ElectrochemistryElectrochemistry study of how electricity produces chemical reactions and chemical

reactions produces electricity

• Involves redox reactions• Electrochemical cellElectrochemical cell: any device which converts

chemical energy into electrical energy or vs.

ElectrochemistryElectrochemistry study of how electricity produces chemical reactions and chemical reactions produces electricity

• Involves redox reactions• Electrochemical cellElectrochemical cell: any device which

converts chemical energy into electrical energy or vs.

IntroductionElectrochemistry is a branch of chemistry that studies

chemical reactions which take place in a solution at the interface of an electron conductor (a metal or a semiconductors) and an ionic conductor (the electolyte), and which involve electron transfer between the electrode and the electrolyte or species in solution.

Alessandro Volta's discovery, in 1793, that electricity could be produced by placing two dissimilar metals on opposite sides of a moistened paper.

In 1800, Nicholson and Carlisle, using Volta’s primitive battery as a source, showed that an electric current could decompose water into oxygen and hydrogen.

By 1812, the Swedish chemist Berzelius

could propose that all atoms are electrified, hydrogen and the metals being positive, the nonmetals negative.

Humphry Davy prepared the first elemental sodium by electrolysis of a sodium hydroxide melt.

Michael Faraday, to show that there is a direct relation

between the amount of electric charge passed through the solution and the quantity of electrolysis products

Chemical reactions where electrons are transferred between molecules are called oxidation/reduction (redox) reactions. In general, electrochemistry deals with situations where oxidation and reduction reactions are separated in space or time, connected by an external electric circuit to understand each process.

Electron transfer reactions are oxidation-reduction or redox reactions.Electron transfer reactions are oxidation-reduction or redox reactions.

Results in the generation of an electric current (electricity) or be Results in the generation of an electric current (electricity) or be

caused by imposing an electric current. caused by imposing an electric current.

Therefore, this field of chemistry is often calledTherefore, this field of chemistry is often called

ELECTROCHEMISTRYELECTROCHEMISTRY..

Electron Transfer ReactionsElectron Transfer Reactions

Terminology for Redox ReactionsTerminology for Redox ReactionsOXIDATIONOXIDATION :loss of electron(s) by a species; increase in oxidation :loss of electron(s) by a species; increase in oxidation

number; increase in oxygennumber; increase in oxygen..

REDUCTIONREDUCTION:: Gain of electron(s); decrease in oxidation number; Gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen.decrease in oxygen; increase in hydrogen.

OXIDIZING AGENT: OXIDIZING AGENT: Electron acceptor; species is reduced.Electron acceptor; species is reduced.

REDUCING AGENT: REDUCING AGENT: Eelectron donor; species is oxidizedEelectron donor; species is oxidized..

OXIDATION-REDUCTION REACTIONS

Direct Redox ReactionDirect Redox Reaction

Oxidizing and reducing agents in direct contact.Oxidizing and reducing agents in direct contact.

Cu(s) + 2 AgCu(s) + 2 Ag++(aq) ---> Cu(aq) ---> Cu2+2+(aq) + 2 Ag(s)(aq) + 2 Ag(s)

• BatteriesBatteries• CorrosionCorrosion• Industrial production of Industrial production of

chemicals such as Clchemicals such as Cl22, NaOH, , NaOH,

FF22 and Al and Al

• Biological redox reactionsBiological redox reactions

The heme groupThe heme group

Why Study Electrochemistry?Why Study Electrochemistry?

Classification of ConductorsThese may be divided into three main categories; they are: (I) gaseous (II) metallic or electronic (III)

electrolytic.Gases conduct electricity with difficulty and only under

the influence of high potentials or if exposed to the action of certain radiations.

Metallic or electronic conductors : Conductors which transfer electric current by transfer of electrons, without transfer of any matter, are known as metallic or electronic conductors. Metals such as copper, silver, aluminum, etc., non-metals like carbon (graphite - an allotropic form of carbon) and various alloys belong to this class.

.

Electrolytic conductors : (a) Conductors like aqueous solutions of acids, bases and salts in which the flow of electric current is accompanied by chemical decomposition are known as electrolytic conductors.

b)The substances whose aqueous solutions do not conduct electric current are called non-electrolytes. Solutions of cane sugar, glycerine, alcohol, etc., are examples of non-electrolytes

Fig. 1. Illustration of electrochemical terms 18 aug

H+ + 2e- H2 (hydrogen gas at the (-)cathode).

2Cl- - 2e- Cl2 (chlorine gas at the (+)anode).

