CONDUCTANCE Conductors

- Flow of electricity through a conductor involves a 1. ELECTRONIC CONDUCTORS

transfer of electrons from a point of higher - Include solid/molten metals and certain solid salts
negative potential to one of lower negative - Example: CuS and CdS
potential
- Conduction takes place by direct migration of
- The mechanism varies for different conductors. electrons through the conductor under the
influence of an applied potential

- Atoms and ions composing the conductor are not

involved in the process
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Conductors Electrolysis

2. ELECTROLYTIC CONDUCTORS - The splitting of a substance by the input of

- Include solutions of strong and weak electrolytes electrical energy
- Often used to decompose a compound into its
- Electron transfer takes place by migration of ions elements
(accomplished by a transport matter)
- (+) and (-) ions do not carry equal portions of the - In predicting the product, the general rule is that
current, thus, a concentration gradient develops
▪ the more easily oxidized species (stronger reducing
agent) reacts at the anode and
- Current flow in electrolytic conductors is ▪ the more easily reduced species (stronger oxidizing
accompanied by a chemical reaction (electrolysis) agent) reacts at the cathode
at each electrode.
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 Identification of Reaction at the Electrode
Electrolytic Cell
in an Electrolytic Cell

- A cell in which non-spontaneous redox reaction

1. ELECTROLYSIS OF PURE MOLTEN SALT
is made to occur by the action of an external - Predicting the product is simple because the
potential electrolyte is the molten salt itself
CATION – Reduction; ANION – Oxidation
- Involved in key industrial - Example: Industrial preparation of metallic calcium
production steps for isolating and Cl2 from CaCl2
some of the most commercially
Anode: 2Cl-(l) → Cl2(g) + 2e-
important elements, including
chlorine, aluminum, copper, etc. Cathode: Ca2+(l) + 2e- → Ca(s)
OCR* Ca2+ + 2Cl-(l) → Ca(s) + Cl2(g)
*OCR: Overall Cell Reaction
Photo from: By © Nevit Dilmen, CC BY-SA
5 3.0, 6
https://commons.wikimedia.org/w/index.php?curid=10959462

continuation… continuation…
2. ELECTROLYSIS OF MIXED MOLTEN SALT 2. ELECTROLYSIS OF MIXED MOLTEN SALT
- Electrolyte is a mixture of molten salts which is
then electrolyzed to produce a specific metal CATION – Reduction ANION - Oxidation
- knowledge of PERIODIC ATOMIC TRENDS is Periodic Atomic Trend:
significant in predicting which ion gains or loses IONIZATION ENERGY (IE) ELECTRONEGATIVITY (EN)
electrons more easily
If a metal holds its electrons If a nonmetal holds its electrons
more tightly than the other less tightly than the other
- NOTE: Tabulated E° values cannot be used to tell the relative • then it has a higher IE • then it has a lower EN
strengths of oxidizing and reducing agents since those values • as a cation, it gains electrons • as an anion, it loses more
refer to the change from aqueous ion to free element under more easily electrons more easily
standard state conditions • it is the stronger oxidizing • it is the stronger reducing
▪ ex. Mn+(aq) + ne- → M(s) agent agent
▪ there are no aqueous ions in the molten salt • and is reduced at the cathode • is oxidized at the anode
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 continuation…
Sample Problem:
3. ELECTROLYSIS OF AQUEOUS SOLUTIONS
1. A chemist and a chemical engineer melt a
naturally occurring mixture of NaBr and MgCl2 and - Water may participate in the reduction/oxidation
decompose it in an electrolytic cell. Predict the half-reactions
substance formed at each electrode and write the - Tabulated E° values can be used to identify the
balanced half-reactions and the overall relative strengths of oxidizing and reducing agents
electrolytic reaction.
because those values refer to the change from the
aqueous ion to free element:
2. A sample of AlBr3 contaminated with KF is melted
Mn+(aq) + ne- → M(s) (under std state condition)
and electrolyzed. Determine the electrode
products and the overall cell reaction.

