Entry Test Preparation 2015
Chemistry
Chapter # 7
CHEMICAL
EQUILIBRIUM
Reversible Reaction:
The reactions, which do not got to completion and occur in both
directions simultaneously, are called reversible reactions. These are the reactions
which proceed in both forward and backward directions and take infinite time
for completion.
H2(g) + 12 ↔2HI (g)
Equilibrium
of Chemical Reaction
Chemical equilibrium is the state when rates of two opposite reactions
(i.e. forward and reverse) are the same and the concentration of reactants and
products do not change with time.
Dynamic Equilibrium:
Suppose a reversible reaction occurs as, Let the rate of forward
reaction (Rf) be initially maximum and decreases with time. Let the
rate of backward reaction Rr be zero initially and then increases.
After certain time, a state is reached when Rf becomes equal to Rr
This is called the state of chemical equilibrium. That is, at equilibrium,
Rf = Rr
· At the state of
equilibrium, certain observable properties like pressure, concentration and
density become constant.
· Chemical equilibrium can
be approached from either side.
· A catalyst can cause the state of equilibrium to be reached
faster, but does not alter the state of equilibrium.
Homogeneous Equilibrium:
If the phases of the reactants and products are same in an equilibrium
system, then that type of system is said to have homogeneous equilibrium.
Heterogeneous Equilibrium:
If the phases of the reactants and products are different in an
equilibrium system, then that type of system is said to have heterogeneous
equilibrium.
Rate of Reaction:
The change in concentration of reactants or products is called rate of
reaction.
Active Mass:
The mass of a substance, which takes part in the chemical reaction, is
called the active mass of the substance. The active mass of those species,
which have greater reactivity, is very greater than that of those species which
have lesser reactivity.
Activation Energy:
The minimum amount of energy required in addition to average energy of
reacting species, just to convert reactants into products is called activation
energy. It is represented by Ea. Its units are kJ or Kcal.
Law of Mass Action:
The rate at which a substance reacts is directly proportional to its
active mass and the rate of a chemical reaction is proportional to the multiple
of the active masses of the reacting substances.
Consider the reversible reaction where A and B are reactants and C and
D are products. The equilibrium concentrations of A, B, C and D are represented
in square brackets like [A] [B] [C] and [D] respectively and they are expressed
n moles dm-3. According to the law of mass action,
Rf 
[A][B]
[A][B]
Rf =kf [A][B]
Where kf is the proportionality constant and is called rate
constant for forward reaction and Rf
is rate of forward reaction. Similarly, the rate of reverse reaction (Rf)
is given by
Rf = kf [C][D]
Where kr is the proportionality constant and is called the
rate constant for backward reaction. At equilibrium,
Rf = Rr
Kf [A][B] = kr [C][D]
Kc =
= 
=
The constant kc is called the equilibrium constant of the
reaction.
Kc = 
and
and
Kc = 
Units of Equilibrium Constant:
Equilibrium constant is the ratio
of the products of the concentrations of the products to the product of
concentrations of the reactants. If the reaction has equal number of moles on
the reactant and product sides, then equilibrium constant has no units. When
the number of moles is unequal then it has units related to the concentration
or pressure. But it is a usual practices that units with kp or kc
values are not written.
Application of Equilibrium Constant:
The value of equilibrium constant
is specific and remains constant at a particular temperature. The study of
equilibrium constant provides us the following information:
Direction of Reaction:
The value of [product] /
[reactants ratio leads to one of the following three possibilities.
·
If this ratio is less than
Kc, this implies that more of the product is required to attain the
equilibrium; therefore, the reaction will proceed in the forward direction.
·
If this ration is greater
than Kc, it means that the reverse reaction will occur to attain the
equilibrium.
·
If this ratio is equal to Kc,
then the reaction is at equilibrium.
Extent of Reaction:
·
If equilibrium constant is
very large, this indicates that the reaction is almost complete.
·
If the value Kc
is small, it reflects that the reaction does not proceed appreciably in the
forward direction.
·
If the value of Kc
is in fraction, this shows a very little forward reaction.
The Effect of Conditions on the position of Equilibrium:
Kc is equilibrium
constant and has constant value at a particular temperature whereas the ratio
of products to the reactants in equilibrium mixture is described as the
position of equilibrium and it can change if the external conditions e.g.
temperature, pressure and concentration are altered. If Kc is large
the position of equilibrium lies on the right if it is small, the position of
the equilibrium lies on the left, for a reversible reaction.
The Le Chatelier’s Principle
If a system at equilibrium is
disturbed by some change, the system will shift so as to counteract the effect
of the change.