Mechanisam of electrolytic conduction and electrolysis

The overall reaction is2NaCl(aq) + 2H2O(l) 2Na+

(aq) + 2OH-(aq) + Cl2(g)+ H2(g)

Electrolysis of sodium chloride solution

NaCl ↔ Na+ + Cl-

H2O ↔ H+ + OH-

At cathode At AnodeH+ + e- → H Cl- → Cl + e-

2H → H2 2Cl → Cl2Electrolysis of copper sulphate solution using platinum electrodes

CuSO4 ↔ Cu2+ + SO42-

H2O ↔ H+ + OH-At cathode At Anode

Cu2+ + 2e- → Cu 2OH- → H2O + O + 2e- O + O→O2

The laws, which govern the deposition of substances (In the form of ions) on electrodes during the process of electrolysis, is called Faraday's laws of electrolysis. These laws given by Michael Faraday in 1833.

Faraday's first law: It states that, the mass of any substance deposited or liberated at any electrode is directly proportional to the quantity of electricity passed.

W α Q W = Mass of ions liberated in gm, Q = Quantity of electricity passed in Coulombs = Current in Amperes ( i ) × Time in second (t) W α i t W = Z i tWhere, Z = constant, known as electrochemical equivalent (ECE) of the ion deposited

• Where Z is the consant known as the Electrochemical equivalent of the substance (electrolyte).

• If I= 1 ampere and t = 1 scond, then m= Z• Thus, the electrochemical equivalent is the

amount of a substance deposited by 1 ampere current passing for 1 second (I.e., one coulomb)

• The Electrical unit FaradayIt has been found experimentally that the

quantity of electricity required to liberate one gram-equivalent of a substance is 96,500 coulombs. This quantity of electricity is known as Faraday and is denoted by the symbol F.

• it is obvious that the quantity of electricity needed to deposit 1 mole of the substance is given by the expression.Quantity of electricity = n x F

Where n is the valency of its ion. Thus the quantity of electricity required to discharge.

one mole of Ag+ = 1 x F = 1Fone mole of Cu2+ = 2 x F = 2Fone mole of A13+ = 3 x F = 3F

we can represent the reaction on the cathode as:Ag+ + e = AgCu2+ + 2e =CuA13+ + 3e = A1

• Moles of electrons required to discharge one mole of ions Ag+, Cu2+ and a13+ is one, two and three respectively. Therefore it means that the quantity of electricity in one Faraday is one mole of electrons. Now we can say that.

1 Faraday = 96,500 coulombs = 1 Mole electrons (1)Importance of first law (2)With the help of first law of electrolysis we

are able to calculate: (3)(1) the value of electrochemical equivalents

of different substances.(4)(2) the masses of different substances

produced by passing a known quantity of electricity through their solutions.

• Example No. (1) 0.1978 g of copper is deposited by a current of 0.2 ampere in 50 minutes. What is the electrochemical equivalent of copper?

• Example No. (2) what current strength in amperes will be required to liberate 10 g of iodine from potassium iodide solution in one hour ?

Faraday's second law: It states that, when the same quantity of electricity is passed through different electrolytes, the masses of different ions liberated at the electrodes are directly proportional to their chemical equivalents (Equivalent weights).

W α E

E α Z or E = FZ or E = 96500 × Z

Faraday's law for gaseous electrolytic product for the gases, we use V = It Ve/96500

Where, V = Volume of gas evolved at S.T.P. at an electrode Ve = Equivalent volume = Volume of gas evolved at an electrode at

S.T.P. by 1 Faraday charge

Importance of the second law

• The second law of electrolysis helps to calculate:

• (1) the equivalent weights of metals • (2) the unit of electric charge • (3) the Avogadro’s number •

conductance and its measurement

Ohm’s lawMetallic as well as electrolytic conductors obey Ohm’s law which

states the strength of current (I) flowing through a conductor is directly proportional difference (V) applied across the conductor and is inversely proportional to the resistance (R ) of the conductor

I = V/R R - Resistance in V/A = Ω (Ohm) V - Voltage or potential difference in Volts, V I - Current in Amperes, A

If a material has a resistance of 1 Ω, it means that when applying a potential difference of 1 V, the current in the material is 1 A.

