Electron, Electricity and Electrochemistry.pptx

SENIOR FIVE PHYSICAL CHEMISTRY ONLINE LESSON 280621
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Tuesday, 28 May 2024
1:31 PM
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CHEMISTRY
DEPARTMENT
BY: KAMAGA RASULI

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CHEMISTRY
DEPARTMENT
ELECTROLYSIS AND
ELECTROCHEMISTRY
Electrolytic Conductivity
And
Molar Conductivity

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Factors affecting electrolytic
conductivity of a solution
The factors include:
–Temperature
–Concentration

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The effect of the above
factors depends on the nature
of the electrolyte i.e. strong or
Weak in nature.

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Temperature
For strong electrolytes,
increase in temperature increase
electrolytic conductivity,
Why? As temperature increases, the average
kinetic energy of the conducting ions increases. The
viscosity of the water decreases and the ionic
mobility increases

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Temperature
For weak electrolytes,
the effect depends on enthalpy
changes accompanied with ionization of
the weak electrolytes i.e. gives out heat
(Exothermic) or absorbs heat
(Endothermic)

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Temperature
For a weak electrolyte
that ionizes exothermically, increase in
temperature decreases the degree of dissociation
according to Le Chatelier’s principle, decreasing the
number of conducting ions per unit volume of
solution thus decreasing the electrolytic
conductivity

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Temperature
For a weak electrolyte
that ionizes endothermically, increase in
temperature increases the degree of dissociation
according to Le Chatelier’s principle, increasing
the number of conducting ions per unit volume of
solution thus increasing the electrolytic
conductivity.

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CHEMISTRY
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Molar Conductivity

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Factors affecting Molar Conductivity
• Temperature of the
solution
• Magnitude of the
charge on the ions
• Ionic radius
• Viscosity of the
solvent
• Concentration of
the solution
• Temperature

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1.Temperature of solution
For strong electrolytes
molar conductivity increases with increase in
temperature
Explanation
Increase in temperature increases the average
kinetic energy of conducting ions, decreasing the
viscosity of water, increasing the ionic mobility.

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For weak electrolytes
the effect of temperature depends on the enthalpy
change accompanying ionization of the electrolyte.
For weak electrolyte that ionizes exothermically,
increase in temperature decreases the degree of
ionization according to Le Chatelier’s principle,
decreasing the number of conducting ions per unit
volume, thus decreasing the molar conductivity

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For a weak electrolyte that ionizes
endothermically,
increase in temperature increases the degree of
dissociation according to Le Chatelier’s principle,
increasing the number of conducting ions per unit
volume of solution thus increasing the molar
conductivity.

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Trial Question
a)Distinguish between electrolytic
conductivity and molar conductivity
b)The molar conductivity of ethanoic acid
containing is 8.50 units at 18 and 14.7
units at 100 . Explain the difference in
molar conductivities at the two
temperatures.

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2. Magnitude of the charge on the ions
Molar conductivity increases with increase in the
magnitude of ionic charge.
Why?
Ions with high charges are more strongly
attracted by the oppositely charged electrode and move
faster.

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3. Ionic radius
Molar conductivity increases with increase in ionic
radius.
The higher the ionic radius, the lower the charge
density and the less hydrated the ion.
The less hydrated ions move more faster than heavily
hydrated ions hence ions with smaller ionic radius are
more conducting than ions with higher ionic radius

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Trial Questions
Explain why the:
(a) magnesium ion has a higher molar
conductivity than the sodium ion
(b) sulphate ion has a higher molar conductivity
than the chloride ion.
(c) the molar conductivity of the lithium ion is
smaller than that of the sodium ion.

