Topic 21: Electrochemistry

Question 1

Description of reduction reactions (3)

Answer 1

a) Gain of e- / O
b) Oxidation number decreases
c) Loss of H

Question 2

Description of oxidation reactions (3)

Answer 2

a) Loss of e- / O
b) Oxidation number increases
c) Gain of H

Question 3

What happens in a redox reaction?

Answer 3

One substance always becomes reduced while the other one oxidized

Question 4

Function of a voltaic cell (2)

Answer 4

a) Use spontaneous redox (ΔG < 0) reactions to produce electrical energy
b) System does work on the surroundings

Question 5

Structure of a voltaic cell

Answer 5

Two half-cells joined by a wire and a salt bridge

Question 6

Definition of half-cell

Answer 6

Metal in contact with an aqueous solution of its own ions

Question 7

Function of a salt bridge

Answer 7

Ions flow through the half-cells to balance the charges and maintain the potential difference.

Question 8

Electrodes based on REDOX in voltaic cells

Answer 8

a) Anode | Oxidation / - / Left
b) Cathode | Reduction / + / Right

Question 9

Electrodes based on activity

Answer 9

a) Active - Metals are components of half-reactions
b) Inactive - No reaction components used as electrodes (Pt / Graphite)

Question 10

Process of voltaic cells
a) External circuit
b) Salt bridge

Answer 10

a) Electricity is produced because e- flow from the oxidized substance to the reduced species where e- are gained
b) The salt bridge completes the circuit and neutralizes any buildup of charge by the following ion movement

Question 11

Description of ion flow in salt bridge

Answer 11

a) Anions in the salt bridge move from the cathode to the anode
b) Cations move in the salt bridge move from the anode to the cathode

Question 12

Cell Notation Examples

Answer 12

Pt(s)|H2(g),H+(aq) ‖ Fe3+(aq),Fe2+(aq)| Pt(s)
Zn(s)|Zn2+(aq) ‖ Cu2+(aq)|Cu(s)

Question 13

Explanation for spontaneity in voltaic cells

Answer 13

Electrons move spontaneously from areas of higher PE to areas of lower PE
a) Each electrode tends to lose or gain e- when it is in contact with the solution of its ions
b) A potential difference between the electrodes will allow the flow of e-

Question 14

Standard electrode potential Eθ(half-cell)

Answer 14

Electrical potential of a half-reaction to reduce with all components in their standard states

Question 15

Standard cell potential Eθ(cell)

Answer 15

Potential difference between two electrodes in a voltaic cell to undergo reduction under standard conditions
a) No current flowing

Question 16

Standard conditions

Answer 16

298K
100kPa
1M
All elements in their standard state

Question 17

Calculation + Units

Answer 17

Eθcell = Eθ(cathode) - Eθ(anode)
V = J/C

Question 18

Does cell potential depend on the amount of substance?

Answer 18

No. It is an intensive property

Question 19

Standard hydrogen electrode definition

Answer 19

Half-cell with solution of 1 M H+ ions at 298 K, H2(g) at 100 kPa and Pt electrode

Question 20

Importance of SHE

Answer 20

a) Reference point to measure electrode potential of other half cells
b) Linked to a second half-cell through an external circuit

Question 21

Reaction of SHE

Answer 21

2H+ (aq;1M) + 2e- ↔ H2 (g;1atm)
Eθ = 0 V

Question 22

Relation between reduction electrode potential and oxiding/reducing strength

Answer 22

a) The higher the electrode potential, the stronger the oxidizing strength
b) The higher the electrode potential, the weaker the reducing strength

Question 23

Explanation of activity series using electrode potential
a) Metals that can displace H2 from acid
b) Metals that cannot displace H2 from acid
c) Metals that can displace H2 from water
d) Metals that displace other metals from solution

Answer 23

a) Lower electrode reduction potential
b) Higher electrode reduction potential
c) Very low electrode potential
d) Lower electrode potential

Question 24

Relationship between Ecell and spontaneity
a) Ecell > 0
b) Ecell < 0
c) Ecell = 0