For metals:

Ohm’s Law R: resistance

Dimension: Ohm,

Conductance is the ability of a material to pass electrons

C = 1 / R

R = V/I

Specific conductance or conductivity The resistance of any conductor varies directly as its length (l) and

inversely as its cross sectional area (a), i.e.,

R α l/a or R = ρ l/a , Here ρ = specific resistance

If l = 1 cm and a = 1 cm2, then R = ρ Κ= 1/ρ, Κ = kappa - the specific conductance ρ = a/l. R or 1/ρ = 1/a.1/R

K = 1/a×C (1/z = cell constant) Specific conductance = cell constant x Conductance The unit of specific conductance is ohm-1 cm-1.

Specific conductance or conductivity

Specific conductance depend on the number of ions present in unit volume (1 ml ) of solution

-

+

-

anode

+Cathode

1 cm1 cm

Solution

-

+

-

anode

+Cathode

1 cm1 cm

Solution

-

+

-

anode

+Cathode

-

+

-

anode

+Cathode

+

-

anode

+Cathode

+

-

anode

+Cathode

+

-

anode

+Cathode

+

-

anode

+Cathode

+

-

anode

+Cathode

-

anode

+Cathode

-

anode

+Cathode

-

anode

+Cathode

-

anode

+Cathode

anode

+Cathode

anode

+Cathode

+Cathode

+Cathode

+Cathode

+CathodeCathode

1 cm1 cm

Solution

Representation of specific conductance

To understand the meaning of equivalent conductance, imagine a rectangular trough with two opposite sides made of metallic conductor (acting as electrodes) exactly 1 cm apart, If 1 cm3 (1 mL) solution containing 1 gram equivalent of an electrolyte is places in this container is measured.

/\ = KV

In case, if the concentration of the solution is c g equivalent per liter, then the volume containing 1 g equivalent of the electrolyte will be 1000/C.

So equivalent conductance /\ k 1000/c /\ = k × 1000/N

Where N = normality The unit of equivalent conductance is ohm-1 cm-2 equi-1.

Equivalent conductance (/\)

One of the factors on which the conductance of an electrolytic solution depends is the concentration of the solution. In order to obtain comparable results for different electrolytes, it is necessary to take equivalent conductances.

Equivalent conductance is defined as the conductance of all the ions produced by one gram equivalent of an electrolyte in a given solution.

1 cm

1 cm

1 cc

1 cm

1 cm

1 cm

1 cm

1 cc

Representation of Equivalent conductance

Molar conductanceThe molar conductance is defined as the conductance of all the ions

produced by ionization of 1 g mole of an electrolyte when present in V mL of solution. It is denoted by.

Molar conductance Λ m = k ×V

Where V is the volume in mL containing 1 g mole of the electrolyte. If c is the concentration of the solution in g mole per litre, then

Λ m = k × 1000/c It units are ohm-1 cm2 mol-1. Equivalent conductance = (Molar conductance)/n Where n = (Molecular mass) / (Equivalent mass)

Effect of dilution on equivalent conductance

Conductance’s of electrolytes of different type

Kohlrausch’s law of independent ionic mobilities

At time infinite dilution (m) , the molar conductivity of an electrolyte can be expressed as the sum of the contributions from its individual ions

Λ∞m = v+ λ∞ + + v- λ∞- v+ and v- are the number of cations and anions per formula unit of electrolyte respectively and, λ∞+ and λ∞- are the molar conductivities of the cation and anion at infinite dilution respectively 24 Aug

Applications of Kohlrausch's law

Determination of Λ∞m for weak electrolytes

Determination of the degree of ionization of a weak electrolyte

Determination of the ionization constant of a weak electrolyte

Determination of the solubility of a sparingly soluble salt

Charging a BatteryCharging a BatteryWhen you charge a battery, you When you charge a battery, you are forcing the electrons are forcing the electrons backwards (from the + to the -). backwards (from the + to the -). To do this, you will need a higher To do this, you will need a higher voltage backwards than voltage backwards than forwards. This is why the forwards. This is why the ammeter in your car often goes ammeter in your car often goes slightly higher while your battery slightly higher while your battery is charging, and then returns to is charging, and then returns to normal.normal.

• In your car, the In your car, the battery charger is battery charger is called an alternator. If called an alternator. If you have a dead you have a dead battery, it could be the battery, it could be the battery needs to be battery needs to be replaced OR the replaced OR the alternator is not alternator is not charging the battery charging the battery properly. 24 (aug) evnproperly. 24 (aug) evn

HH22 as a Fuel as a Fuel

Cars can use electricity generated by HCars can use electricity generated by H22/O/O22 fuel fuel cells.cells.HH22 carried in tanks or generated from carried in tanks or generated from hydrocarbonshydrocarbons