NCERT Book Class 11 Chemistry Chapter 6 Equilibrium

Unit 6

Equilibrium

Chemical equilibria are important in numerous biological
and environmental processes. For example, equilibria
involving O2 molecules and the protein hemoglobin play
After studying this unit you will be
a crucial role in the transport and delivery of O2 from
able to
our lungs to our muscles. Similar equilibria involving CO
• identify dynamic nature of molecules and hemoglobin account for the toxicity of CO.
equilibrium involved in physical
and chemical processes; When a liquid evaporates in a closed container,
• state the law of equilibrium; molecules with relatively higher kinetic energy escape
• explain characteristics of the liquid surface into the vapour phase and number of
equilibria involved in physical liquid molecules from the vapour phase strike the liquid
and chemical processes; surface and are retained in the liquid phase. It gives rise
• write expressions for equilibrium to a constant vapour pressure because of an equilibrium in
constants;
which the number of molecules leaving the liquid equals the
• establish a relationship between
Kp and Kc; number returning to liquid from the vapour. We say that
• explain various factors that the system has reached equilibrium state at this stage.
affect the equilibrium state of a However, this is not static equilibrium and there is a lot of
reaction; activity at the boundary between the liquid and the vapour.
• classify substances as acids or Thus, at equilibrium, the rate of evaporation is equal to the
bases according to Arrhenius, rate of condensation. It may be represented by
Bronsted-Lowry and Lewis
concepts; H2O (l) H2O (vap)
• classify acids and bases as
The double half arrows indicate that the processes
weak or strong in terms of their
ionization constants; in both the directions are going on simultaneously. The
• explain the dependence of degree mixture of reactants and products in the equilibrium state
of ionization on concentration is called an equilibrium mixture.
of the electrolyte and that of the
common ion; Equilibrium can be established for both physical
• describe pH scale for representing processes and chemical reactions. The reaction may be
hydrogen ion concentration; fast or slow depending on the experimental conditions and
• explain ionisation of water and the nature of the reactants. When the reactants in a closed
its duel role as acid and base; vessel at a particular temperature react to give products,
• describe ionic product (Kw ) and the concentrations of the reactants keep on decreasing,
pKw for water;
while those of products keep on increasing for some time
• appreciate use of buffer
after which there is no change in the concentrations
solutions;
• calculate solubility product
of either of the reactants or products. This stage of the
constant. system is the dynamic equilibrium and the rates of the
forward and reverse reactions become equal. It is due to

Unit 6.indd 168 9/12/2022 11:58:20 AM