DEGREE OF IONISATION /DISSOCIATION (DOD)

We know that electrolytes are two types STRONG and WEAK electrolyte . Strong electrolytes are excellent conductor of electricity due to 100% ionization it means their degree of dissociation is  very high or we can say it approaches unity.

Other hand weak electrolytes are not completely ionized theirfore they are bad conductor of electricity for example CH3COOH , H3PO4 , H3BO3, NH4OH etc.

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Degree of dissociation  may be defined as “fraction of total number of molecules which dissociates into ion”. It is represented by alpha.

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The extent of dissociation of a substance is expressed in terms of degree of dissociation.

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(1) ARRHENIUS THEORY OF DISSOCIATION: the theory deals with behaviour of weak electrolyte in aqueous phase .

The theory state that when a weak electrolyte ( acid ,base and salts ) is added in water , a chemical equilibrium exist between undissociate  molecules and ions produce by dissociation.

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Where Kd is called dissociation/Ionization constant

For strong electrolyte Kd is greater than one and DOD is equal to one .

For weak electrolyte Kd is lower than one and DOD is also lower than one .

Kd is also known as 

For acids  Ka , for bases Kb and for ions it is represented as Kh.

(2) OSTWALD DILUTION LAW OF DISSOCIATION:

Ostwaled dilution law is applicable for only weak electrolyte not for strong electrolyte.

"The rule state that  at infinite dilution a weak electrolyte behave as strong electrolyte due to complete dissociation and it is called Ostwald dilution law "

Consider AB is a weak electrolyte dissociate as below 

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FACTER'S AFFECTING DOD( @)

(1) Nature of electrolyte (solute): DOD of strong electrolyte is greater than weak electrolyte.

(2) Nature of Solvent: DOD also depends upon nature solvent for example.

(i) acetic acid (CH3COOH) behave as weak acid in water (H2O)but it behave as strong acid in liquid Ammonia (NH3)

(ii) HCl is strong acid in water but weak acid in acetic acid (CH3COOH) and HF. and behave non volatile in benzene.

(iii) NaCl is strong electrolyte in water but act as weak electrolyte in liquid ammonia (NH3).

(3) Effect of temperature on DOD :

(i) for endothermic dissociation DOD increases on increasing temperature according to le chateliers principle (LCA).

(ii) for exothermic dissociation DOD  decrease on increasing temperature.

(4) Effect of concentration ( Dilution): when dilution take place concentration of all the species are decrease but here greater concentration decrease in product side so according to Le chateliers principle equilibrium shifted forward direction and finally DOD increases.

ILLUSTRATIVE EXAMPLE (1):

ILLUSTRATIVE EXAMPLE (2):

ILLUSTRATIVE EXAMPLE (3):

ILLUSTRATIVE EXAMPLE (4):

ILLUSTRATIVE EXAMPLE (5):

(5) Common ion effect on DOD: In presence of common ions the DOD of weak electrolyte decrease,this phenomenon is known as common ion effect.

For example when HCl is added to the aqueous solution of CH3COOH  then concentration of common ion (H+) increases so according to Le chateliers principle equilibrium  of weak electrolyte (CH3COOH) shift in backward direction and finally DOD of CH3COOH decrease.

(6) Opposite ion effect on DOD: In the presence of opposite ion (OH-) concentration of H+ decrease and according to Le chateliers principle equilibrium shifted in forward direction  hence DOD of weak electrolyte H2S increases , this phenomenon is known as opposite ion effect.

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IONIC EQUILIBRIUM

An equilibrium involving between dissociate electrolyte and the ions produced by the dissociation called ionic equilibrium. " the ionic equilibrium deals the equilibrium of a substance with its degree of dissociation (DOD) of a solute in a particular solvent   (generally water) ,factors affecting  DOD, and PH calculation" 

Depending upon DOD The substances (solute) are classified  as  follows:

1.   Electrolytes and

2.   Non-electrolytes

(1): Electrolyte (conducting)

The substances which produce ions in aqueous solution or molten state and can conduct electricity are known as electrolyte..

Example: NaCl_(S) on  heating above its melting point  it produce ions and also produce ions when dissolved in water at 25 degree centigrade . and other KCl, HCl, NaCl etc.

On the basis of conducting units conductors are of two types :

Metallic or Electric Conductors: Electricity conduct due to the presence of free and mobile electron which act as electricity conducting unit called metallic or electric conductors. eg. Metals, Alloys, Graphite, Gas, Carbon etc.

Ionic Conductors or Electrolytes: Conductors in which the current is passes through due to the presence of free ions are called Ionic Conductors or Electrolyte or Electrolytic conductors.