Answer 24

a) Spontaneous
b) Nonspontaneous
c) Equilibrium

Question 25

Relationship between free energy and standard cell potential

Answer 25

∆G = w(max) = -Ecell × charge ∆G = -nFEcell

Question 26

Relationship between equilibrium constant and standard cell potential

Answer 26

Ecell = (RT/nF) ln⁡(K) E°cell = (0.0592/n) log⁡(K) at 298.15 K

Question 27

Effect of concentration in cell potential

Answer 27

(a) ∆G = ∆G° + RT ln⁡(Q) (b) Ecell = E°cell - (RT/nF) ln⁡(Q)

Question 28

Principles of concentration cell

Answer 28

a) Voltaic cell of a concentrated solution of a substance with a dilute solution b) Final solution has an intermediate concentration c) Ecell depends on the ratio of concentrations

Question 29

Process of concentration in a) Anode (Dilute) b) Cathode (Concentrated)

Answer 29

a) Oxidation causes solution to become more concentrated b) Reduction causes solution to become less concentrated

Question 30

Reaction that measures pH

Answer 30

2H+ (aq;1M) → 2H+ (aq;unknown)

Question 31

Calculation of pH using Ecell

Answer 31

Ecell = -0.0592/n log⁡([H+unknown)]^2)/([H+standard]^2) Ecell = 0.0592×pH

Question 32

Composition of a pH electrode

Answer 32

a) Glass electrode - Ag/AgCl half-reaction immersed in HCl solution -Enclosed by a thin glass membrane sensitive to H+ ions b) Reference electrode - Pt wire immersed in calomel (Hg2Cl2) paste, liquid Hg, and saturated KCl

Question 33

Function of electrolytic cells

Answer 33

Use electrical energy from an external source to drive a nonspontaneous redox reaction

Question 34

Description of electrodes in electrolytic cells

Answer 34

Made up of an inert, heat resistant material like graphite or Pt

Question 35

Electrodes based on REDOX in electrolytic cells

Answer 35

a) Anode (Oxidation | +) b) Cathode (Reduction | -)

Question 36

Structure of electrolytic cells

Answer 36

Electrodes are placed in an electrolyte and connected to the power source by electrical wires a) Power source = Battery b) Electrodes cannot touch each other c) Electrolyte = Liquid substance

Question 37

Definition of electrolysis

Answer 37

Splitting of a substance by the input of electrical energy into its elements

Question 38

General reaction during electrolysis

Answer 38

a) Ions attracted by the oppositely charged electrodes b) Cation is reduced while anion is oxidized

Question 39

In mixed molten salts, which species is reduced at the cathode?

Answer 39

The more easily reduced Metals with high Ionization energy

Question 40

In mixed molten salts, which species is oxidized at the cathode?

Answer 40

The more easily oxidized Nonmetals with lower electronegativity

Question 41

Are E values useful in assessing the reduction/oxidation of molten salts?

Answer 41

No. As those refer to species in aqueous solutions

Question 42

Can pure water easy to electrolyze?

Answer 42

It proceeds rapidly by adding small amount of a salt that cannot be electrolyzed

Question 43

Oxidation reaction of water

Answer 43

2H2O(l) → O2(g) + 4H+(aq) + 4e- E = 0.82 V (1.4 V with overvoltage)

Question 44

Reduction reaction of water

Answer 44

2H2O(l) + 2e- → 2OH-(aq) + H2(g) E = -0.42 V

Question 45

Definition of overvoltage

Answer 45

Increment above the expected voltage Common in the production of gases

Question 46

Predicting electrode products in cathode

Answer 46

a) Cations of less active metals are reduced (Au/Ag/Cu/Cr/Pt/Cd) b) Cations of more active metals are not reduced (G1/G2/Al) => Production of H2(g) + OH-(aq)

Question 47

Predicting electrode products in anode

Answer 47

a) Anions that are oxidized include halides, except F-(aq) b) Anions that are not oxidized include F-(aq) and common oxoanions - Production of O2(g) and H+(aq)

Question 48

Faraday law

Answer 48

The amount of substance produced at each electrode is directly proportional to the quantity of charge flowing through the electrolytic cell

Question 49

Relationship between current and charge

Answer 49

Charge (C) = Current (A) x Time (t)