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Monday, October 1, 2018

Chemical Equilibrium : Part-1

(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
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.
Symbolic representation of any chemical change in terms of reactants and products is called chemical reaction.
On the basis of process chemical reactions are two types: 
(1) Reversible Reaction 
(2) Irreversible 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 + H2


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) Neutralization reaction of strong acid and strong base

      (1) HCl + NaOH àNaCl + H2O


 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:



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 ( KC):
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 Kf and Kb  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
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


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
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. ?

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 ?

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