agents have a greater tendency for itself to be reduced, therefore it has a more positive standard electrode potential as the forward reaction is favoured

Question 31

Does a stronger reducing agent have a more positive or more negative standard electrode potential?

Answer 31

Stronger reducing agents have a greater tendency for itself to be oxidised, thus the backward reaction is favoured and it has a more negative standard electrode potential

Question 32

How to determine the relative stabilities of metal ions in different oxidation states?

Answer 32

If it is reduction, standard electrode potential more negative, more stable If it is oxidation, standard electrode potential more positive, more stable

Question 33

What is standard cell potential?

Answer 33

Electromotive force of a galvanic cell which consists of two half-cells connected under standard conditions. It is the difference between the two standard electrode potentials

Question 34

What is the equation to calculate standard cell potential?

Answer 34

E cell = E (red) - E (ox) *note that both values are standard reduction potential, meaning that you don't flip the sign when you flip the equation

Question 35

How to predict the spontaneity of redox reaction with standard cell potential?

Answer 35

- If E(cell) > 0, forward reaction is energetically feasible, spontaneous under standard conditions - If E(cell) < 0, forward reaction is not energetically feasible, the reverse reaction is expected to be spontaneous under standard conditions - If E(cell) = 0, equilibrium is established

Question 36

What is the equation linking Gibbs free energy and electrode potential?

Answer 36

delta G naught = - nFE(cell) naught n: amount of electrons transferred

Question 37

What are the limitations of standard electrode potentials?

Answer 37

- E(cell) value indicates whether a reaction is possible (i.e. energetic stability) but not the rate of reaction (i.e. kinetic stability), eg. high activation energy - E(cell) value relate only to standard conditions, changes in temperature, pressure and concentration will affect the values of electrode potential

Question 38

How does fuel cell differ from conventional electrochemical cells?

Answer 38

- Fuel cells have a steady supply of reactants to produce an electric current - Fuel cell (reactants) are continuously replaced as they are consumed and the products are continuously removed as they are formed

Question 39

What are the surface of electrodes of fuel cells coated with?

Answer 39

Surface of porous graphite electrodes are coated with Ni and NiO which act as catalysts (Pt and Pd can also be used)

Question 40

What are the advantages of fuel cell?

Answer 40

- Pollution free - High power to mass ratio - Highly efficient, with at least 70% of chemical energy converted into electricity

Question 41

What are the disadvantages of fuel cell?

Answer 41

- Expensive due to large amounts of Ni and Pt required as catalysts - catalyst are easily poisoned by impurities and are unable to function efficiently for long periods of time - A high temperature is required to initiate a reaction in a fuel cell