Sunday, October 21, 2007


Post updated on 29 August 2009

JEE Syllabus

Chemical equilibrium:
Law of mass action;
Equilibrium constant,
Le Chatelier's principle (effect of concentration, temperature and pressure); Significance of DG and DGo in chemical equilibrium;
Solubility product, common ion effect,
pH and buffer solutions;
Acids and bases (Bronsted and Lewis concepts);
Hydrolysis of salts.

The syllabus has two main components: Equilibrium among ions and equilibrium among compounds

Main topics in TMH Book Chapter

Chemical equilibrium

Some chemical reactions appear to go only in one direction and they are said to go to completion. The reactants are totally consumed or some of the reactants are totally consumed and because of that reaction comes to a stop.

But there are many reactions, wherein a reaction among products takes place and reactants are formed due to it. In such reactions, initially in the reaction products keep forming and their concentration increases to certain level and then an equilibrium is reached from which point there is no change in the concentration of products or reactants.

Reversible reactions

All chemical reactions are theoretically reversible. But in some reactions, the reverse reaction is so slight and takes place at such a low rate that for all practical purposes the reaction is considered to be irreversible.

Example: Formation of water.

But there are reactions where one can see the reverse action also to be active and see the equilibrium point. In this case

A + B → C + D and

C+D → A+B both reactions keep taking place.

At the point of equilibrium the rate of both reactions is same. Formation of A+B is equal to consumption of A+B.

From the rate law (Refer Chemical Kinetics chapter), we can write

Rate of forward reaction = k-f[A][B]

Rate of reverse reaction or backward reaction = k-r{C]{D]

Therefore k-f[A][B] = k-r[C]{D]

This gives k-f/k-r = [C]{D]/[A][B]

This is called as equilibrium constant. When the concentrations of C and D and A and B reach this proportion equilibrium is reached in the reaction.

The equilibrium constant is always written as products by reactants.

For the a general reaction

aA + bB ↔ cC+dD (Normal two arrows are used for reversible reaction. Only one arrow with heads on both sides is used here to tide over the inability to show two arrows.)

k-eq = [C]^c[D]^d/[A]^a[B]^b

The equilibrium constant may or may not have units. As we know concentrations are in mol/liter.

In the case of 2A ↔ 2B +C

The units of equilibrium constant are going to be: (mol/l)^2(mol/l)/(mol/l)^2

= mol/l

For any reaction mechanism at equilibrium, the equilibrium constant is equal to the concentration of products over the concentration of the reactants, all raised to the power of the coefficients from the balanced equation.


Solubility product, common ion effect, pH and buffer solutions; Acids and bases (Bronsted and Lewis concepts); Hydrolysis of salts.

Syllabus rearranged

Acids and bases (Bronsted and Lewis concepts);
buffer solutions;
common ion effect,
Solubility product,
Hydrolysis of salts.

Ionic Equilibrium – Introduction

Acids, basess and salts when dissolved n water dissociate to some extent and form ions. In the ion formation, an equilibrium is established between ionized and unionized (whole) molecules as this ionization is a reversible reaction. Such an equilibrium that involves ions is called ionic equilibrium.

Acids and bases definitions (Arrhenius, Bronsted and Lewis)

Arrhenius defined acid as a hydrogen compound which in water solution give hydrogen ions.
He defined base as a hydroxide compound which in water solution gives hydroxide ions.

Lowry and Bronsted came out with a different concept to broaden the definition of Arrhenius.

An acid is defined as a substance having a tendency of lose or to donate one or more protons.
A base is defined as a substance having a tendency to accept or add a proton

Lewis Theory of Acids and Bases

Acid: An acid is any substance (molecule, ion or atom) that can accept a lone pair of electrons to form a coordinate bond (*Remember coordinate bond and lone pair topics in chapter on Bonding)

Base: Base is any species (molecule, ion or atom) that can donate a lone pair of electrons to form a co-ordinate bond.

According to this concept acid-based reaction involves the formation of a co-ordinate covalent bond.

Dissociation of acids and bases into ions

Degree of dissociation: It is defined as the fraction of the total number of moles of an acid or base or electrolyte that dissociates into ions in acqueous solution when the equilibrium is attained. It is represented by α.

α = Total mole of acid or base dissociated/Total mole of acid or base present in the solution

The greater the degree of dissociation, the stronger the acid.

Strong acids HCl, H2SO4, HNO3
Weak acids H2CO3, H2S, CH3COOH

Strong bases NaOH, KOH
Weak bases NH4OH, Mg(OH)2, Ca(OH)2

Dissociation constants of weak acids

Represent acid by HA (H is hydrogen and A remaining portion of the acid)

HA + H2O ↔ H+ + A- (HA dissociates into H+ and A-)

Equilibrium constant from law of mass action = Ka = [H+][A-]/[HA]

Ka is also termed as dissociation constant of acids

Dissociation of bases

Bases are represented as BOH and they dissociate into B+ and OH- ions.

Equilibrium constant for dissociatin of bases from law of mass action =
Kb = [B+][OH-]/[BOH]

Kb is also termed as dissociation constant of bases.

Ostwald’s Dilution law

This law is the relationship between degree of dissociation and the dissociation constant.

If one mole of a weak electrolyte be dissolved in v litres of a solution. At equilibrium let α be the degree of dissociation.

Hence BA will be 1- α moles
B will be α moles and A will be α moles

As K = [B][A]/[BA] = (α/v)( α/v)/{(1- α)/v}
= α²/(1- α)v

But the concentration C = 1/v

Therefore K = α²C/(1- α)

If α is very small, 1- α can be assumed as equal to one and K becomes equal to α²C.

And α = SQRT(K/C)

In case of acids it can be written as

α = SQRT(Ka/C)

In case of bases it can be written as

α = SQRT(Kb/C)

web sites
Reversible Reactions, Chemical Equilibrium,


JEE Question 2007 Paper II

23. Consider a reaction aG+bH--> Products. When concentration of both the reactants G and H is doubled, the rate increases by eight times. However, when concentration of G is doubled keeping the concentration of H fixed, the rate is doubled. The overall order of the reaction is
(A) 0
(B) 1
(C) 2
(D) 3

Solution: D

This is a question of chapter chemical kinetics.
Reason: When G alone is doubled, rate is doubled. That is the exponent of [G] is one in rate law. When both are doubled, rate increases by 8 times, telling us that exponent of [H] is 2 in rate law. So overall order of reaction is sum of the two exponents that is 3.

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