CMT552CMT552ELECTROCHEMISTRY ELECTROCHEMISTRY

AND CORROSION AND CORROSION SCIENCESCIENCE

ELECTROLYTE ELECTROLYTE CONDUCTANCECONDUCTANCE

What is electrolyte?Any substance that produce ions when

dissolved in a solvent (usually water) is an electrolyte.

It is the electrically conductive solution that must be present for corrosion to occur.

Types of electrolytesStrong electrolyteWeak electrolyteNon-electrolyte

Strong Electrolytes

Strong electrolytes are substances that only exist as ions in solution.

They completely dissociate to their ions when dissolved in solution.

Ionic compounds are typically strong electrolytes.

Strong acids, strong bases and salts are strong electrolytes.

They conduct electricity when molten or in aqueous solution.

Example: Hydrochloric acid, Sodium chloride

ClNaOHNaCl 2

ClOHOHHCl 32

Weak Weak ElectrolytesElectrolytes

A weak electrolyte only partially dissociates in solution and produces relatively few ions (exist in water as a mixture of individual ions and incontact molecules).

Polar covalent compounds are typically weak electrolytes.

Weak acids and weak bases are weak electrolytes.

They conduct electricity weakly.Example: Acetic acid, ammonia

HCOOCHOHCOOHCH 323

OHNHOHNH 423

Non-Non-electrolyteselectrolytes

A non-electrolyte does not dissociate at all (present entirely as intact molecules) in solution and therefore does not produce any ions.

Non-electrolytes are typically polar covalent substances that do dissolve in water as molecules instead of ions.

They do not conduct electricity at all.Example: Sugar

1122122112212 OHCOHOHC

AcidAcidss

Are molecular compounds which ionize or turn into ions in water.

The properties of acids were due to the presence of hydrogen ions, H+.

All acids are soluble in waterSome acids are strong electrolytes

and some are weak electrolytes.No acids are non-electrolytes.

BaseBasess

Can be molecular compounds or ionic compounds.

Some bases are soluble and some are not.

The soluble bases ionize or dissociate into ions in water.

The properties of bases were due to the presence of hydroxide ions, OH-.

All of the ionic bases which are soluble are also strong electrolytes.

SaltSaltss

Are ionic compounds which are not acids or bases.

In other words, the cation is not hydrogen and the anion is not hydroxide.

Some salts are soluble in water and some are not.

All of the salts which are soluble are also strong electrolytes.

Electricals Electricals TermsTerms

SI Term SI Symbol

SI Unit

Electrical Current I Ampere (A)

Quantity of Electricity Q Coulomb (C)

Electric Potential V Volt(V)

Electric Resistance R Ohm()

Resistivity m

Conductance G Siemens (S);ohm-1

Conductivity Sm-1; -1m-1; -

1cm-1

Molar conductivity Sm2mol-1

Molar Conductivity of Ion

Sm2mol-1

Electric Mobility of Ion u m2V-1s-1

Transport Number of Ion

t

Other Symbols and Terms

Symbol Term

C Molar Concentration, mol dm-3 (with :mol m-3)

Degree of dissociation

l Length

A Area

Kcell Cell Constant

° Molar conductivity at infinite dilution or Limiting Molar Conductivity

Electrolytic conductanceElectrolytic conductance

Electrolytic conductance occurs when a voltage is applied to the electrode dipped into an electrolyte solution, ions of the electrolyte move and electric current flows through the electrolytic solution.

This power of the electrolyte to conduct electricity is known as conductance or conductivity.

Electrolytic solution also obey Ohm’s Law just like metallic conductor.

Ohm’s Law: It states that the current flowing through a conductor is directly proportional to the potential difference across it:

where, V = applied potential (V) I = current measured (A)R = solution resistance () between the

two electrodes

V=IR

Solution Solution Resistance (R)Resistance (R)

The increase the [ions] presence in the solution, the lower the solution resistance, R, will be.

A strong electrolyte like KCl is dissolve in water, the no. of ions per unit volume increase and the solution resistance, R, is lowered, thus increasing the current measured for a particular applied potential.

Thus, current can be related to the [ions] in a particular solution.

However, the distance between the electrodes, the surface area of the electrodes and the identity of the ions also affect the solution resistance, R.

Solution Solution Conductance (G)Conductance (G)

The reciprocal of solution resistance (1/R) is called Conductance, G.

Conductance is expressed as Siemens (S) or ohm-1 (-1) or mho.

Where, A = surface area of each electrode

l = distance btwn electrode = conductivity

l

A

RG

1

Values of conductivity,, increased with T and concentration.