On the basis of ions produce in molten or liquids state are two type:

True electrolyte : Those electrolytes which can produced ions and conduct electricity even in molten state  as well as when dissolved  in solvent are called true electrolytes like NaCl. KCl ete

Potential electrolyte:Those electrolytes which do not produce ions and do not conduct electricity into their molten or liquid state but produce ions in appropriate solvent are called potential  electrolytes. Example  HCl and CH₃COOH ete

(2) Non Electrolyte (non-conducting) :The substance which dissolved in solvent but do not produce  any ions is known as non electrolytes  and don’t conduct electricity. For example  glucose, urea   sucrose, benzene ete

On the basis of  extent of DOD electrolytes are two type:

(A)Strong electrolyte: Those susbstance whose aqueous solution or molten form conduct electricity to a greater extent. They almost completely ionised in water.

            e.g. NaCl, H₂SO₄, HCl, NaOH, NH₄Cl

            Since strong electrolyte completely ionised in aqueous solution so their ionisation is represented as    NaCl(S)----> Na+ (aq) + Cl- (aq)

(B) Weak electrolyte: Those substance whose aqueous solution or molten form conducts electricity to a lesser extent. They do not completely ionised in water i.e. partially ionised. They behaves as poor conductor of electricity.e.g. when CH₃COOH is dissolved in water, it is ionised partly and an equilibrium is setup between the ions and the unionised electrolyte.

(2) Non-electrolyte      

      Those substance whose aqueous solution or molten form does not conduct electricity to any extent. They are bad conductor of electricity.

      e.g. aqueous solution of sugar, urea, etc. do not conduct electricity.

       continue reading.......

   Acid base concepts:

(1) Arrhenius acid base concept:

(2) Bronsted Lowery acid base concept:

(3) Lewis acid base concept:

(4) The Solvent System (Self or Auto Ionization of Solvent):

(5) The Lux-Flood Acids-base concept:

(6) The Usanovich Acid-base concept:

(1) Oxy acids of Boron:

(2) Oxy acids of Silicon:

(3) Oxy acids of Nitrogen:

(4) Oxy acids of Phosphorous:

(5) Oxy acids of Sulphure:

(7) Oxy acids of Chlorine:

Related Questions: 

(1) What is Use of Boric Acid?

(2) What is use of Orthoboric acids?

(3) What is basicity of "Boric acid" ?

(4) Why Boric acid exist in solid state ?

(5) What is structure of solid Ortho Boric acid ?

(6) What is effect of heat on Borax?

(7) What is the structure of trimetaboric acid and trimetaborate ion?

(8) What is the Sodium per borate ,give the structure and its uses?

(9) Why aqueous solution of borax reacts with two moles of acids ?

(10) What is the molecular formula of Borax ?

(11) Why Boric acid become strong acid in the presence of cis 1,2-diol or 1,3-diol ?

(12) Why Borazine is more reactive than benzene towards Electrophic Aromatic substitution reactions ?

(13) Why Borazine (B3N3H6) is also known as inorganic benzene ?.

Chemical Equilibrium : Part-1

 CHAPTER TOPICS:

(1) Type of chemical reactions-reversible and irreversible reactions

(2) Physical and Chemical Equilibrium and nature of equilibrium

(3) Homogeneous and Heterogeneous equilibrium

(4) Equilibrium constant Kp ,Kc and Kx (Law of mass action) 

(5) Relation between Kp and Kc and their units

(6) Characteristic of equilibrium constant

(7) Application of equilibrium constant

(8) Factor affecting equilibria

INTRODUCTION:

Equilibrium is the state of the any process including chemical reactions at which two opposing forces are balance. We know whenever a chemical reaction carried out in a closed vessel, do not go to completion due to equilibrium state. Under this state a chemical exist in a state of equilibrium balancing forward and backward reactions. At equilibrium state the reactants may still be present but they do not appear to change into products any more. Equilibrium arise in a reversible reactions.

CHEMICAL REACTION:

Symbolic representation of any chemical change in terms of reactants and products is called chemical reaction.

(1)TYPE OF CHEMICAL REACTIONS:

On the basis of process chemical reactions are two types: 

(1) Reversible Reaction 

(2) Irreversible reaction:

(1) REVERSIBLE REACTION:

A reaction is said to be reversible if the composition of reaction mixture on the approach of equilibrium at a given temperature is the same irrespective of the initial state of the system, i.e. irrespective of the fact whether we starts with reactants or the products 

Characteristic feature of reversible reaction is given as:

(i) Chemical reaction in which products can be converted back into reactants 

         (1) N₂ + 3H₂ à2NH₃

         (2) 3Fe + 4H₂Oà Fe₃O₄ + 4H2 

         (3) H₂ + I₂ à 2HI                       

(ii) Proceed in forward as well as backward

(iii) Possible in closed container

(iv) These attain equilibrium

(v) Reactant are never completely converted  into products

(vi) Generally thermal dissociations are held in closed vessel 

         

      (1) PCl₅(g) àPCl₃(g) + Cl₂(g)

      (2) 2HI(g)-à H₂(g) + I₂(g)

(vii) Neutralization reaction except of strong acid strong base

            HCl + NH₄OH-->NH₄Cl + H₂O 

(2) IRREVERSIBLE REATION:

The chemical reactions which are goes forward direction only and the reverse reaction does not take place ie reactants react to form 100% product. This type of reaction is said to be irreversible reaction.   

