CHEMICAL EQUILIBRIUM- 09                             

Q: What is difference between Reversible and Irrevrsible reaction?

Ans:  1. Reversible Reactions:

The reactions, in which the products do not recombine to form reactants, are called irreversible reactions. They are supposed to complete and are represented by putting a single arrow (         ) between the reactants and products.

“Reversible Reactions: A reaction in which the products can react together to re-form the original reactants is called reversible reaction”                   OR

“A reaction which proceeds in the forward direction as well as in the reverse direction under the same conditions is called a reversible reaction. These reactions never go to completion”.

The double arrow (                   ) in the chemical equation shows that the reaction is reversible.

For example:

Q: How Reversible Reaction takes place in fizzy drinks?

When fizzy drinks are made, CO2 is dissolved in the liquid drink under pressure and sealed. When you remove lid of the bottle, bubbles of CO2 suddenly appear. When you put the lid back on the bottle, the bubbles stop. This is due to the following equilibrium.

CO2(g)         CO2(aq)

The forward reaction happens during manufacturing and the reverse reaction happens on opening.

Q: What is Chemical Equilibrium Define?

Ans:         A state of a chemical reaction in which forward and reverse reactions take place at the same rate is called chemical equilibrium.

At equilibrium state there are two possibilities.

1. When reaction ceases to proceed, it is called static equilibrium.

2. When reaction does not stop, only the rates of forward and reverse reactions become equal to each other but take place in opposite directions. This is called dynamic equilibrium state. Dynamic means reaction is still continuing at dynamic equilibrium state:

Q: What is Law of Mass Action?

Ans: Guldberg and Waage in 1869 put forward this law. According to this law 

The rate at which a substance reacts is directly proportional to its active mass and the rate of a reaction is directly proportional to the product of the active masses of the reacting substances”.

Generally, an active mass is considered as the molar concentration having units of mol dm-3, expressed as square brackets [ ].

Mathematical Derivation:

   aA(g) + bB(g)                 cC(g)  +  dD(g)

According to the law of mass action;

                        Rate of forward reaction  [A]a [B]b

Rate of forward reaction =      kf [A]a [B]b


Rate of reverse reaction  [C]c [D]d   

Rate of reverse reaction= kr[C]c [D]d

         Where kf and krare the rate constants for forward and the reverse reactions respectively

 At equilibrium state:
Rate of F. reaction = Rate of reverse reaction

Thus                                                      kf [A]a [B]b     =     kr[C]c [D]d

where k is the rate constant for the forward reaction.

On rearranging                          =    

                                                   Kc      =     

                        Where Kc = and is known as equilibrium constant,

  • above equation is called as equilibrium constant expression.
  • The square brackets indicate the concentration of the chemical species at equilibrium in moles/dm-3.       
  • Equilibrium constant is defined as the ratio of the product of concentration of products to the product of concentration of reactants each raised to the power equal to the coefficient in the balanced chemical equation
  • The value of Kc depends only on temperature, it does not depend on the initial concentrations of the reactants and the products.

Derive Kc expression:

CO (g)+3H2 (g)              CH4 (g) +    H2O (g)

Q: What are conditions for equilibrium?

Ans:  When equilibrium is reached, this leads to a situation where the concentration of reactants and products remain the same indefinitely, as long as the following conditions are observed:

1.Concentration of none of the reactants or products is changed.

2. Temperature of the system is kept constant.

3. Pressure or volume of the system is kept constant.

Q:  What are ways to Recognize Equilibrium?


By determining concentration of reactants and products at regular intervals. When constant concentration of products and reactants are observed, the reaction is at equilibrium. This canbe done by

  • by physical methods
  •  Chemical methods such as titration, spectroscopy.

Q: What is Importance of Equilibrium Constant?


Equilibrium constant for a reaction can be used to predict many important features of a chemical reaction. It can be used to

  • determine the equilibrium concentration of equilibrium mixture knowing the   initial concentration of reactants.
  •  predict the direction of a chemical reaction.
  •  predict the extent of a chemical reaction.
  • predict the effect of change in conditions of the chemical reaction on the equilibrium state.

