It is an observation that most of the reactions carried out in closed vessels, do not acquire completion, under a given set of temperature and pressure. Infact, in all such cases, in the initial state, only the reactants are there but as the reaction proceeds, the concentration of reactants decreases and concentration of products increases. Finally a stage is acquired when no further change in concentration of the reactants and products is seen.
This state at which the concentration of reactants and products is stable with time is termed as a state of chemical equilibrium.
The amount of reactants which remain unused depends on the experimental conditions like concentration, temperature, pressure and the nature of the reaction.
The reactions which can take place, under same conditions, in both the forward and the backward directions, are called reversible reactions.
A reaction is considered reversible, if the reaction mixture contains the reactants as well as the products and if both the forward and the backward reactions are taking place under the experimental conditions.
Reversible reactions are represented by a pair of half headed arrows .
CaCO3 (s) CaO (s) + CO2 (g)
Some examples for reversible reactions are:
N2 (g) + 3 H2 (g) 2 NH3 (g)
2 SO2 (g) + O2 (g) 2 SO3 (g)
H2 (g) + I2 (g) 2 HI (g)
PCl5 (g) PCl3 (g) + Cl2 (g)
N2 (g) + O2 (g) 2 NO (g)
CH3COOH (l) + C2H5OH (l) CH3COOC2H5 (l) + H2O (l)
A reversible reaction does not go to completion in either direction. In the beginning forward reaction only takes place and the reverse reaction will not be there.
The rate of forward reaction is high initially and with the progress of the reaction it gradually decreases with time since the reactants are consumed. The rate of forward reaction attains a constant value after certain time.
But at sufficient reaction time after the start of the reaction, the reverse reaction also occurs with measurable rate. Unlike the rate of forward reaction, the rate of the backward reaction increases with time due to the formation of the products. The rate of the reverse reaction attains a constant value after this time .
The chemical reaction in which the products formed do not combine to give the reactants is termed as irreversible reactions.
Reactions in which whole amount of the reactants is converted into products is said to be irreversible reaction.
(a) These reactions proceed only in one direction i.e. in forward direction
(b) These reactions can proceed to completion of reaction
(c) In an irreversible reaction, deltaG <0,
(d) The arrow (–>) is present between reactants and products
Potassium chlorate decomposes on heating and forms potassium chloride and oxygen.
2K Cl O3(S) --à 2K Cl(S) + 302(g)
But, the products cannot combine to form potassium chlorate. In case of irreversible processes, the change occurs only in forward direction and the processes acquire completion. On the other hand, the reversible processes do not go to completion and attain state of chemical equilibrium; even then, some starting materials are left.
Some examples of irreversible reactions are:
(a) Neutralisation between strong acid and strong base:
Eg: NaOH + HCl –> NaCl + H2O + 13.7 kcal
(b) Double decomposition reactions or precipitation reactions:
Eg: BaCl2 (aq) + H2SO4 (aq) –> BaSO4 (s) ↓ + 2HCl (aq)
(c) Thermal decomposition: Eg: 2K Cl O3 (S) --à 2K Cl (S) + 302 (g)
(d) Redox reactions: Eg: SnCl2 (aq) + 2FeCl3 (aq) –> SnCl4 (aq) + 2FeCl2 (aq)