Electrochemistry

Question 1

Charge: Symbol? SI unit?

Answer 1

Charge:

Symbol = Q

SI unit = Coulomb, C

Question 2

Voltage: Symbol? Measures? SI unit?

Answer 2

Voltage:

Symbol = V

Measures: energy per unit charge

SI unit = Volt, V

Question 3

Current: Symbol? Measures? SI unit?

Answer 3

Current:

Symbol = I

Measures: the rate of flow of charge (electrons or ions)

SI unit = Amps, A

Question 4

Resistance: Symbol? Measures? SI unit?

Answer 4

Resistance:

Symbol = R

Measures: How difficult it is to get current to flow through something

SI unit = Ohm, Ω

Question 5

effect on current and voltage if resistance is high?

Answer 5

If resistance = high, then current = low, and voltage won’t fall as quickly.

Question 6

effect on current and voltage if resistance is extremely high?

Answer 6

If resistance = extremely high, then only a tiny about of current will flow, so voltage is at its maximum – this maximum = zero-current cell potential or electromotive force (emf), symbol = Ecell.

Question 7

OILRIG meaning?

Answer 7

oxidation is loss of electron, reduction is gain of electrons

Question 8

What does a salt bridge do?

Answer 8

The solutions are connected by a salt bridge where the charge is conducted by ions in solution.

Question 9

Roles of the salt bridge:

Answer 9

To balance the charge:

It is extremely unfavourable to allow net charge to build up

With just the wire, the right hand side would become negative

…so repel electrons back

To balance the counterions:

Overall reaction: Zn(s) + Cu2+(aq) + SO4 2- (aq)→ Zn2+(aq) + SO4 2- (aq) + Cu(s)

Zinc + Copper sulphate → Zinc sulphate + Copper

Sulphate goes from being counter-ion of Cu2+ to counter-ion of Zn2

Question 10

What does a voltmeter do?

Answer 10

measures the potential difference.

Question 11

What is an electrolytic cell?

Answer 11

Apparatus which consumes electricity.

Question 12

What is a Daniell cell? Who invented it? What type of electrode is it?

Answer 12

The set-up involving Cu and Zn = a Daniell Cell.

It is named after John Daniell who first used it in 1836.

it’s an example of a metal-ion electrode.

Question 13

What is a redox electrode?

Answer 13

When metal ions exist in multiple oxidation states, e.g. Fe2+ and Fe3+, the metal can switch between both oxidation states, this redox couple can provide a half reaction for an electrode = redox electrode.

Fe2+ (aq) → Fe3+ (aq) + e−.

In order to collect the electron from a redox electrode an inert metal, Pt, is used.

No phase boundary.

Question 14

What is a hydrogen electrode?

Answer 14

Half reactions involving gases can be used – hydrogen electrode.

2 H+ (aq) + 2 e− → H2 (g)

2 phase boundaries.

Question 15

What are the two ways that the term “electrode” can be used?

Answer 15

Electrode can be used in 2 ways:

Refers to the half-cell producing or consuming electrons.

The piece of metal that conducts electrons in and out of the solution and where the transfer of electrons takes place.

Question 16

Notation in representing cell-diagrams:

Answer 16

For example: Zn (s)|Zn2+ (aq) = shows the difference between a solid and a solution.

|| = represents the salt bridge that connects the two half-cells.

= represent a boundary between phases.

Question 17

What is an Anode?

Answer 17

where oxidation occurs – is placed on the left.

Question 18

What is a Cathode?

Answer 18

where reduction occurs – is placed on the right

Question 19

What is the standard hydrogen electrode (SHE) used for?

Answer 19

Using a standard value allows you to compare the potential of the desired half-cell to the standard one and therefore get a value of the half-cell potential.

The standard chosen is the standard hydrogen half-cell, also known as the standard hydrogen electrode (SHE).

Under standard conditions ( a H2 gas pressure of 1 bar and [H+] =1.00M ) at 298K, the potential is measured to be 0.00V - so anything compared with hydrogen will be the total potential value for that half-cell.

Question 20

How are electrode potentials generally written as?

Answer 20

By convention, electrode potentials are always written as reduction potential, so they go in the direction of the reduction reaction.

Question 21

What is the standard reduction potential?

Answer 21

The standard reduction potential, E, is the potential of a reduction half-cell relative to the standard hydrogen electrode, set at E= 0.00 V, at 298 K and 1 bar with all species in their standard states.

Question 22

Properties of Half-cells with highly positive E° values:

Answer 22

accept electrons readily = good oxidising agents.

stronger tendency to occur in the forward reaction.

F is the strongest oxidising agent – has the greatest tendency to be reduced.

Question 23

Properties of Half-cells with highly negative E° values:

Answer 23

donate electrons readily = good reducing agents.

Stronger tendency to occur in the reverse direction.

Li is the weakest oxidising agent – has the greatest tendency to occur in the reverse direction – will likely be oxidised as it is a stronger reducing agent.

Question 24

How to calculate E°(cell):

Answer 24

E°(cell) = E°(cathode) - E°(anode)

Question 25

standard conditions for E°(cell)

Answer 25

Solid; solution concentration = 1 mol dm-3

gas (partial) pressure = 1 bar.

Question 26

What is Electrical work?

Answer 26

Electrical work: moving a charge Q through a potential difference ΔV.

Question 27

What is resistance?

Answer 27

Resistance is the opposition that a substance offers to the flow of electric current

Question 28

Electrical work equation:

Answer 28

Electrical work = -(charge x potential difference)

Question 29

What is Faradays constant?

Answer 29

F = 96485 C/mol.

Question 30

Equation for charge (involving Faradays constant)

Answer 30

If a reaction produces z mol of electrons, the total charge, Q, produced is given by:
Q = zF

Question 31

Equation for the Standard Gibbs energy change for the reaction (involving Faradays constant)

Answer 31

ΔG° = -Z x F x E(cell)

Question 32

Standard Gibbs energy change and the thermodynamic equilibrium constant are related by what equation?

Answer 32

−zFEcell = −RT ln K

Question 33

What is the reaction quotient given by?

Answer 33

Q=[C]c[D]d / [A]a[B]b

The sum of the activities of the products over the sum of the activities of the reactants.

Question 34

What is the Nernst equation?

Answer 34

E(cell) = E°(cell) - (RT / zF) lnQ

Question 35

What happens when Q = K?

Answer 35

the system is at equilibrium.

Question 36

What does electrolysis do?

Answer 36

It drives the oxidation and reduction at each electrode.

Question 37

What is the Activity coefficient?

Answer 37

γX is called the “activity coefficient” of X: correction factor to account for non-ideality (effective concentration less than actual concentration).