CTEVT Chemistry : Physical(V)-Electrochemistry and Acids, Bases and Salts.

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Chapter 10 : ELECTROCHEMISTRY

The branch of physical chemistry which study about the relation between electricity and chemical process involved is called electrochemistry.

Electrolytes:

Electrolytes are the aqueous solution of chemical substance like acid, base and salt which conduct electricity in aqueous medium.

For examples:- Aqueous solution of H2SO4, NaOH, NaCl, etc.

Electrolytes are ionized into charge particles (ie. Cation and anion) when electricity is passed through them.

Depending upon the strength of electrolytes, they can be classified into two types.

  1. Strong electrolytes
  2. Weak electrolytes

Strong electrolytes:-

The electrolytes which are completely ionized in aqueous medium are called strong electrolytes. It can conduct electricity easily.

Examples- Solution of H2SO4, HNO3, HCl, NaOH, NaCl,etc.

Weak electrolytes:-

The electrolytes which are partially ionized in aqueous medium are called weak electrolytes. It conducts electricity partially.

Examples- Solution of CH3COOH, NH4OH, H2CO3, etc.

Non – Electrolytes :

Non – electrolytes are the aqueous solution of chemical substance which do not conduct electricity in aqueous medium.

Examples – Solution of glucose, sugar, urea etc.

Electrolysis :

The process of chemical decomposition of an electrolytes in solution by using electric current is called electrolysis.

This process is carried out in a vessel called electrolytic cell or voltameter. The two metallic rods are connected to two terminals of battery in electrolytic solution with the help of electric wire. These metallic rods are called electrodes. The electrode connected to positive terminal of battery is called anode and the electrode connected to negative terminal of battery is called cathode. In cathode reduction takes place whereas in anode oxidation takes place.

CTEVT chemistry : Electrolysis

If the electrolyte is NaCl solution, then,

At cathode : Reduction

At anode : Oxidation

Faraday’s Laws of Electrolysis :

Faraday’s first laws of electrolysis :

This law state that, “The mass of the substance deposited at the electrode during electrolysis is directly proportional to the quantity of electricity passed through the solution.”

Mathematically, w α Q

But, we know, Q=It, then,

w α It

Or, w= ZIt

Where, w = mass(weight) of the substance deposited at the electrode in gram.

Q = quantity of electricity passed through the solution in coulomb.

I= current in ampere

t= time in second

Z= constant known as electrochemical equivalent (ECE).

If I = 1 ampere and t = 1 sec then,

[ w = Z ]

Hence, electrochemical equivalent is the mass of the substance deposited at the electrode by passing 1 Ampere current for 1 second.

{ Note : Z = E/F also. Where F = 1 Faraday’s charge i.e. 96500 coulombs charge and E = Equivalent weight of substances.

i.e. Z= E/96500

1 Faraday’s charge : Charge of one mole of electrons.

We know, charge of an electron = 1.602x10-19

1 mole of electron = 6.023x1023 electrons

Therefore, charge of 1 mole electrons = 6.023x1023x1.602x10-19

= 9.6488x104 = 96488  = 96500 coulombs

i.e. 1 faraday’s charge = 96500 coulombs }

Faraday’s Second laws of electrolysis :

This law states that ” the mass(weight) of different substances deposited or liberated at electrode by the same amount of electricity is directly proportional to the equivalent weight of substances”.

Mathematically, W α E

Or, W = K E

Or, W/E = K

Therefore, Weight of substance (W) / Eq. wt. of substance (E) = Constant (K)

For example, when the same current is passed through the solution of H2SO4, CuSO4 and AgNO3 for the same period of time as shown in figure, then,

Wt. of the H2 liberated/ Equivalent weight of H2 = Constant(K) – – – -(i)

Wt. of the Cu deposited / Equivalent weight of Cu = Constant(K) – – – (ii)

Wt. of the Ag deposited / Equivalent weight of Ag = Constant(K) – – – -(iii)

On combining (i), (ii) and (iii),

CTEVT chemistry : Faradays second law

It is found that when 1.008 gram of H2 is evolved from acidified water, then the masses of copper and silver deposited are 31.75 gram and 108 gram respectively, which are the equivalent weight of copper and silver respectively. Hence, this verifies Faraday’s second law of electrolysis.

Applications of Electrolysis :

  1. Electrolysis can be used to extract the pure metals in electroplating, electro refining, etc.
  2. It can be used to manufacture oxygen and hydrogen gas from water.

