7.1 electrolyte and electrolytic solution -...

§7.1 Electrolyte and electrolytic solution

Out-class reading:

Levine, pp. 294-310

Section 10.6 solutions of electrolytes

Section 10.9 ionic association

pp. 512-515

Section 16.6 electrical conductivity of

electrolyte solutions.

Contents of solution chemistry

(1) non-electrolytic solution: Colligative Property and activity

(2) electrolytic solution: conductivity and mean activity

(3) Solution reaction: kinetics and mechanism

Main contents:

1) Electrolyte: origin of the concept

2) Existence of ions in solution

3) Ion-dipole interaction--Hydration theory

4) Interionic interaction

5) Motion under electric field

6) Conducting mechanism

7) Faraday’s law and its application

§7.1 Electrolyte and electrolytic solution

An electrolyte is a substance that, when dissolved in solvent,

produces a solution that will conduct electricity.

1) Definition of electrolyte

Progress of the definition:

(1) molten salt;

(2) solid-state electrolyte: Al2O3, PEO and Nafion;

(3) room-temperature ionic liquids (RTIL).

§7.1 Electrolyte and electrolytic solution

In 1886, van’t Hoff published hisquantitative research on the colligativeproperties of solution.

For sucrose, the osmotic pressure () canbe expressed as:

= c R T

But for some other kind of solvates suchas NaCl, the osmotic pressure had to bemodified as:

= i c R T

i , van’t Hoff factor, is larger than 1.

2) Dissociation of substance

In the paper written in Achieves Neerlandaises (1885) and Transactions of the Swedish.

Academy (1886), van't Hoff showed analogy between gases and dilute solutions.

§7.1 Electrolyte and electrolytic solution

In 1887, Svant A. Arrhenius postulated

that, when dissolved in adequate

solvent, some substances can split into

smaller particles, the process was

termed as dissociation.

AB A+ + B –

molecule cation anion

positive ion negative ion

The charged chemical species are named as ions and the process is

termed as ionization.

+ +

3) Dissociation theory for weak electrolytes

§7.1 Electrolyte and electrolytic solution

New definitions:

Ion, cation, anion;

Dissociation, ionization

Weak / strong electrolyte?

True / potential electrolyte?

Theory of Electrolytic Dissociation

Acid-base theory

Cf. Levine p.295

§7.1 Electrolyte and electrolytic solution

Solvated (hydrated) ion

2. Ions in solution

In what state do ions exist in solution?

ion

Primary hydration shell

secondary hydration shell

Disordered layer

Bulk solution

Solvation shells

§7.1 Electrolyte and electrolytic solution

Hydration of ion

Coordination number:

Li+: 4, K+: 6

Primary solvation shell:

4-9, 6 is the most common number

Secondary solvation shell:

6-8, for Al3+ and Cr3+: 10-20

The water molecules in the hydration sphere and bulk water have

different properties which can be distinguished by spectroscopic

techniques such as nuclear magnetic resonance (NMR), infrared

spectroscopy (IR), and XRD etc.

§7.1 Electrolyte and electrolytic solution

3. Hydration Theory / Solvation Theory

H/

kJ

mo

l-1

4NaCl(s)

Na+(aq) + Cl(aq)

Na+(g) + Cl(g)

788 784

hydration energy:

784 kJ mol-1

1948, Robinson and Storks

Why does NaCl only melt at higher temperature, but dissolve

in water at room temperature?

§7.1 Electrolyte and electrolytic solution

The interionic distance for NaCl crystal is 200 pm, while for 0.1

moldm-3 solution is 2000 pm.

To draw Na+ and Cl apart from 200 nm to 2000 nm, the work

is:

W (/kJ) = 625 / r

for melting: r =1, W = 625 kJ, m.p. = 801 oC。

for dissolution in water: r = 78.5, W = 8 kJ.

Therefore, NaCl is difficult to melt by easy to dissolve in

water at room temperature.

2

0

21

4 r

qqF

r Long-range forces

4. Interionic interaction

§7.1 Electrolyte and electrolytic solution

2

0

21

4 r

qqF

r

At low

concentration

At medium

concentration

At high

concentration

+ + +

Cf. Levine, p. 304 In equilibrium -- Bjerrum

§7.1 Electrolyte and electrolytic solution

Owing to the strong interaction, ionic pair forms in

concentrated solution.

ionic pair vs free ion

In an ionic pair, the cation

and anion are close to each

other, and few or no solvent

molecules are between them.

Therefore, HCl does not ionize

and NaCl does not dissociate

completely in solvents.

§7.1 Electrolyte and electrolytic solution

solution present species

0.52 mol·dm-3 KCl 95% K+ + 5% KCl

0.25 mol·dm-3 Na2SO4 76 % Na+ + 24% NaSO4¯

0.1 mol·dm-3 CuSO4 44% CuSO4

Some facts about strong electrolytes

Degree of association

Activity coefficient is essential even for quite dilute solutions

For concentration-dependence of ion pair, see Levine p. 305, Figure 10.10

§7.1 Electrolyte and electrolytic solution

•Electric transfer of ion in

solution under electric field

+

+

+

+

+

+

+

+

+

Motion of ions in the solution:

1) diffusion: due to difference in

concentration

2) convection: due to the difference in

density

3) transfer: due to the effect of electric

field

How can current cross the

electrode / solution interface ?

I

E

5. Conducting mechanism of electrolyte

§7.1 Electrolyte and electrolytic solution

Cl

e

e

e

At cathode:

2H+ + 2e H2

Cl

Cl

Cl

Cl

Cl

Cl

Cl

H+

e

H+

e

H+

e

H+

H+

H+

H+

H+

H+

Cl

At anode:

2Cl 2e Cl2

H+

Cl

Conducting mechanism:

1) Transfer of ion in solution under electric field;

2) electrochemical reaction at electrode/solution interface.

§7.1 Electrolyte and electrolytic solution

6. Law of electrolysis

where m is the mass of liberated matter; Q the

electric coulomb, z the electrochemical

equivalence, F a proportional factor named as

Faraday constant, M the molar weight of the

matter.

MzF

Qm

For quantitative electrolysis:

Micheal Faraday

Great Britain 1791-1867

Invent the electric motor

and generator, and the

principles of electrolysis.

Faraday’s Law

Faraday’s constant

F = (1.6021917 10-19 6.022169 1023 )

C·mol-1 = 96486.69 C·mol-1 usually round off

as 96500 C·mol-1, is the charge carried by 1

mole of electron.

§7.1 Electrolyte and electrolytic solution

Current efficiency ()

effective

ltheoretica

Q

Q

ltheoretica

effective

m

m

Current efficiency is lower than 100% due to side-reactions.

For example, evolution of hydrogen occur during electro-

deposition of copper.

§7.1 Electrolyte and electrolytic solution

1) Definition of ampere:

IUPAC: constant current that would deposit 0.0011180 g of

silver per second from AgNO3 solution in one second: 1

ampere.

Application of Faraday’s law

2) Coulometer: copper / silver / gas (H2, O2) coulometer

3) Electrolytic analysis – electroanalysis

Q ↔m ↔ n ↔ c

§7.1 Electrolyte and electrolytic solution

7. Transfer of ion under electric field

1) Ionic mobility

d

d

E

l

d

d

EU

l

Ionic mobility (U) :

the ionic velocity per unit electric field, is a constant.

Rate of electric transfer: Ionic velocity

How do we describe the motion of ions under electric field?

§7.1 Electrolyte and electrolytic solution

MA, MA’ have an ion in common.

The boundary, rather different in

color, refractivity, etc. is sharp.

measure ionic mobility using moving boundary method

xv

t

v xU

V tE

l

§7.1 Electrolyte and electrolytic solution

I = I＋ + I－

Q = Q＋ + Q－

j j

j

I Qt

I Q

t+ + t- = ?

8. Transference number

Transference number (transfer/ transport number), is the

fraction of the current transported by an ion.

plane A

I－

I＋

I

+ ++

I Qt

I Q - -

-

I Qt

I Q

Supporting electrolyte?

§7.1 Electrolyte and electrolytic solution

For time t:

Q+ = A U+t c+ Z+ F

Q = A Ut c Z F

(1) Principle for measuring transference number

Owing to electric migration, on the left side of plane A,

there are more anions, while on the right side, more cations.

Is this real?

A

I＋

BC

tU

§7.1 Electrolyte and electrolytic solution

(1) Principle of Hittorf method (1853)

Example: Electrolysis of HCl solution

When 4 Faraday pass through the electrolytic cell

anodic region cathodic regionbulk solution

+ + + + + + + + + + + + + + + + + +

+ = 1 F

+ + + + + + + + + + + + + + + + + +

4Cl- -4e- 2Cl2 4H+ +4e- 2H23 mol H+1 mol Cl-

3 mol H+1 mol Cl-

§7.1 Electrolyte and electrolytic solution

anodic region cathodic regionbulk solution

+ + + + + + + + + + + + + +

For anodic region:

transferedreactedinitialresidual cccc

final

result

+ + + + + + + + + + + + + + + + + +

4Cl- -4e- 2Cl2 4H+ +4e- 2H23 mol H+1 mol Cl-

3 mol H+1 mol Cl-

§7.1 Electrolyte and electrolytic solution

EXAMPLE

Pt electrode, FeCl3 solution:

In cathode compartment:

Initial: FeCl3 4.00 mol·dm-3

Final: FeCl3 3.150 mol·dm-3

FeCl2 1.000 mol·dm-3

Calculate the transference

number of Fe3+

Hittorf’s transference cell

Anode

chamber

Cathode

chamber

Cock stopper

What factors will affect the

accuracy of the measurement?

§7.1 Electrolyte and electrolytic solution

§7.1 Electrolyte and electrolytic solution

(2) Principle for the moving-boundary method

When Q coulomb passes, the boundary

moves x, the cross-sectional area of the

tube is A, then:

xAcZ+F = Q+ = t+Q

Why is the moving-boundary method

more accurate that the Hittorf method?

Are there any other methods for measuring

transfer number?

§7.1 Electrolyte and electrolytic solution

(1) Temperature and (2) Concentration

0.000 0.005 0.01 0.02

15 0.4928 0.4926 0.4925 0.4924

25 0.4906 0.4903 0.4902 0.4901

35 0.4889 0.4887 0.4886 0.4885

Transference number of K+ in KCl solution at different

concentration and temperature

T /℃

c /mol·dm-3

(3) Influential factors

§7.1 Electrolyte and electrolytic solution

(3) Co-existing ions

Electrolyte KCl KBr KI KNO3

t+ 0.4902 0.4833 0.4884 0.5084

Electrolyte LiCl NaCl KCl HCl

t– 0.6711 0.6080 0.5098 0.1749

Table transference number on co-existing ions

Problem:

Why does the transference number of certain ion differ a lot in

different electrolytes?

§7.1 Electrolyte and electrolytic solution