The conductivity of a solution of water is highly dependent on its concentration of dissolved salts and sometimes other chemical species which tend to ionize in the solution.

Electrical conductivity of water samples is used as an indicator of how salt free or impurity free the sample is; the purer the water, the lower the conductivity.

Solution Electric Conductivity

(Sm-1)

Seawater 5

Drinking water

0.0005 to 0.05

Deionized water

5.5 x 10-6

Molar Molar Conductivity (Conductivity ())

Defined by:

Example 1: Molar conductivity of 0.005 M KCl is 144 Scm2

mol-1. Calculate its electrolytic conductivity in SI

units (Sm-1).

*(Hint: 1m2 = 104 cm2; mol/L or mol/dm3 convert to mol m-3).

C

Units: Sm-1

mol m-3

1-

331

3

3

1224

212

Sm 0.072

50.0144

m mol 5m)10(1

dm 1

dm

mol 0.005

molSm 0.144cm 10

m 1molScm 144

c

Measurement of Measurement of ConductivityConductivity

The conductivity of a solution is measured in a cell called conductance cell or conductivity cell.

Since l and A are difficult to measure, the usual procedure is to treat as a cell constant, K cell

A

l

R

1

A

l

Therefore,

Example 2 In a certain conductivity cell, the

resistance of a 0.01 M KCl solution is 150 . The known molar conductivity of the solution is 141.27 -1 cm2 mol-1. Calculate the cell constant (Kcell).

*(Kcell unit is cm-1)

cellcell GKKR

1

1

3

2119.0

1015001.027.141

cm

cR

Rkcell

Exercise 1 Using the same conductance cell as in

example 2, a student measured the resistance of a 0.10 M NaCl solution to be 19.9 . Calculate the experimental value of the molar conductivity of this solution.

Use the same value of kcell

12

3

48.106

10106485.0

9.191.02119.0

molScm

cR

Rkcell

ExIn order to determine the molar

conductivity of a 0.05 M solution of AgNO3, you need to measure the solution resistance in a conductivity cell and found that R = 75.8 . Then, in the same cell, a 0.02 M KCl solution had a resistance of 157.9 . Given that the accepted molar conductivity of the KCl solution is 0.013834 -1 m2 mol-1, calculate the molar conductivity of the AgNO3 solution.

12

123-

1-

cell3

1-

3-

cell

molSm 0.01153

molSm10 11.53Λ

75.80.05Λm 43.7

ΛcRk:AgNO

m 43.7

10157.902.0013834.0

k :KCl

cR

Variation of Molar Variation of Molar Conductivity with Conductivity with

ConcentrationConcentrationMolar conductivity () of electrolytes

increases with dilution.The variation is different for strong and

weak electrolytes.

Strong electrolytes Fully ionized in solution increases slowly with dilution and

there is a tendency for to approach a certain limiting value when the concentration approaches zero(i.e. When dilution is infinite).

The when the concentration approaches zero (infinite dilution) is called molar conductivity at finite dilution or limiting molar conductivity (°).

= ° when C → 0 (at infinite

dilution)

For strong electrolytes molar conductivity increase slowly with dilution and can be represented by:C DEBYE HUCKEL

ONSAGER equation

= Molar conductivity at a given concentration

° = Molar conductivity at infinite dilution

= constant

C

From the graph, it has been noted that the variation of molar conductivity () with concentration (C) is small so that the plot can be extrapolated to zero concentration.

The intercept is equal to (°) and the slope is -.

b) Weak electrolytes Not fully ionized in solution In weak electrolyte like acetic acid

they have low degree of dissociation as compared to strong electrolyte.

However, the variation of molar conductivity () with concentration (C) is very large and we can’t obtain molar conductivity at infinite dilution (°) by extrapolation of versus C plots.

Explanation for the Explanation for the variation of Molar variation of Molar Conductivity with Conductivity with

concentrationconcentration1. Conductance behaviour of strong

electrolyte: No increase in the no. of the ions with

the dilution ( completely ionized in the solution at all concentration).

In concentrated solution: strong inter-ionic forces Molar conductivity is low

In dilute solution:Inter-ionic forces lowMolar conductivity increases with dilution

When concentration very low, inter-ionic interaction becomes almost negligible and molar conductance approaches the limiting value, °.

2. Conductance behaviour of weak electrolyte:

The no. of ions produced in solution depends upon the degree of dissociation with dilution.

Higher the degree of dissociation, larger is the molar conductance.

With increase in dilution Degree of dissociation increases as a

result molar conductivity increases. At infinite dilution, the electrolyte is

completely dissociated so that the degree of dissociation become one.

