Chapter 30 - Electrochemistry

Question 1

What is electrochemistry?

Answer 1

The study of relationships between chemical reactions and electrical energy

Question 2

What to electrochemical reactions include? What do both reactions involve? What does it result in?

Answer 2

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

Question 3

What is an electrochemical cell?

Answer 3

Contained systems in which redox reaction occurs, either to provide electrical energy or to use electrical energy for a useful purpose

Question 4

What are the two types of electro chemical cells?

Answer 4

1) Galvanic (Voltaic) Cells
- Spontaneous

2) Electrolytic Cells
- Non-spontaneous

Question 5

Describe a Galvanic Cell:

Answer 5

• 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

Question 6

What are the rules used to construct a cell diagram?

Answer 6

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

Question 7

Describe and Electrolytic Cell:

Answer 7

• 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

Question 8

How are charges designated in an electrolytic cell?

Answer 8

• Anode is considered positive because it is attached to positive pole of the battery and attracts anions

Question 9

How are charged designated in a Galvanic Cell?

Answer 9

• Anode is considered negative because the spontaneous oxidation reaction is originally the source of the cells negative charges

Question 10

Where does oxidation take place in both cells?

Answer 10

Anode

Question 11

How is charge created in a Galvanic cell?

Answer 11

It is created spontaneously as electrons are released by the oxidizing species at the anode
- Anode is always considered negative

Question 12

How is charge created in an electrolytic cell?

Answer 12

Electrons are forced through the cathode where they encounter the oxidizing agent
- Since the cathode provide electrons, it is considered negative

Question 13

What is electrophoresis?

Answer 13

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

Question 14

How is the number of moles determined in an electrochemical cell ?

Answer 14

From the balance reaction for that cell

Question 15

What is the Faraday constant?

Answer 15

96 487 C/mol e

Question 16

What is the equation for the number of moles transferred by awn electrochemical cell?

Answer 16

i x t = n x F

Question 17

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)

Answer 17

6 x 10-5 mol Cu

Question 18

How does one determine the species in a redox reaction that will be oxidized or reduced?

Answer 18

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

Question 19

What is reduction tendency measured in and defined relative to?

Answer 19

Volts and standard hydrogen electrode (SHE)

Question 20

What is the standard reduction potential (E) measured under?

Answer 20

Standard conditions:

• 25C
• 1M concentration for each ion participating
• 1atm partial pressure
• Metals in their pure state

Question 21

What does a higher (E) vs. lower (E) represent?

Answer 21

Higher (E) = greater tendency for reduction to occur

Lower (E) = greater tendency for oxidation to occur

Question 22

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

Answer 22

Ag+ reduced to Ag (s)

Tl oxidized to Tl+ (aq)

Due to E-values

Question 23

What is the standard electromotive force (EMF)? How is i calculated?

Answer 23

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)

Question 24

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)

Answer 24

+2.85V

Question 25

What is Gibbs Free Energy (delta G)?

Answer 25

It is a measure of the maximum amount of useful work produced by a chemical reaction
- Determines the spontaneity of a reaction

Question 26

How is deltaG and Ecell related?

Answer 26

DeltaG = -nFEcell

• Galvanic Cell - negative G and positive E
• Electrolytic Cell - positive G and negative E

Question 27

What is used when concentration varies?

Answer 27

Nernst Equation:

Ecell = Ecell - [RT/nF] (lnQ)

Where Q is the reaction quotient:

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