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4. Viscosity of the solvent
The higher the viscosity of the solvent,
the lower the ionic mobility hence the lower
the molar conductivity

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5. Concentration of the solution
The sketches below show the variation of molar conductivity with the square root
of concentration for both a strong electrolyte and a weak electrolyte

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Explanation of the shapes of the graphs
For potassium chloride (strong electrolyte),
increase in concentration, decreases molar
conductivity. WHY?
Increase in concentration increases the total
number of conducting ions per unit volume,
decreasing inter ionic distance, increasing ionic
interference decreasing ionic mobility.

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At zero concentration (infinite dilution),
molar conductivity reaches a maximum
value called molar conductivity at infinite
dilution because ionic interference is
negligible

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Note:
Molar conductivity at infinite dilution is obtained
from the graph by extrapolation.

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For Ethanoic acid (Weak electrolyte),
Increase in concentration decreases molar conductivity.
WHY? Being a weak electrolyte,
Increase in concentration decreases the degree of
dissociation decreasing the number of conducting ions
per unit volume of solution.
Instead of concentration, reciprocal of concentration(
1
𝐶
),
also called dilution can be used.

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5. Concentration of the solution
The sketches below show the variation of molar conductivity
with the dilution for both a strong electrolyte and a weak
electrolyte.

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For potassium chloride (Strong electrolyte),
Increase in dilution increases molar conductivity
to a maximum value at which it remains constant at
infinite dilution. WHY?
Increase in dilution increases inter-ionic distance
decreasing ionic interference, increasing ionic
mobility.

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Note:
At zero concentration (infinite dilution), the
ionic interference is negligible.
Molar conductivity at infinite dilution is
obtained from the graph by extrapolation.

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For Ethanoic acid (Weak electrolyte),
Increase in dilution increases molar
conductivity.
WHY? Being a weak electrolyte,
Increase in dilution increases degree of
dissociation increasing the number of conducting
ions per unit volume of solution.

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Graphs
NOTE:
1. Neither the graph of molar conductivity
against concentration nor those against dilution
begin from the origin.
2. The graphs DO NOT coincide anywhere
too.

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Trial Questions
1. The table below shows values of molar conductivity of
ethanoic and potassium chloride at different
concentrations.
Concentration (moldm-
3)
0.0001 0.001 0.01 0.1 1 2 3
𝚲𝐜 𝐊𝐂𝐥 𝛀𝐜𝐦𝟐
𝐦𝐨𝐥−𝟏 129.1 127.3 122.4 112.0 98.3 92.6 88.3
𝚲𝐜 𝐂𝐇𝟑𝐂𝐎𝐎𝐇 𝛀𝐜𝐦𝟐𝐦𝐨𝐥
107.0 41.0 14.3 4.6 1.32 0.80 0.54

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(a). On the same axes, plot graphs for molar
conductivity against the square root of
concentration.
(b). State and explain the shapes of the
graphs in (a) above.

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UNEB QUESTION BANK
For more Questions, Kindly Check:
1. 2006/P1/No.11
2. 2005/P2/No.4
3. 1996/P1/No.15
And more,

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KOHLRAUSCH’S LAW OF
INDEPENDENT MIGRATION
OF IONS AND ITS
APPLICATION.

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KOHLRAUSCH’S LAW OF INDEPENDENT
MIGRATION OF IONS
Kohlrausch Friedrich investigated the molar
conductivity at infinite dilution of a large
number of strong electrolytes,
found that it was a sum of two quantities i.e.
anion and cation
Today the terms are called “molar
conductivities of ions

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MIGRATION OF IONS
States that “the molar conductivity at infinite
dilution of an electrolyte is equal to the sum of
the molar conductivities at infinite dilution of
the ions produced by the electrolyte” i.e.
For Sodium Chloride (NaCl)
Λ∞(𝑁𝑎𝐶𝑙) = Λ∞(𝑁𝑎+
) + Λ∞(𝐶𝑙−
)
Λ∞ (𝑁𝑎𝐶𝑙)=𝜆∞(𝑁𝑎+)+𝜆∞(𝐶𝑙−)

