Chapter 30 - Electrochemistry Flashcards
What is electrochemistry?
The study of relationships between chemical reactions and electrical energy
What to electrochemical reactions include? What do both reactions involve? What does it result in?
Spontaneous reactions that produce electrical energy and non-spontaneous reactions that use electrical energy to produce a chemical change
- Both reactions involve a transfer of electrons through the processes of oxidation and reduction, and like other chemical reactions result in the conservation of charge and mass
What is an electrochemical cell?
Contained systems in which redox reaction occurs, either to provide electrical energy or to use electrical energy for a useful purpose
What are the two types of electro chemical cells?
1) Galvanic (Voltaic) Cells
- Spontaneous
2) Electrolytic Cells
- Non-spontaneous
Describe a Galvanic Cell:
- Negative delta G and therefore spontaneous
- Supply energy and used to do work
- Oxidation and reduction half-reaction are in separate containers that drive the flow of electrons creating a circuit
- Cations from the salt bridge flow to the cathode and anions flow to the anode
What are the rules used to construct a cell diagram?
1) Reactants and products are used in constructing a cell diagram
Anode | Anode Solution || Cathode Solution | Cathode
2) A single vertical line indicates a phase boundary
3) Double vertical line indicates presence of salt bridge or some type of barrier
Describe and Electrolytic Cell:
- Positive G and therefore non-spontaneous
- Electrical energy is required to induce reaction
- Oxidation and reduction half-reaction can be placed into same container since non-spontaneous
- No salt bridge required
- Cations move toward cathode, anions move toward anode
How are charges designated in an electrolytic cell?
- Anode is considered positive because it is attached to positive pole of the battery and attracts anions
How are charged designated in a Galvanic Cell?
- Anode is considered negative because the spontaneous oxidation reaction is originally the source of the cells negative charges
Where does oxidation take place in both cells?
Anode
How is charge created in a Galvanic cell?
It is created spontaneously as electrons are released by the oxidizing species at the anode
- Anode is always considered negative
How is charge created in an electrolytic cell?
Electrons are forced through the cathode where they encounter the oxidizing agent
- Since the cathode provide electrons, it is considered negative
What is electrophoresis?
A technique used to separate amino acids based on their isoelectric points
- Positively charged amino acid cations migrate toward the cathode
- Negatively charged amino acid anions migrate toward the anode
How is the number of moles determined in an electrochemical cell ?
From the balance reaction for that cell
What is the Faraday constant?
96 487 C/mol e
What is the equation for the number of moles transferred by awn electrochemical cell?
i x t = n x F
How many moles of solid copper are produced during a redox reaction that generates 4A of current for 3s according to the reaction (page 349):
Zn(s) + Cu2+ (aq) -> Zn2+ (aq) + Cu (s)
6 x 10-5 mol Cu
How does one determine the species in a redox reaction that will be oxidized or reduced?
Determined from the reduction potential of each species, defined as the tendency of a species to acquire electrons and be reduced
- More positive the potential, more tendency to be reduced
What is reduction tendency measured in and defined relative to?
Volts and standard hydrogen electrode (SHE)
What is the standard reduction potential (E) measured under?
Standard conditions:
- 25C
- 1M concentration for each ion participating
- 1atm partial pressure
- Metals in their pure state
What does a higher (E) vs. lower (E) represent?
Higher (E) = greater tendency for reduction to occur
Lower (E) = greater tendency for oxidation to occur
Given the following half reactions and E-values, determine which species would be oxidized and which would be reduced (page 349):
Ag+ (aq) + e- –> Ag(s) E = +0.80
Tl+ (aq) + e- –> TI(s) E = -0.34
Ag+ reduced to Ag (s)
Tl oxidized to Tl+ (aq)
Due to E-values
What is the standard electromotive force (EMF)? How is i calculated?
The difference in potential between two half-cells
Determined by added the standard reduction potential of the reduced species and oxidized species:
E(cell) = E(redox) + E(oxid)
Give that the standard reduction potential for Sm3+ and [RhCl6]3- are -2.41V and +0.44V, respectively, calculate the Cell of the following reaction (page 350):
Sm3+ (aq) + Rh(s) + 6Cl- (aq) -> [RhCl6]3- (aq) + Sm(s)
+2.85V
What is Gibbs Free Energy (delta G)?
It is a measure of the maximum amount of useful work produced by a chemical reaction
- Determines the spontaneity of a reaction
How is deltaG and Ecell related?
DeltaG = -nFEcell
- Galvanic Cell - negative G and positive E
- Electrolytic Cell - positive G and negative E
What is used when concentration varies?
Nernst Equation:
Ecell = Ecell - [RT/nF] (lnQ)
Where Q is the reaction quotient:
Q = [C]c [D]d / [A]a [B]b