For example 

         HCl + NaOH à NaCl+H₂O

The arrow points only forward and there is no reverse arrow. This reaction is irreversible reaction. Irreversible reaction takes place spontaneously. other Example. 

        NaCl_(aq) + AgNO_3(aq) → NaNO_3(aq) + AgCl_(s)

Characteristic feature of reversible reaction is given as:

(i) Chemical reaction in which products cannot be convert back into reactants

       (1) AgNO₃ + NaClàAgCl + NaNO₃  

  

       (2) NaCl + H₂SO4àNaHSO₄ + HCl

       (3)  Zn + H₂SO₄àZnSO₄ + H₂

(ii) Proceed only in one direction (forward direction direction)

(iii) Generally possible in open container

(iv)These do not attain equilibrium

(v)Reactants are nearly completely converted into products.

(vi)Generally thermal decompositions are held in open vessel

      (1) KClO3(s) à2KCl(s) + 3O2(g

      (2) CaCO_3(S) à CaO_(S)+CO_2g

(vii) Neutralization reaction of strong acid and strong base

      (1) HCl + NaOH àNaCl + H2O

(2) EQUILIBRIUM AND ITS DYNAMIC NATURE:

 Definition: "Equilibrium is the state at which the concentration of reactants and products do not change with time. i.e. concentrations of reactants and products become constant."

Characteristics : Following are the important characteristics of equilibrium state,

 

(i) Equilibrium state can be recognised by the constancy of certain measurable

 properties such as pressure, density, colour, concentration etc. by changing these conditions of the system,we can control the extent to which a reaction proceeds.

(ii) Equilibrium state can only be achieved in close vessel, but if the process is carried out in an open vessel equilibrium state cannot be attained because in an open vessel, the reverse process may not take place.

(iii) Equilibrium state is reversible in nature.

(iv) Equilibrium state is also dynamic in nature.Dynamic means moving and at a microscopic level,the system  is in motion .The dynamic state of equilibrium can be compared to water tank having an inlet and outlet.Water in tank can remain at the same level if the rate of flow of water from inlet (compared to rate of forward reaction) is made equal to the rate of flow of water from outlet (compared to rate of backward reaction).Thus, the water level in the tank remains constant, though both the inlet and outlet of water are working all the time.

(v) At equilibrium state:

Rate of forward reaction = Rate of backward reaction

Graphical representation of chemical equilibrium:

 

(4)_{EQUILIBRIUM CONSTANT Kp ,Kp AND Kx (LAW OF MASS ACTION)}

EQUILIBRIUM CONSTANT Kp ,Kc AND Kx (LAW OF MASS ACTION)

Law of mass action is applicable for only reversible chemical reactions and it is an imperial law.

The law state that “ At a fixed temperature the rate of a chemical reaction is directly proportional to the product of reactive mass of reactants raised to the their respective Stoichiometric cofficients ” The law of mass action is by Guldberg and Waage.

(1) Equilibrium Constants in term of concentration ( K_C):

At the constant temperature, let us consider the following reversible reaction

According to law of mass action - The rate of forward reaction

The rate of reverse reaction-

Where K_f and K_b  is the rate constant of the forward reaction and backward reaction respectively

We know at equilibrium, the two rates of forward as well as backward are equal. ie

Rate of reaction = Rate of forward reaction – Rate of backward reaction = 0

Kc=Kf/Kb

Unit of Kc= (Conc)ng

(2) Equilibrium Constants in term of Pressure( Kp):

Consider the general chemical  reaction taking place at constant temperature.

From law of mass action- rate forward reaction is directly proportional  to product of active mass of reactants and rate backward reaction is directly proportional  to product of active mass of products.

For an ideal gas PV=nRT

Where 

P= Pressure in atm

V=Volume in liters

n=Number of gaseous moles

R=Gas constant  

  =  0.0821 L atm/mol/K or 1/12 L atm /mole/K

T=Temperature in kelvin

                                      = total number of moles of gaseous products -total number of moles of gaseous reactants

EXAMPLE (1): At 700 K, the equilibrium constant Kp, for the reaction

is 1.8 × 10³  kPa What is the numerical value of Kc for this reaction at the same temperature

SOLUTION:

EXAMPLE (2) : Methanol (CH₃OH) is manufactured industrially by the reaction

 CO(g) + 2H₂(g) ⇌ CH₃OH(g) The equilibrium constant (Kc) for the reaction is 10.5 at 220°C. What is the value of Kp at this temperature ?

SOLUTION: Give Data ,Kc = 10.5 ,T= 220oC = (220 + 273)K = 493 K

EXAMPLE(3): For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) The Kp is 4.3 × 10^-4 at 375°C. Calculate Kc for the reaction. ?

SOLUTION:

EXAMPLE(4):   The value of Kc for the reaction is 0.50 at 400º C.

What will be the value of Kp at 400ºC when concentration are expressed in mole litre.1 and pressure in atmosphere ?

SOLUTION: 

EXAMPLE(5):

EXAMPLE(6):

SOLUTION: