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Thursday, 9 February 2017

Equilibrium II

1. Units of Equilibrium Constant: The value of equilibrium constant Kc can be calculated by substituting the concentration terms in mol/L and for Kp partial pressure is substituted in Pa, kPa, bar or atm. This results in units of equilibrium constant based on molarity or pressure, unless the exponents of both the numerator and denominator are same. 
For the reactions :
(i) H2(g) + I2(g) ---> 2HI Kc and Kp have no unit.
(ii) N2O4(g) ---> 2NO2Kc has unit mol/L and Kp has unit bar.

2. Characteristics Of Equilibrium Constant:
• Equilibrium constant is applicable only when concentrations of the reactants and products have attained their equilibrium state.
• The value of equilibrium constant is independent of initial concentrations of the reactants and products.
• Equilibrium constant is temperature dependent having one unique value for a particular reaction represented by a balanced equation at a given temperature.
• The equilibrium constant for the reverse reaction is equal to the inverse of the equilibrium constant for the forward reaction.
• The equilibrium constant K for a reaction is related to the equilibrium constant of the corresponding reaction, whose equation is obtained by multiplying or dividing the equation for the original reaction by a small integer.

3. Applications of equilibrium constant:
• Predict the extent of a reaction on the basis of its magnitude.
• Predict the direction of the reaction, and
• Calculate equilibrium concentrations.

4. Le Chatelier’s Principle: 
  • It states that if a system in equilibrium is subjected to a change of concentration, temperature or pressure, the equilibrium shifts in a direction that tends to undo the effect of the change imposed.
  • Effect of change of concentration: When the concentration of any of the reactants or products in a reaction at equilibrium is changed, the composition of the equilibrium mixture changes so as to minimize the effect of concentration change.
  •  For eg:- H2(g) + I2(g) ---> 2HI(g)
  • If H2 is added to the reaction mixture at equilibrium, the equilibrium of the reaction is disturbed. In order to restore it, the reaction proceeds in a direction whereas H2  is consumed i.e more of H2 and I2  react to form HI and finally the equilibrium shifts in forward direction.
  • Effect of change of pressure: When the pressure is increased the equilibrium shifts in the direction in which the number of moles of the gas decreases.
  • Consider the reaction, CO (g) + 3H2 (g) ---> CH4 (g) + H2O (g) Here, 4 mol of gaseous reactants (CO + 3H2 become 2 mol of gaseous products (CH4 (g) + H2O). so by Le Chatelier‘s principle. The increase in pressure will shift the equilibrium in the forward direction, a direction in which the number of moles of the gas or pressure decreases
  • Effect of change of Temperature: When a change in temperature occurs, the value of equilibrium constant changes. In general, the temperature dependence of the equilibrium constant depends on the sign of H for the reaction. The equilibrium constant for an exothermic reaction (-ve H) decreases as the temperature increases. The equilibrium constant for an endothermic reaction (+ve H) increases as the temperature increases. When the Temperature is increased the equilibrium shifts in the direction in of endothermic reaction.
  • Consider a reaction N2(g) + 3H2(g) ---> 2NH3(g) H =  -92.38Kj/mol
  • According to Le Chatelier‘s principle, raising the temperature shifts the equilibrium to left (backward direction i.e direction of endothermic reaction) and decreases the equilibrium concentration of ammonia Effect of Inert Gas Addition: If the volume is kept constant and an inert gas such as argon is added which does not take part in the reaction, the equilibrium remains undisturbed. It is because the addition of an inert gas at constant volume does not change the partial pressures or the molar concentrations of the substance involved in the reaction. The reaction quotient changes only if the added gas is a reactant or product involved in the reaction.
  • Effect of a Catalyst: A catalyst increases the rate of the chemical reaction by making available a new low energy pathway for the conversion of reactants to products. It increases the rate of forward and reverse reactions that pass through the same transition state and does not affect equilibrium. Catalyst lowers the activation energy for the forward and reverse reactions by exactly the same amount. Catalyst does not affect the equilibrium composition of a reaction mixture. It does not appear in the balanced chemical equation or in the equilibrium constant expression.
5. Summary of Le Chatelier’s Principle:

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