Q : What is Le- Chatelier Law and its Applications?


“If you impose a change in concentration, temperature or pressure on a chemical system at equilibrium, the system responds in a way that opposes the change”


With the application of this principle, components of air i.e N2 and O2 can be used successfully in producing important chemicals, ammonia and sulphuric acid in 98% yield. Both these processes involve reversible reactions, so inadequate amount of products are formed under normal conditions.

As such these reactions are uneconomical with only 33% of yield ,

However, Le Chatelliers’s principle has made it possible to get maximum amount of products. First, equilibrium is established in the presence of catalyst in minimum time and then by increasing pressure and decreasing temperature, equilibrium is shifted towards right. Such conditions tend to increase the yield of NH3 and SO3 to about 98%.

Q: Differentiate between Forward and reverse reaction?

Forward reaction Reverse reaction
1. It is written from left to right. 2. Reactants produce products. 3. Rate is fastest in the beginning and gradually slows down 1.It  is written from right to left 2.Products produce reactants 3. Its rate is zero in the beginning and gradually speeds up.
Self-Assessment Exercise 9.3  

Determine the units of equilibrium constants for the following reactions.

  1. N2(g) + O2 (g)                          2NO(g)
  • H2 (g) + CO2 (g)                                 H2O(g) + CO (g)
  • PCI5 (g)                                          PCl3 (g)  +  Cl2 (g)
  • CO (g) + 2H2 (g)                                           CH3OH (g)

(do by yourself)

  • mportant Short Questions.
  • Differentiate between forward and reverse reactions. (See P# 3)
  • What is chemical equilibrium? See P# 1
  • State the law of Mass Action. See P#1
  • State conditions for equilibrium.        See P# 2
  • What is the importance of equilibrium constant for a chemical reaction?  See P# 2
  •  Following reaction can occur during lightning storms

3O2 (g                                                                         2O3 (g)

For this reaction write

  •       Equilibrium constant expression.
  •    units of equilibrium constant.
  • Forward and reverse reactions.
  • Coal reacts with hot steam to form CO and H2. These substances react further in the presence of a catalyst to give methane and water vapour.

CO (g) + 3H2 (g)                           CH4 (g) + H2O (g)

  •   Write forward and reverse reactions for it.
  •      Derive Kc expression for the reaction.
  • Determine units for Kc
  •  Determine the units of equilibrium constants for the following reactions.
  •  COCl2 (g)                       CO (g) + Cl2 (g)
  •             H2 (g) + I2                       2HI (g)
  •  2H2 (g) + O2 (g)                            2H2O (g)
  •  N2 (g) + 2O2 (g)                     2NO2 (g)
  • State the ways that equilibrium can be recognized.

Ans: By determining concentration of reactants and products at regular intervals. When constant concentration of products and reactants are observed, the reaction is at equilibrium. This canbe done by

  • by physical methods
  •  Chemical methods such as titration, spectroscopy.
  • Describe the macroscopic characteristics of an equilibrium reaction. See P# 3
  •  Bromine chloride (BrCl) decomposes to form chlorine and bromine. For this reaction write.
  • Chemical equation
  • Kc expression
  • Units of Kc

            Write chemical equation for this reaction and derive the units of Kc.

  1.  For which of the following reactions are both reactants and products likely to be found when the reaction appears to be complete. Explain.
  • C(s) + O2(g)                                     CO2(g)
  • 2HF(g)                         H2(g) + F2(g)
  1.  Cobalt chloride forms pink crystals (CoCl3.6H2O). When they are heated water is evolved and they turn blue (CoCl3). Explain how you could use Cobalt chloride as a test for water.

Ans: The colour of anhydrous cobalt chloride paper is blue. When this paper touch the moisture or water it turns into pink.

CoCI3 + H2O        CoCI3.6H2O

So, cobalt chloride is used to detect water contents on any surface. Therefore, presence of moisture and water particles are determined by the use of cobalt chloride paper.