Electrochemistry Flashcards
Charge: Symbol? SI unit?
Charge:
Symbol = Q
SI unit = Coulomb, C
Voltage: Symbol? Measures? SI unit?
Voltage:
Symbol = V
Measures: energy per unit charge
SI unit = Volt, V
Current: Symbol? Measures? SI unit?
Current:
Symbol = I
Measures: the rate of flow of charge (electrons or ions)
SI unit = Amps, A
Resistance: Symbol? Measures? SI unit?
Resistance:
Symbol = R
Measures: How difficult it is to get current to flow through something
SI unit = Ohm, Ω
effect on current and voltage if resistance is high?
If resistance = high, then current = low, and voltage won’t fall as quickly.
effect on current and voltage if resistance is extremely high?
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.
OILRIG meaning?
oxidation is loss of electron, reduction is gain of electrons
What does a salt bridge do?
The solutions are connected by a salt bridge where the charge is conducted by ions in solution.
Roles of the salt bridge:
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
What does a voltmeter do?
measures the potential difference.
What is an electrolytic cell?
Apparatus which consumes electricity.
What is a Daniell cell? Who invented it? What type of electrode is it?
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.
What is a redox electrode?
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.
What is a hydrogen electrode?
Half reactions involving gases can be used – hydrogen electrode.
2 H+ (aq) + 2 e− → H2 (g)
2 phase boundaries.
What are the two ways that the term “electrode” can be used?
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.
Notation in representing cell-diagrams:
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.
What is an Anode?
where oxidation occurs – is placed on the left.
What is a Cathode?
where reduction occurs – is placed on the right
What is the standard hydrogen electrode (SHE) used for?
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.
How are electrode potentials generally written as?
By convention, electrode potentials are always written as reduction potential, so they go in the direction of the reduction reaction.
What is the standard reduction potential?
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.
Properties of Half-cells with highly positive E° values:
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.
Properties of Half-cells with highly negative E° values:
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.
How to calculate E°(cell):
E°(cell) = E°(cathode) - E°(anode)
standard conditions for E°(cell)
Solid; solution concentration = 1 mol dm-3
gas (partial) pressure = 1 bar.
What is Electrical work?
Electrical work: moving a charge Q through a potential difference ΔV.
What is resistance?
Resistance is the opposition that a substance offers to the flow of electric current
Electrical work equation:
Electrical work = -(charge x potential difference)
What is Faradays constant?
F = 96485 C/mol.
Equation for charge (involving Faradays constant)
If a reaction produces z mol of electrons, the total charge, Q, produced is given by:
Q = zF
Equation for the Standard Gibbs energy change for the reaction (involving Faradays constant)
ΔG° = -Z x F x E(cell)
Standard Gibbs energy change and the thermodynamic equilibrium constant are related by what equation?
−zFEcell = −RT ln K
What is the reaction quotient given by?
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.
What is the Nernst equation?
E(cell) = E°(cell) - (RT / zF) lnQ
What happens when Q = K?
the system is at equilibrium.
What does electrolysis do?
It drives the oxidation and reduction at each electrode.
What is the Activity coefficient?
γX is called the “activity coefficient” of X: correction factor to account for non-ideality (effective concentration less than actual concentration).
