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CHEMICAL EQUILIBRIUM

 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
(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) Neutralisation reaction except of strong acid strong base
            HCl + NH4OH-->NH4Cl + H2

(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) + AgNO3(aq)  NaNO3(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) CaCO3(S) à CaO(S)+CO2g
(vii)Neutralisation reaction of strong acid and strong base
      (1) HCl + NaOHàNaCl + 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:

 


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