Postulates of Arrhenius theory of ionization :

The main postulates of Arrhenius theory of ionization are given below.

  • When an electrolyte is dissolved in water or any polar solvent, it’s molecules are dissociated into charged particles called ions. The process of breaking down of molecules into ions is called ionization.
  • Positively charged ions are called cations and negatively charged ions are called anion.
  • Number of cations and anions in the solutions is always equal. So, the solution becomes electrically neutral.

         

  • Ions have tendency to reunite to form unionized molecules too. So an equilibrium exists between the ions and unionized molecules.

           

  • The degree of ionization vary with concentration . Lower the concentration of dissolved substance, greater is the degree of ionization.

         

Thus complete ionization(dissociation) may be expected to take place only in infinitely dilute solutions.

  • The properties of an electrolyte in solution are the properties of it’s ions.

pH : pH may be defined as negative logarithm of hydrogen ion concentration.

i.e. pH = -log [H+]

Auto – ionization of water/ Relation between pH and pOH :

Water is a weak electrolyte which ionizes weakly as,

Applying law of mass action,

Or, k[H2O] = [H+] [OH–]

Or, kw = [H+] [OH–]

Where Kw is a constant called ionic product of water and is defined as the product of molar concentration of H+ and OH– ions at temperature of 250C. it’s value at 250C is found to be 1×10-14

Therefore, [ H+ ] [ OH– ] = 1×10-14

Taking –log on both sides,

-log [ H+ ] – log [ OH– ] = -log [ 1×10-14 ]

pH + pOH = 14 …………………. ( i )

This equation ( i ) is the relation between pH and pOH.

pH Scale :

The scale or the instrument which is used to measure the pH of the solution is called pH scale.

CTEVT chemistry : pH scale

If a solution have:

pH = 7 ( Then solution is neutral. )

pH< 7 ( Then solution is acidic.)

pH> 7 ( Then solution is alkaline.)

Importance of pH

  1. pHplays important role in the digestion of food and other biochemical activities. Examples : pH of gastric juice is 1 – 2 which help in digestion of food.
  2. Enzymes function effectively at certain pH. Example : Trypsin acts best in alkaline pH.
  3. pH of blood is maintained at the pH of 7.35 – 7.42 which is due to the buffer action of bicarbonate and carbonic acid system.

Buffer solution :

The solution which can resist the pH of the solution when small amount of acid or base is mixed in it is called buffer solution.

OR, The solution of equimolar mixture of a weak acid and its salt with strong base or weak base and its salt with strong acid is called buffer solution.

There are two types of buffer solution.

  • Acidic Buffer : It is the solution of equimolar mixture of a weak acid and its salt with strong base. Eg. Equimolar mixture of CH3COOH and CH3COONa . The pH value of acidic buffer is less than 7.
  • Basic(alkaline) Buffer: It is the solution of equimolar mixture of a weak base and its salt with strong acid. Eg. Equimolar mixture of NH4OH and NH4Cl. The pH value of acidic buffer is more than 7.

Applications of buffer solution:

  1. Buffer action maintains the pH of blood at about 7.35 – 7.42. If pH is slightly changed then it may cause death.
  2. It is used in many industrial processes like electroplating, manufacture of dyes, food, etc.
  3. It is used in analytical chemistry.
  4. It is used in bacteriological research.

Common – ion effect :

When a strong electrolyte having a ion common to weak electrolyte is mixed, then the ionization of weak electrolyte is totally suppressed. This effect of ion is called common ion effect.

This effect is used in soap industries to make the precipitate of soap from soap solution.

Numerical solved problems :

  • Find the pH of 0.1 Molar (M) of HCl.

Solution:-

HCl      →    H+ +    Cl–

0.1M        0.1M     0.1M

We know that,

pH = -log [ H+ ]

= -log [0.1]     = 1 ans.

  • Calculate the pH of 0.5 M solution of sulphuric acid.

Solution:-

H2SO4     →   2H+   +   SO4—

0.5M         2×0.5M      0.5M

We know,

pH = -log [ H+ ] = – log [2×0.5] = 0 ans.

  • Calculate the pH of solution containing [OH–] ions concentration of 10-6 mol L-1

Solution:

Given, [OH–] = 10-6 mol L-1

pH = ?

we know pOH = -log[OH–]

= – log10-6      = 6

Now , pH + pOH = 14

Therefre, pH = 14 – pOH = 14 – 6 = 8 ans.