Thus, if = Molar conductivity at a given

concentration° = Limiting molar conductivity or

molar conductivity at infinite dilutionThen, degree of dissociation

= ° (at C → 0)

Consider an aqueous solution of a weak binary electrolyte, AB, of concentration C mol dm-3 and degree of dissociation of .

At equilibrium:

Ostwald Dilution Law & Ostwald Dilution Law & Dissociation Constant of Dissociation Constant of

Weak ElectrolyteWeak Electrolyte

AB (aq) ↔ A+ (aq) + B- (aq)

Initial/mol dm-3 C 0 0

Equilibrium/mol dm-3 C(1-) C C

][

]][[

AB

BAK

)1(

))((

C

CCKc

1

2CKc

Therefore, the dissociation constant can be expressed as:

Ostwald Dilution Law

However, for weak electrolyte; is very small.

Hence, (1- ) 1Therefore,

Since H+ = C [H+ ] = C =

2CKa

CKH

C

KC

a

a

][

C

Ka

KOHLRAUSCH’S LAWKOHLRAUSCH’S LAW

At infinite dilution the ions act completely independently, and the ° obeys a rule of additivity:

where AX, AY, BX and BY are strong electrolytes.

BYBXAYAX

° for a weak electrolyte can be deduced from ° values obtained from strong electrolytes.

For example, consider CH3COOH denoted as HAc’:

where HX, Mac and MX are strong electrolytes.

MXMAcHXHAc

Table 1: Limiting Molar Table 1: Limiting Molar Conductivity, Conductivity, °, of some °, of some

strong electrolytesstrong electrolytes

Electrolyte ° (S cm° (S cm22 mol mol-1-1))

HCl 426.16

HBr 428.10

NaCl 126.45

KBr 151.80

KCl 149.86

NaNO3 121.55

KNO3 144.96

NH4Cl 149.70

KHCO3 118.00

Exercise 2 Calculate ° for a weak electrolyte

NH4OH from the ° values for these strong electrolytes: NH4Cl: 149.7; NaCl: 126.5 and NaOH: 248.10

3.271

5.12610.2487.149

)()()()( 44

NaClNaOHClNHOHNH

Kohlrausch also stated at infinite dilution when the dissociation complete, each ion makes a definite contribution towards molar conductance of the electrolyte irrespective of the nature of the other ion with which it is associated.

It means that the molar conductivity at infinite dilution for a given salt can be expressed as the sum of the individual contributions from the ions of the electrolyte.

where v+ and v-: stoichiometric coefficients for

the cation and anion in the electrolyte.

°+ and °-: ionic conductance of individual ions (cation and anion)

vv

Example 3For NH4OH electrolyte: v+ = 1 and v- = 1

Since 1NH4+ ion present for each OH- ion

present in solution.

Example 4For K4Fe(CN)6 electrolyte: v+ = 4 and v- =

1Since there are 4K+ ions present for each Fe(CN)4-

6 ion present in solution.

Thus, the limiting ionic conductivities represent the contributions to the total solution conductivity made per mole of each ion present in a dilute solution.

Exercise 3Calculate the ° of the following

electrolytes:1)Acetic acid2)Hydrochloric acid3)Potassium Chloride

Ionic Conductivities at Ionic Conductivities at Infinite Dilution at 25°C Infinite Dilution at 25°C

Cation °+ / Scm2mol-1 Anion °- / Scm2mol-1

H+ 349.6 OH- 197.8

Li+ 38.7 Cl- 76.4

Na+ 50.1 Br- 78.2

K+ 73.5 I- 76.8

Fe2+ 108.0 CH3COO-

40.9

Fe3+ 204 CO2-3 138.6

NH4+ 73.4 NO-

3 71.5

Ba2+ 127.3 SO2-4 160.0

12

3

5.390

)9.40(1)6.349(1

)(

molScm

vvCOOHCH

Example 5Molar conductivity for 0.10 M NaCl is 107

Scm2 mol-1. Calculate the degree of

dissociation for the Solution.

1)Calculate the limiting molar conductivity for NaCl

2)Use formula

846.0 5.126

107

5.126

)4.76(1)1.50(1

)(

NaCl

Exercise 4 At 25° C, = 3.40 10-3 Sm-1 for

0.001 M NH4OH. Values of ° are NH4Cl = 0.01497, NaOH = 0.02481, NaCl = 0.01265 Sm2mol-1. Calculate the dissociation constant, K, of ammonium hydroxide.

1253.002713.0

1040.3

1040.310001.0

1040.3

02713.0

01265.002481.001497.0

)()()()(

3

333

44

c

NaClNaOHClNHOHNH