Chemical Equilibrium

Chemical Equilibrium

Chemical Reaction is the process where one or more substances(reactants) collide with each other and forms a product. There are two types of chemical reaction. They are:

  1. Irreversible Reaction
    The reaction in which the reactants gets converted into product but product can’t get converted into reactant is called irreversible reaction. They are represented by uni-direction arrow.

     
  2. Reversible Reaction
    The reaction in which both the reactants can form products as well as products can be converted into reactant is known as reversible reaction.

     

Chemical Equilibrium

Chemical Equilibrium is the condition in which the concentration of reactants and products does not change with time. Their concentration remains constant because the rate of forward reaction gets equal to rate of backward reaction. Due to this , there is no any observable change in properties of the system.

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Characteristic of Chemical Equilibrium

  1. It is always dynamic in nature. Since, the rate of forward and rate of backward reaction are same , we observe to be no reaction, but actually reaction is going on.
  2. Equilibrium can be obtained from either side.
  3. At equilibrium , the concentration of each reactant and products remains constant.
  4. Initial concentration has no effect on relative concentration at equilibrium.
  5. Equilibrium can be obtained only in closed vessel.
  6. Catalyst enhance both forward and backward reaction.

Law of Mass Action state that ” The rate of a reaction is directly proportional to active mass of reactants.”

Let us consider a reversible reaction,
aA + bB \rightleftharpoons cC + dDaA+bBcC+dD
This reaction can be showed into two steps:

  1. Forward Reaction
    aA + bB \rightarrow cC + dDaA+bBcC+dD
    Rate of forward reaction = k_{f} [A]^{a} [B]^{b}kf[A]a[B]b
  2. Backward Reaction
    cC + dD \rightarrow aA + bBcC+dDaA+bB
    Rate of backward reaction = k_{b} [C]^{c} [D]^{d}kb[C]c[D]d

At equilibrium,
Rate of forward reaction = Rate of backward reaction

k_{f} [A]^{a} [B]^{b} = k_{b} [C]^{c} [D]^{d}kf[A]a[B]b=kb[C]c[D]d
\frac{k_{f}}{k_{b}}kbkf​​ = \frac{[C]^{c} [D]^{d}}{[A]^{a} [B]^{b}}[A]a[B]b[C]c[D]d
k_{e}ke = \frac{[C]^{c} [D]^{d}}{[A]^{a} [B]^{b}}[A]a[B]b[C]c[D]d
Here, k_{e}ke= equilibrium constant
[A] = active mass of A
[B] = active mass of B
[C] = active mass of C
[D] = active mass of D

Equilibrium constant ( k_{e}ke ) is of two types;

  1. When active mass are represented in terms of concentration
    k_{c}kc = \frac{[C]^{c} [D]^{d}}{[A]^{a} [B]^{b}}[A]a[B]b[C]c[D]d
  2. When active mass are represented in terms of partial pressure
    We know that,
    PV=nRT
    P = 
    \frac{n}{V}Vn RT
    where 
    \frac{n}{V}Vn= Molar concentration. Thus , partical pressure of individual gases are,
    P_{A} =[A]RT , P_{B} =[B]RT ,P_{C} =[C]RT , P_{D} =[D]RTPA=[A]RT,PB=[B]RT,PC=[C]RT,PD=[D]RT
    k_{p}kp = \frac{[C]^{c} (RT)^{c} [D]^{d} (RT)^{d}}{[A]^{a}(RT)^{a} [B]^{b} (RT)^{b}}[A]a(RT)a[B]b(RT)b[C]c(RT)c[D]d(RT)d
    or, k_{p}kp = \frac{[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}[A]a[B]b[C]c[D]d (RT)^{(c+d)-(a+b)}(RT)(c+d)−(a+b)
    or, k_{p}= k_{c} (RT)^{\triangle n}kp=kc(RT)n

Le Chatlier’s Principle

Le- Chatlier’s Principle state that “If the system of equilibrium is disturbed, the equilibrium shift to minimize the change.”

  1. Effect of concentration
    The increase in concentration of the substances favours the reaction to that side in which concentration of added substance will decrease.

    For eg : N_{2} + 3H_{2} \rightleftharpoons 2NH_{3} + 22.4KCl

     
    1. When conc. of N_{2} or H_{2} is increased, to decrease that conc. the formation of ammonia increases.
    2. When conc. of NH_{3} is decreased , to maintain that decreased conc. the formation of ammonia increases.
  2. Effect of Pressure
    The increase in pressure shifts the system in the direction in which there is decrease in volume.
    Here in the above reaction, the 4 volumes of reactants(1 volume N_{2} + 3 Volume H_{2}) gives 2 volumes of the product( 2 volume of NH_{3}). Hence increase in pressure increases the formation of ammonia.
  3. Effect of temperature
    The increase of temperature favours the reaction which takes place with an absorption of heat.
    Since, the reaction is exothermic, decomposition of NH_{3} takes place with absorption of heat.
  4. Effect of inert materials
    • At constant volume
      There will not be any effect of inert materials at constant volume.
    1. At constant pressure
      The addition of inert materials at constant volume shifts the system in the direction in which there is increase in volume.

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