Sunday, February 3, 2008

IIT JEE Revision - Ch. 7. CHEMICAL EQUILIBRIA - Core Points

Chemical equilibrium

JEE Syllabus

Chemical equilibrium:
Law of mass action;
Equilibrium constant,
Le Chatelier's principle (effect of concentration, temperature and pressure); Significance of ΔG and ΔGo 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

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.

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]

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.

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

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
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.

Ostwald’s Dilution law
α = SQRT(K/C)

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

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