Intro, cell potentials and thermodynamics

Question 1

Name the 4 energy storage categories

Answer 1

Potential, kinetic, thermal and electrochemical

Question 2

True or False: Fuel cells store energy

Answer 2

False - they convert it

Question 3

Which reaction occurs at the cathode?

Answer 3

Reduction

Question 4

What are the units of current?

Answer 4

Ampere

Question 5

What are the units of potential?

Answer 5

Volt

Question 6

What are the units of resistance?

Answer 6

ohm

Question 7

What are the units of electrical conductance?

Answer 7

siemens

Question 8

What are the units for electric charge?

Answer 8

coulomb

Question 9

What does Faraday’s law describe?

Answer 9

The relationship between the amount of current that flows through the external circuit and the amount of material that is either consumed or produced in a half-cell reaction.

Question 10

Write the equation for total charge passed (Q) at a constant current

Answer 10

Q = current x time

Question 11

Define ‘faradaic efficiency’. What does it help to determine?

Answer 11

The fraction of the total current that drives the desired reaction. It helps to identify the desired reaction from the multiple reactions that take place at a single electrode.

Question 12

What is current density? What does it help to identify.

Answer 12

The current divided by the area of the electrode. It helps to understand the reactions that occur on the electrode surface.

Question 13

Define electrostatic potential

Answer 13

The work required to move a unit charge from infinity to a position in the metal or solution in Volt

Question 14

Write the equation relating current density and electrostatic potential. What does this characterise?

Answer 14

How well the material moves charge from a high potential to low potential

Question 15

By what transport mechanism does current transfer from high to low potential?

Answer 15

Convection

Question 16

How is the potential drop/potential difference across the electrolyte calculated?

Answer 16

Modified ohms law - integration

Question 17

What are the two categories of electrochemical cells?

Answer 17

Galvanic and electrolytic

Question 18

What is the difference between a galvanic and electrolytic cell?

Answer 18

Electrical work/energy is the output of a galvanic cell whereas it is the input of an electrolytic cell.

Question 19

Is the chlor-alkali process galvanic or electrolytic?

Answer 19

Electrolytic - DC supplied electrical work

Question 20

True or false: You can control both potential and current.

Answer 20

False - you cannot control both simultaneously

Question 21

What happens to the current (rate of production) when potential is increased? What is this mode of operation called?

Answer 21

Current (rate of production) also increases. This is potentiostatic mode.

Question 22

What is the mode of operation when the current (rate of production) is set? What happens to the cell potential?

Answer 22

Galvanostatic.

Question 23

Picturing an I-V curve, what happens to the cell potential in galvanic mode vs electrolytic mode? Why?

Answer 23

Cell potential is below the equilibrium potential in galvanic mode whereas it is above in electrolytic mode. This is due to irreversible losses when chemical energy is converted to electrical energy in galvanic mode (i.e. discharging)

In electrolytic mode, the applied potential must be greater than the equilibrium voltage to generate and move current.

Question 24

What does equilibrium mean in an electrochemical system? What is this condition called?

Answer 24

No net current flow. This is called open circuit

Question 25

Define equilibrium potential

Answer 25

zero external current + general dynamic equilibrium within cell

Question 26

Describe the reactions that occur at each of the electrodes and whether an electron is being accepted or gained.

Answer 26

** DURING DISCHARGING ** Anode - oxidation - electrons are lost - ionic charge becomes more positive Cathode - reduction - electrons are gained - ionic charge becomes more negative

Question 27

How do you calculate the thermodynamic potential?

Answer 27

* If no external voltage is being supplied * The difference in potential, subtracting the more negative potential from the more positive. This is usually cathode minus anode.

Question 28

True or false: During discharging, electrons flow from low to high potential. This is natural due to its negative charge.

Answer 28

True

Question 29

What are half-cell reaction potentials measured against?

Answer 29

A reference electrode called the standard hydrogen electrode (SHE)

Question 30

What is the Faraday constant?

Answer 30

The conversion factor between the mole and the coulomb. The Faraday constant is the quotient of avogadros constant divided by one coulomb equivalent (1/e). = 96485 C/eq

Question 31

How do you calculate the maximum amount of work available in a closed reversible system at a constant temperature and pressure?

Answer 31

W (electrical) = G = - n F U

Question 32

How do you calculate thermodynamic potential at standard conditions using Gibbs energy data?

Answer 32

Gibbs for the reaction = sum ( stoic x Gibbs for component i) ** Only use either the ionic species data OR neutral aqueous, NOT both ** U = - (Gibbs reaction)/ nF

Question 33

What is approximated value for activity of pure solid components, solvents (water) and electrons?

Answer 33

1

Question 34

Why is a simplified activity correlation used in the thermodynamic potential equation?

Answer 34

To correct the cell potential with the products on the top and reactants on the bottom.

Question 35

What is the value of the thermodynamic potential (and Gibbs) at equilibrium?

Answer 35

0

Question 36

What is the purpose of the reference electrode?

Answer 36

Provides a known and stable potential which other potentials can be measured against.

Question 37

True or false: Current passes through a reference electrode.

Answer 37

False, it doesn't. This is so that it remains at its equilibrium potential.

Question 38

What are the 4 characteristics of a reference electrode?

Answer 38

1. Reversible reactions 2. Stable and well-defined potential 3. Ion(s) in the reference electrode reaction is present in solution 4. No offset in potential caused by liquid junctions

Question 39

Give 3 examples of reference electrodes.

Answer 39

Hydrogen electrode, calomel electrode and silver-silver chloride electrode.

Question 40

Draw a Daniel Cell with copper and Pb(II)

Answer 40

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