Effect of change of temperature:
For endothermic reaction
T 
and T 
and T
For exothermic reaction
T and T
and T
Effect of change of Concentration:
C reactant and C reactant
and C reactant
C Product and C Product
and C Product
Effect of change of pressure
P if 
n < 0
if n < 0
P if n > 0
if n > 0
Por no effect if n = 0
or no effect if n = 0
Reverse is the case for decreases
in pressure
Effect of Catalyst on Equilibrium Constant:
In most of the reversible
reactions, the equilibrium is not always reached within a suitable short time.
So an appropriate catalyst is added. A catalyst does not affect the equilibrium
position of the reaction. It increases the rates of both forward and backward
reactions and this reduces the time to attain the state of equilibrium.
Ionic product of water:
H2O is amphoteric and
gives H3O+ and OH-
H2O + H2O ↔
H3O+ + OH-
Kw = [H3O+]
[OH-
At 25oC, in pure water
[H3O+] [OH-
The equilibrium constant for the
self ionization of water is called ionic product of water or auto ionization
constant (Kw).
At 25oC
Kw = [H3O+]
[OH-
Kw increases with the rise of
temperature due to the increases dissociation of water.
pH Scale:
This is defined as the negative
of logarithm of [H3O+] or [OH+
pH = -log [H+] 
-log 10-7
-log 10-7
Similarly pOH = -log [OH] -log 10-7
-log 10-7
Buffer Solutions or Buffers:
A solution which resists any
change in its pH even after the addition of a small amount of acid or alkali is
called a buffer. These are of two types. Acidic buffer is a mixture of weak
acid and its conjugate base (one of its salts).
Example:
CH3COOH + CH3COONa OR
CH3COOH + CH3COO-
HCN +NaCN OR
HCN + CN- etc.
Basic buffer is a mixture of a
weak base and its conjugate acid (one of its salts).
Example:
NH3 + NH4Cl
OR NH3 + NH4+
Ionization constant of acids (Ka)
and bases (Kb):
pH
=pKa + log [salt]/[acid]
Buffer capacity:
The amount of acid or base which
a buffer solution can absorb without significant change in pH is called buffer
capacity. It depends upon actual molarities of components.
Acids and bases when dissolved in
water may not be completely dissociated. Many acids are weak electrolytes and
they ionize to an extent which is much less than 100%. The value of Ka called
the dissociation constant of acid, is the quantitative measure of the strength
of the solid.
% Ionization =
Amount of acid
ionized x 100
Amount of acid initially
available
The percentage ionization of weak
acids depends upon the extent of dilution of their aqueous solutions. The
ionization constant of bases is denoted by Kb. Small is the value of
Kb weaker is the base.
Lowery-Bronsted Concept of Acid and Base:
According to lowry-Bronsted
concept of an acid an a base, the conjugate base of a strong acid is always
weak. So pKa + pKb = pKw where pKa
and pKb are the parameters to measure the strength of acids and
bases.
Common Ion Effect:
The decrease in the ionization of
a weak electrolyte by the presence of a common ion from a strong electrolyte is
called common ion effect.
·
Ionization of CH3COOH (weak acid) is suppressed by the addition of
CH3COONa (CH3COO- being common icon)
·
Solubility of a sparingly
soluble salt (for example, AgCl) is decreased by the addition of common ion
(AgNO3, or KCl)
Effect of common Ion on Solubility:
The presence of a common ion
decreases the solubility of a slightly soluble ionic compound.
Solubility and Solubility Product:
The product of the concentrations
of the positive and negative ions in a saturated solution is in equilibrium
with the solid state is the solubility product.
For a sparingly soluble salts
like AgCl, PbI2, BaSO4 and so on, ionization is very
small and concentration of salt may be considered as constant. Thus, for AgCl
AgCl(s) 
Ag+ (aq) +
Cl- (aq)
Ag+ (aq) +
Cl- (aq)
Solubility product
(Ksp)=[Ag+][Cl-]
But if solubility of AgCl at
given temperature is S mol lit-1, then, [Ag+] = S = [Cl-]
So, Ksp = S x S
Ksp = S2 or S=
Thus, solubility product of an
electrolyte solution may be defined as the product of its ionic concentration,
in its saturated solution at a constant temperature
Conditions for precipitation:
If for a solution of an
electrolyte
·
Ionic product <Ksp
solution is
unsaturated. (that is, no precipitation)
solution is
unsaturated. (that is, no precipitation)
·
Ionic product = Ksp
solution is saturated
(that is, no precipitation)
solution is saturated
(that is, no precipitation)
·
Ionic product >Ksp
solution is
supersaturated. (That is, precipitation occurs).
solution is
supersaturated. (That is, precipitation occurs).
Molecularity of a Reaction:
Total number of moles of the
reactants involved in the reaction as shown by a balanced chemical equation is
termed as molecularity.
H2 + I2 
2HI
2HI
The molecularity of this reaction
= 1 + 1 = 2
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