  • A current of 2.5 Ampere is passed through a solution of ZnSO4 for 30 minutes and deposits 1.52 gram at cathode. Calculate the equivalent weight of zinc.

Solution:-

Given,  Current (I) = 2.5 A

Time (t) = 30 minutes = 30×60 sec = 1800 seconds

Weight of zinc deposited( w ) = 1.52 gram

Equivalent weight of Zinc ( E ) = ?

We know from first law of electrolysis,

w = ZIt

w = E/96500 x It

1.52 = E/96500 x 2.5 x 1800

E = 32.59

Thus, equivalent weight of zinc is 32.59.

  • 0.383 gram of divalent metal was deposited by passing 2 Ampere of current for 50 minutes. Calculate the atomic weight of metal.

Solution:-

Given,

Weight of metal ( w ) = 0.383 gram

Current ( I ) = 2 A

Time ( t ) = 50 minutes = 50×60 sec = 3000 second

Valency of metal ( V ) = 2

Atomic weight of Metal ( At. Wt.) = ?

We know,

m = Zit

m = E/96500 x It

0.383 = E/96500 x 2 x 3000

E = 6.15

Again,

Equivalent weight ( E ) = Atomic weight/Valency

6.15 = At. Wt./2

At.Wt. =12.30

Therefore, atomic weight of zinc is 12.30 gram.

  • How many coulombs of electricity is required to discharge 0.1 M of sodium.

Solution:-

We know,

1 mole of Na discharge 1 Faraday’s coulombs charge.

i.e. 1 mole of Na discharges 96500 coulombs.

0.1 M of Na discharge 96500 x 0.1 Coulombs.

= 9650 coulombs

Therefore, 9650 coulombs of electricity is required to discharge 0.1 M of sodium.

 

CHAPTER 11  : Acids, Bases and Salt:

Different Concepts of Acids and Bases :

1. Arrhenius Concept of Acids and Bases :

  • An acid is a compound which gives H+ ions in water. Eg.

   

  • A base is a compound which gives OH– ions in water. Eg.

   

 2. Btonsted – Lowry Concept of acids and bases :

  • An acid is a substance that can donates a proton.
  • A base is a substance that accepts a proton. Eg.

   

Here HCl donates proton and H2O accepts proton. Hence HCl is an acid and H2O is base.

 

 3. Lewis concept of acids and bases:

  • An acid is a substance which can accept a pair of electrons.
  • A base is a substance which can donate a pair of electrons.

 

Conjugate Acid- Base pair :

A pair of acid and base that differs from each other by a single proton is known as conjugate acid – base pair.

In this reaction Hydrochloric acid and chloride ion are conjugate acid- base pair. Similarly, hydronium ion and water are another conjugate acid – base pair.

Some examples of Acids and Bases :

Strong acids : HI > H2SO4 > HCl > HNO3 , etc.

Strong bases : NaOH > KOH > Mg(OH)2 > Ca(OH)2 , etc.

Weak acids : HCOOH, CH3COOH, HF, H2S, H2CO3, etc.

Weak bases : NH3, NH4OH, etc.

Salt and it’s types :

A salt is an ionic compound that results from the neutralization reaction of an acid and a base.

Eg. NaCl, KCl, Na2CO3, CuSO4 etc.

Arrhenius concept: Those substances which gives cation except H+ and anion except OH- when dissolves in water is Arrhenius salt. Eg. NaCl.

 1. Acidic salt : Salt formed by the reaction between strong acid and weak base is called acidic salt.

 2. Basic salt : Salt formed by the reaction between strong base and weak acid is called acidic salt.

 3. Neutral salt : Salt formed by the reaction between strong acid and strong base is called acidic salt.

Antacids , Antabases and their medical uses :

 1. Antacid : An antacid is a substance which neutralizes acidity of our body i.e. stomach acidity. It generally contains weak bases like magnesium hydroxide, aluminium hydroxide, sodium carbonate, etc.

Medical use : It is used to relieve occasional heartburn, indigestion and gastritis.

2. Antabase : An antabase is a substance which neutralizes basicity of our body. It generally contains weak acids like acetic acid, citric acid, etc.

Medical use : It controls and neutralizes the base present in our body. It is used to relief the pain of wasp bite. It is used to maintain the pH value of urine and prevents kidney diseases.

   CTEVT chemistry : acids, bases and salts

REFERENCES :

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