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MIGRATION OF IONS
For Sodium sulphate, (Na2SO4)
Λ∞ 𝑁𝑎𝟐𝑆𝑂4 = 𝟐𝜆∞ 𝑁𝑎+
+ 𝜆∞(𝑆𝑂4
2−
)
For Aluminum Sulphate (Al2(SO4)3)
Λ∞(𝐴𝑙𝟐(𝑆𝑂4)𝟑= 𝟐𝜆∞ 𝐴𝑙3+
+ 3𝜆∞ 𝑆𝑂4
2−
For ethanoic acid (CH3COOH)
Λ∞ 𝐶𝐻3𝐶𝑂𝑂𝐻 =𝜆∞ 𝐻+
+ 𝜆∞(𝐶𝐻3𝐶𝑂𝑂−
)

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APPLICATION OF KOHLRAUSCH’S
LAW
1. Indirect determination of molar conductivity at
infinite dilution for weak electrolyte,
2. Determine solubility of sparingly soluble
electrolytes,
3. Determine solubility product of sparingly soluble
electrolyte,
4. Determine the degree of dissociation and
equilibrium constants of weak electrolyte.

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Determination of
molar conductivity at
infinite dilution for
weak electrolyte

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Infinite dilution
The concept of infinite dilution is used in the
chemistry
to analyze the substance's dissolution nature
in solvents.
Infinite dilution is a hypothetical state of the
solution where solute concentration is
considered to be zero.

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Molar conductivity at infinite
dilution
• Also called “limiting molar conductivity”
• Can be obtained if the molar conductivities of
selected strong electrolyte are known.
For instance:

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Example #1
For instance;
For ethanoic acid, molar conductivity at infinite
dilution can be obtained from the molar conductivities
at infinite dilution of HCl, CH3COOK and KCl.
𝛬∞ 𝐾𝐶𝑙 = 𝜆∞ 𝐾+
+ 𝜆∞(𝐶𝑙−
)…………………….(i)
𝛬∞(𝐶𝐻3𝐶𝑂𝑂𝐾) = 𝜆∞ 𝐾+
)….(ii)

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𝛬∞(𝐻𝐶𝑙) = 𝜆∞ 𝐻+
+ 𝜆∞(𝐶𝑙−
)………………….(iii)
𝛬∞(𝐶𝐻3𝐶𝑂𝑂𝐻) = 𝜆∞ 𝐻+
)….(iv)
Equation (iv) can be obtained by adding (ii) and (iii)
and subtracting equation (i)
𝛬∞(𝐶𝐻3𝐶𝑂𝑂𝐻)=[𝜆∞ 𝐾+
)] +
[𝜆∞ 𝐻+
+ 𝜆∞(𝐶𝑙−
)] − [𝜆∞ 𝐾+
+ 𝜆∞(𝐶𝑙−
)]

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Cont’d
𝛬∞(𝐶𝐻3𝐶𝑂𝑂𝐻) = Λ∞ 𝐶𝐻3𝐶𝑂𝑂𝐾 + Λ∞(HCl)- Λ∞(KCl)
Given the molar conductivities at infinite dilution of
some electrolytes are as shown below.
Compound 𝚲∞(𝑺𝒄𝒎𝟐
𝒎𝒐𝒍−𝟏
)
Sodium chloride 113.0
Ammonium chloride 134.1
Sodium hydroxide 225.2
Sodium ethanoate 101.2
Hydrochloric 397.8

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Cont’d
(a). Calculate the molar conductivity at infinite dilution
of:
(i). Methanoic acid (𝐻𝐶𝑂𝑂𝐻).
Λ∞ 𝐻𝐶𝑂𝑂𝐻 = Λ∞ 𝐻𝐶𝑂𝑂𝑁𝑎 + Λ∞ 𝐻𝐶𝑙 −
Λ∞ 𝑁𝑎𝐶𝑙
= (101.2+397.8-113.0) Scm2mol-1
= 386 Scm2mol-1

