Equilibrium 2017

Chemical equilibrium
• In a chemical reaction, chemical equilibrium is
the state in which both reactants and products
are present in concentrations which have no
further tendency to change with time. Usually,
this state results when the forward reaction
proceeds at the same rate as the reverse
reaction.

Physical Equilibrium
Physical equilibrium can be defined as
the equilibrium in physical processes.
H2O(s) ⇔ H2O(l)
H2O(l) ⇔ H2O(g)

Chemical Equilibrium
chemical equilibrium can be defined as
the equilibrium in chemical processes.
Examples are
3Fe(s) + 4H2O(g) ⇔ Fe3O4(s) + 4H2(g)
CaCO3(s) ⇔ CaO(s) + CO2(g)
N2(g) + 3H2(g) ⇔ 2NH3(g)

Equilibrium in Chemical Process
(Reversible Reactions)
A reaction in which not only the reactants react to form the
products under certain conditions but also the products
react to form reactants under the same conditions is called
a reversible reaction.
Examples are
3Fe(s) + 4H2O(g) ⇔ Fe3O4(s) + 4H2(g)
CaCO3(s) ⇔ CaO(s) + CO2(g)
N2(g) + 3H2(g) ⇔ 2NH3(g)

Equilibrium in Chemical Process-
Irreversible Reactions
If a reaction cannot take place in the reverse direction, i.e.
the products formed do not react to give back the
reactants under the same condition it is called an
irreversible reaction.
Examples are:
1.AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(g)
2.2M(g) + O2(g) → 2MgO(s)
Note: If any of the products will be removed from the system,
reversible reaction will become irreversible one.

Equilibrium in Chemical Process-
Irreversible Reactions
Generally, a chemical equilibrium is
represented as
Where A, B are reactants and C, D
are products.

homogeneous equilibrium
If all the reactants and products of any reaction under
equilibrium are in same physical state, it is called
a homogeneous equilibrium. For example,
N2(g) + 3H2(g) 2NH3 (g)
Here, all the reactants and products are in same phase

heterogeneous equilibrium
If all the reactants and products of any reaction under
equilibrium are in different physical state, it is called
a heterogeneous equilibrium. For example,
Here, reactant is solid while in product all the solid, liquid
and gaseous matter states are present.
2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(l)

Equilibrium process is dynamic in nature.
Chemical equilibrium is called dynamic equilibrium as the
reactant changes to product and products are converted to
reactant after attaining the equilibrium.
2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(l)
As they are in continuous conversion, they are called dynamic
equilibrium.

Characteristics of chemical Equilibrium
1. It can be attained only if the reversible reaction is carried out in
closed vessel.
2. It can be attained from either side of the reaction.
3. A catalyst can hasten the approach of equilibrium but does not
alter the state of equilibrium.
4. It is dynamic in nature i.e. reaction does not stop but both
forward and backward reactions take place at equal rate.
5. Change of pressure, concentration or temperature favours one
of the reactions (forward or backward) resulting in shift of
equilibrium point in one direction.

Law of Mass Action and Equilibrium Constant
“The rate at which a substance reacts is directly proportional to
its active mass and rate of a chemical reaction is directly
proportional to product of active masses of reactants each raised
to a power equal to corresponding stoichiometric coefficient
appearing in the balanced chemical equation”.
rate of reaction ∝ [A]a.[B]b
rate of reaction = K[A]a[B]b
where K is rate constant or velocity constant of the reaction at that
temperature.
Unit of rate constant (K) = [moles/lit]1–n time–1 (where n= order of
reaction.)

Mathematical expression Law of Mass Action and
Equilibrium Constant
Rate of forward reaction [A]a[B]b
Rf = Kf [A]b [B]b
Similarly for backward reaction
Rb = Kb[C]c [D]d
At equilibrium, Kf[A]a[B]b = Kb[C]c[D]d

Mathematical expression Equilibrium Constant
Equilibrium Constant In terms of Partial Pressures (Kp)
For reaction in equilibrium,
We can write,

Relationship between Kc and Kp
For reaction in equilibrium,
We can write,
Using Ideal Gas Equation we get,
PA = CART = [A] RT
PB = CBRT= [B] RT
Pc = CCRT = [C] RT
Pd = CDRT= [D] RT
PV = n RT
P = n/V x RT
P = CRT

Relationship between Kc and Kp
Substituting the above values we get,
where n = (number of moles of gaseous products) – (number of moles
of gaseous reactants) in the balanced chemical equation. The above
given equation represents relationship between kc and kp.

Le-chatelier principle
If an external stress applied to a resulting system at equilibirium,
The system will adjust itself in such a way that effect of the stress
Reduce.

Effect of change in concentration

Example-
PCl5 PCl3 + Cl2
According to le-chatelier’s principle,
If PCl5 conc added is reduced by shifting left to right.
If PCl3 and Cl2 conc added is reduced by shifting right to
left.
N2 + O2 2NO (-181KJ)
According to le-chatelier’s principle,
If N2 + O2 Temperature added is endothermic by shifting
left to right.
If NO Temp- added is exothermic by shifting right to left.

Ionic equilibrium
The equilibrium established
between the unionised molecules
and the ions in the solution of
weak electrolytes is called ionic
equilibrium.
Eg. H2O(aq) -> H+
(aq) + OH-
(aq)

Degree of dissociation-(α)
α = no. of moles dissociate
Total no. of moles
% α = α x 100
Degree of dissociation-(α)
1. Nature of solute- strong or weak electrolyte
2. Nature of solvent- polarity of solvent. Making bond.
3. Concentration of solution- more concentrated more α.
4. Temperature of solution-

Ionic Product of Water
The product of concentrations of H+ and OH- ions in water
at a particular temperature is known as ionic product of
water.
[H+][OH-] = Kw unit = mol2/dm-6 or mol2litre-2
The constant, Kw, is termed as ionic product of water.

Expression for Ionic Product of Water
Pure water is a weak electrolyte and it undergo self ionization
or auto-protolysis. In this process water molecules splits
into hydrogen ion (H+) and hydroxide ion (OH-).
The equation is shown as:

Expression for Ionic Product of Water
Applying law of mass action at equilibrium, the value of dissociation
constant,
K = [H+] [OH-]/[H2O]
or [H+][OH-] = K[H2O]
Since dissociation takes place to a very small extent, the
concentration of undissociated water molecules, [H20], may be
regarded as constant. Thus, the product [H20] gives another
constant which is designated as Kw. So,
[H+][OH-] = Kw
The constant, Kw, is termed as ionic product of water.

• Neutral buffer - maintain pH value = 7.
Also called Buffer action

How buffer solution is prepared.
Preparation of acidic buffer
Preparation of Basic buffer
Acidic buffer solution is prepared by adding weak acid to the
solution of its salt with strong base. pH < 7.
Basic buffer solution is prepared by adding weak base to the
solution of its salt with strong acid. pH > 7.

Equilibrium 2017

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Equilibrium 2017