Electrochemistry Flashcards
Define the term electrochemistry.
The interconversion of electrical + chemical energy.
How is this interconversion achieved?
Batteries: producing electricity from a chemical reaction.
Electrolysis: forcing an unfavourable reaction to occur by applying a voltage.
Define what is meant by a redox reaction.
Loss of electrons from one species + their gain by another species.
What are the SI units of electrochemistry?
- Electric charge: coloumb C.
- Electric current (I): amps A (1A = 1Cs-1)
- Potenital difference: volts V (V = I x R)
What is an electrochemical cell?
Two half cells in each of which a particular redox reaction is occurring.
What is a galvanic cell?
An electrochemical cell that produces electricity as a result of a spontaneous reaction.
What is an electrolytic cell?
A non-spontaneous reaction is forced by an external source of current.
What is a fuel cell?
A galvanic cell in which reagents are supplied continuously.
What is a gas electrode?
A gas is in equilibrium with a solution of its ions in the presence of an inert metal.
The metal acts as a conductor AND a catalyst.
What is a redox electrode?
An inert metal in a solution in which a species exists in two oxidation states.
The metal acts as a conductor.
What is a metal/metal ion electrode?
A piece of metal in contact with a solution that contains ions of the same metal e.g. Daniell Cell.
What is a metal-insoluble salt electrode?
A metal coated with an insoluble salt of the same metal e.g. silver-silver chloride.
What is a calomel electrode?
An alternative reference electrode.
What is a glass electrode?
An ion-selective electrode, specific for H+ ions. An Ag/AgCl electrode is immersed in a buffer solution, when the electrode is placed in a solution whose pH is different from that of the buffer, a potential difference develops.
Outline the events at the anode (4).
Oxidation occurs.
Negative.
Loss of electrons which travel through the external circuit.
Left hand side.
Outline the events at the cathode (4).
Reduction occurs.
Positive.
Gain of electrons.
Right hand side.
Outline the role of the salt bridge.
Completes the circuit + allows ions to move, ensuring that the electrolyte solutions remain neutral.
For electrochemical cell notation, what does a comma denote?
A comma denotes two species in the same phase.
How do we use standard reduction potentials? (Equations).
RHS Cathode:
Aox + ne- –> Ared
Eo (Aox/Ared)
LHS Anode:
Bred –> Box + ne-
Eo = -Eo (Box/Bred)
Overall: RHS - LHS
Eo(cell): Eo(Aox/Ared) - Eo(Box/Bred)
Outline the reactions in a H2 fuel cell.
Anode: H2 –> 2H+ + 2e-
Cathode: 1/2O2 + 2H+ +2e- –> H2O
Overall: 2H2 + O2 –> 2H2O
State the equation for gibbs free energy change (∆G) and annotate.
∆G = -nFE
n: number of electrons transferred
F: Faraday constant
E: standard cell potential
State the equation that combines standard cell potential + equilibrium constant.
lnK = nF/RT Eocell
What values for this equation favour products/reactants?
Products: E > 0, lnK > 0, K > 1
Reactants: E < 0, lnK < 0, K < 1
State the Nernst equation and annotate.
Ecell = Eocell - RT/nF lnQ
Ecell: cell potential Eocell: standard cell potential R: ideal gas constant T: temperature in K n: number moles of electrons F: Faraday constant Q: reaction quotient
State why ions in solution are not ideal (3).
- Do not move independently of one another
- Are surrounded by an ionic atmosphere
- Have attractive and repulsive interactions (described the activity coefficient gamma)
What is the result of ideals being not ideal?
Solutions behave less concentrated than they are due to the non-ideal affects of ions.
State the equation used when identifying changes in Eo/Gibbs free energy.
∆S = nF (E’ - E)/T’ - T
What is an electrolytic cell?
An electric current is used to drive a non-spontaneous reaction. There is one electrolyte, both electrodes are in one compartment.
Give two examples of electrolytic cells.
- Molten salts with inert graphite electrodes.
2. Soluble salts with copper electrodes.
Define electrolysis of water.
The decomposition of water into oxygen and hydrogen gas, due to an electric current being passed through the water.
At 25oC and pH 7, the potential needed is 1.23V.