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Cont’d
(ii). Ammonia solution 𝑵𝑯𝟒𝑶𝑯 .
Λ∞ 𝑁𝐻4𝑂𝐻 = Λ∞ 𝑁𝐻4𝐶𝑙 + Λ∞ 𝑁𝑎𝑂𝐻 −
= (134.1+225.2-113.0) Scm2mol-1
= 246.3 Scm2mol-1

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Cont’d
(iii). Ammonium methanoate (HCOONH4).
Λ∞ 𝐻𝐶𝑂𝑂𝑁𝐻4 = Λ∞ 𝐻𝐶𝑂𝑂𝑁𝑎 +
Λ∞ 𝑁𝐻4𝐶𝑙 − Λ∞ 𝑁𝑎𝐶𝑙
= (101.2+134.1-113.0) Scm2mol-1
= 122.3 Scm2mol-1

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Cont’d
(b). Explain the difference in your answers in a(i)
and (iii) above.
The molar conductivity at infinite dilution of
methanoic acid is higher than that of ammonium
methanoate because the H+ ion in methanoic acid
has a higher molar conductivity than the
ammonium ion in ammonium methanoate.

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Example #2
The molar conductivity at infinite dilution of
the sulphate ions is 160 Scm2mol-1. if the
molar conductivity at infinite dilution of
Aluminium sulphate is 858 Scm2mol-1.
Calculate the molar conductivity at infinite
dilution of Aluminium ions.

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Example #2
𝛬∞ 𝐴𝑙𝟐(𝑆𝑂4)𝟑 = 𝟐𝜆∞ 𝐴𝑙3+
+ 𝟑𝜆∞(𝑆𝑂4
2−
)
858 = 𝟐𝜆∞ 𝐴𝑙3+
+ 3(160)
𝟐𝜆∞ 𝐴𝑙3+
= 858-480
𝟐𝜆∞ 𝐴𝑙3+
= 378
𝜆∞ 𝐴𝑙3+
=
378
2
𝑆𝑐𝑚2
𝑚𝑜𝑙−1
= 189 𝑺𝒄𝒎𝟐
𝒎𝒐𝒍−𝟏

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UNEB 1995/P1/No.15 (a) (i).
(a) (i). State the Kohlrausch’s law of Ionic
conductivity at infinite dilution.
(ii). Calculate the molar conductivity of
methanoic acid at infinite dilution.
(𝜆∞ 𝐻𝐶𝑂𝑂𝑁𝑎 =9.50x10-2 Scm2mol-1;
𝜆∞ 𝑁𝑎𝐶𝑙 =1.26x10-1 Scm2mol-1; 𝟐𝜆∞ 𝐻𝐶𝑙 =
4.26x10-1 Scm2mol-1).

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Solution
Λ∞ 𝐻𝐶𝑂𝑂𝐻 = Λ∞ 𝐻𝐶𝑂𝑂𝑁𝑎 + Λ∞ 𝐻𝐶𝑙 −
=𝜆∞ 𝐻𝐶𝑂𝑂−
+ 𝜆∞ 𝐻+
− (𝜆∞ 𝑁𝑎+
+ 𝜆∞ 𝐶𝑙−
)
= (9.50x10-2) +(4.26x10-1)-(1.26x10-1) Scm2mol-1
=(0.521-0.126) Scm2mol-1
=0.395 Scm2mol-1
The molar conductivity at infinite dilution is 0.395
Scm2mol-1

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UNEB QUESTION BANK
1.1996/P1/No.15 (c)
2.1999/P1/13 (b)
3.2004/P1/No.12
4.2004/P2/No.2 (b)
5.2006/P1/No.11 (c)
6.2007/P1/No. 17 (b)
And many more.

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Application No.#2
Determination of solubility and
solubility product of sparingly
soluble electrolytes.

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Thanks

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