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continuation…
Sample Problem: 2. Identify possible anode and cathode reactions
from reduction potential table (follow the link
- What reaction occur at each electrode in the Electrochemical Series)
electrolysis of aqueous copper fluoride (CuF2)?
Possible cathode reactions:
• Cu2+(aq) + 2e- → Cu(s) E° = 0.34 V
SOLUTION:
• 2H2O(l) + 2e- → H2(g) + 2OH-(aq) E° = -0.83 V at pH 14
- Species: Cu2+, F-, and H2O
• 2H2O(l) + 2e- → H2(g) + 2OH-(aq) E° = -0.41 V at pH 7
1. Identify species that could be oxidized or reduced
- Oxidized? F- and H2O At the cathode, reduction of copper generates
- Reduced? Cu2+and H2O some voltage, while the reduction of water uses
electrical energy. Copper metal is deposited at the
cathode due to the difference in potential.
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continuation…
Faraday’s Law of Electrolysis
Possible anode reactions:
- Michael Faraday (1791-1867); the great English
• 2F-(aq) → F2(s) + 2e- E° = -2.87 V
physicist and chemist
• 2H2O(l) → O2(g) + 4H+(aq) + 4e- E° = -1.23 V at pH 0
- Determines the amount of electrolysis product that
• 2H2O(l) → O2(g) + 4H+(aq) + 4e- E° = -0.82 V at pH 7 will be produced by passage of a given amount of
electrical current
At the anode, oxygen gas is evolved, because less
voltage is required to oxidize water than to oxidize
fluoride ion.

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First Law of Electrolysis Second Law of Electrolysis

- The mass of a substance altered at an electrode during
electrolysis is directly proportional to the quantity of - when the same quantity of electricity is passed
electricity transferred at that electrode. through several electrolytes, the mass of the
- the amount of chemical change produced by current at substances deposited are proportional to their
an electrode-electrolyte boundary is proportional to respective chemical equivalence or equivalent
the quantity of electricity used weight.
▪ m = mass of the substance ▪ One equivalence of a chemical reaction is
produced by electrolysis produced by the passage of one Faraday, F =
▪ Q = quantity of electric charge
96485 C/equiv
▪ M = molar mass of the
substance ▪ An equivalence of a substance is associated with
▪ F = Faraday’s constant one mole of electrons
▪ n = electrons transferred per ion
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Sample Problems: Sample Problems:
a. Calculate the mass of Al produced in 30 mins by the a. Calculate the mass of Al produced in 30 mins by the
electrolysis of molten AlCl3 if the electric current is electrolysis of molten AlCl3 if the electric current is
10.0 A. 10.0 A.

b. What mass of copper (in g) is deposited by a current b. What mass of copper (in g) is deposited by a current
of 1.5 A in 1.00 hour in the electrolysis of CuSO4 of 1.5 A in 1.00 hour in the electrolysis of CuSO4
solution? solution?

c. Given a series of electrolytic cells consisting of c. Given a series of electrolytic cells consisting of
chlorides of Al, Mg, Li, iron (II), and copper (I). chlorides of Al, Mg, Li, iron (II), and copper (I).
Determine the amount of materials that will be Determine the amount of materials that will be
deposited if a mole of electron is passed through this deposited if a mole of electron is passed through this
system. system.
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Illustration for part c:

Al3+ Mg2+ Li+ Fe2+ Cu+

MM, g/mol 26.98 24.31 6.94 55.85 63.55
EW, g/eq 8.99 12.155 6.94 27.93 63.55
Mass 8.99 12.155 6.94 27.93 63.55
- Given a series of electrolytic cells. When 1 mole of deposited, g
electrons is passed through the system, different No. of eq, 1 1 1 1 1
quantities of material are generated at each of the
mole e-
cathodes. The quantities of reduced materials are
equal to the equivalent weight of cations being Amount 0.333 0.5 1 0.5 1
reduced. deposited,
mole
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Sample Problems:
d. The platinum crucible used in a silver coulometer
gains 0.500 g in a certain electrolysis. What would
be the gain in weight of copper cathode in a cell
filled with potassium cuprocyanide [KCu(CN)2]
placed in the same circuit?

e. The platinum crucible used in aluminum

coulometer gains 2.35 g in a certain electrolysis.
What would be the gain in weight of iron cathode
in a cell filled with FeCO3 placed in the same
circuit? (MM in g/mol: Al = 26.98, Fe = 55.85)
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