phase rule ppt UNIT 4 applicable to both physical and chemical equilibria

UNIT - IV
PHASE RULE
AND
STORAGE DEVICES

Phase rule and Batteries
 Introduction – Phase rule
 One component system - Water
 Two component system – (Pb-Ag System)
 Introduction – Battery
 Primary battery
 Secondary battery
 Fuel battery

Introduction
All the chemical reactions can be broadly classified into the
following types.
Irreversible reaction
Some chemical reactions can occur only in one direction.
These reactions are called irreversible reaction. In this type of
reaction, Reactants can change to the Products but the Products
cannot be changed to reactants.
Zn + H2SO4 → ZnSO4+ H2↑

Reversible reaction
A reversible reaction is a reaction in which the
“conversion of reactants to products and the conversion of
products to reactants occur simultaneously”.
It has two types such as
a. Homogeneous reversible reaction and
b. Heterogeneous reversible reaction.

Homogeneous reversible reaction
In this type,
All the constituents of the reaction are in same
phase.
Example:
In the formation of ammonia all the constituents are in
gaseous phase
N2 (g) + 3H2 (g) 2NH
⇌ 3 (g)

Heterogeneous reversible reaction
In this type of reaction the state (Phase) of the
reactants and products differ from each other.
Example:
CaCO3(s) CaO (s) + CO
⇌ 2 (g)
In the above reaction;
different constituents are involved
(Which involves solid and gaseous state)

Phase rule
The phase rule is a generalization given by
Willard Gibbs, which seeks to explain the
equilibria existing in heterogeneous systems. it
may stated as
“provided the equilibrium between any
number of phases is not influenced by gravity
or electrical , or magnetic forces

Phase Rule
“Provided the equilibrium between any number of
phases is not influenced by gravity or electrical , or
magnetic forces , or by surface action and only by
temperature , pressure and concentration, then the
number of degree of freedom(F) of the system is related
to the number of components (C) and of phases (P) by
the Phase rule equation,
F = C – P + 2

Merits of Phase rule:
a. It is applicable to both physical and chemical
equilibria.
b. It requires no information regarding molecular/micro
structure, since it is applicable to macroscopic
systems.
c. It is a convenient method of classifying equilibrium
states in terms of phases, components and degree of
freedom.

Limitation of phase rule
a. It can be applied only for system in
equilibrium.
b. It applies only to a single equilibrium
system, and provides no information
regarding any other possible equilibria
in the system.

Basic terms involved
Phase (P)
A Phase is defined as
“A homogeneous, physically distinct and
mechanically separable portion of system, which
is separated from other such parts of the system by
definite boundary surfaces”.

For example:
i) At freezing point, water consists of three phases
Ice (s) Water(l) water vapour (g)
⇌ ⇌
ii) A gaseous mixture, being thoroughly miscible in all
proportions, will constitute one phase only.
Thus a mixture of N2 and H2 forms one phase only.

Classification of Phases
Phase is classified into three types namely
a. Solid phase
b. liquid phase and
c. gaseous phase
Solid Phase
Each solid constitutes a separate phase.
The number of solid phase depends on the number of solids
present in it.

Example:
ii) Consider a water system consisting of
three phases.
Ice(s) Water (l) Vapour
⇌ ⇌
(g)



iii. An emulsion of oil in water forms two phases.
iv. Decomposition of MgCO3
MgCO3(s) MgO (s) + CO
⇌ 2 (g)
Liquid Phase
The number of liquid phase depends on the number of
liquids present and their miscibilities.

i) If two liquids are
immiscible, they will form three separate
phases ;
Two liquid phases and One vapour phase.
Example:
benzene and water

ii) If two liquids are completely miscible, they
will form ;
one liquid phase and
one vapour phase.
Example:
alcohol and water
(these two are miscible in nature)

Gaseous Phase
Gaseous mixture are thoroughly miscible in
all proportions and it will form
one phase only
Hence, Gaseous constituents are associates
each other.

Example:
i) Air is a mixture of
O2, H2, N2, CO2 and Vapour, etc.,
constitutes a single phase.

Component (C)
By the term component is meant “the smallest
number of independent variable constituents,
taking part in state of equilibrium, by means
of which the composition of each phase can
be expressed in the form of chemical
equation”.

Examples:
i) In the water system,
Ice (s) Water (l) Water vapour (g).
⇌ ⇌
The chemical composition of all the three phases
is H2O, but is in different physical forms.
Hence, it is one component system.

ii. The Sulphur system consists of four
phases;
rhombic, monoclinic, liquid and vapour,
but the chemical composition of all phases is
Sulphur.
Hence, it is an
one component system.

iii. In the dissociation reaction,
CuSO4.5H2O (s) CuSO
⇌ 4.3H2O(s) + 2H2O
The composition of each phase can be
represented by the simplest components,
CuSO4 and H2O , hence it is a “two
component system”.

iv. In the thermal decomposition of CaCO3
CaCO3 (s) CaO (g) + CO
⇌ 2 (g)
The system consists of three phases namely solid
CaCO3, solid CaO and gaseous CO2., But it is a
two component system, because the composition
of each of the above phases can be expressed in
terms of any two of the three components present.

Phase: CaCO3 = CaCO3 + 0 CaO
Phase: CaO = 0 CaCO3 + CaO
Phase: CO2 = CaCO3 – CaO
Thus it is a two component system.

Degree of Freedom (F) or Variance
Degree of freedom is defined as, “the
minimum number of independently variable
factors, such as temperature, pressure and
composition of the phases, which must be
arbitrarily specified in order to represent
perfectly the condition of a system”

Example:
For a system consisting of water in contact with its
vapour,
Water (l) Vapour (g)
⇌
We must state either the temperature or pressure to
define it completely.
Hence, degree of freedom is one or system is
Univariant.

One component system (Water)
The water system consists of three phases, viz,
ice, water and water Vapour
Ice(s) Water (l) Water Vapour (g)
⇌ ⇌
Since water is the only chemical compound
involved, therefore , it is single or one
component system.

The phase diagram contains
curves, areas and triple point.
From the phase rule, when C = 1,
F = C – P + 2
F = 1- P+2
F = 3- P
The most convenient variables are the
pressure and temperature.

Areas:
AOB,AOC,and BOC are the fields of
existence of vapour,liquid and ice phase
respectively. Within these single-phase areas,
the system in bivariant,because to locate any
point in an area ,temperature as well as
pressure co-ordinates need to be known.

Curve OA :Vapour pressure curve or
Vapourisation curve
The Curve OA, dividing the liquid from the
Vapour region, is called Vapour pressure curve
of liquid water
In which water Vapour is in equilibrium with
liquid water
Water (l) ⇌ Water vapour (g)

F =C-P+ 2 , F=1-2+2
So F=1 (Univariant system)
Curve OB : Sublimation curve of ice.
It gives the conditions under which water Vapour is
in equilibrium with solid ice.
Water(s) ⇌ Water vapour (g)
F =C-P+ 2, F=1-2+2 so F=1 (Univariant system)

Curve OC:
Melting curve or Fusion curve of ice.
Which divides the solid ice region from the
liquid water region is called melting curve.
Water (s) ⇌ Water (l)
P=2 and C=1

Point ‘O’:Triple point
The curves OA, OB, and OC meet at the triple
point ‘O’ where all the three phases are in
equilibrium. This occurs at 0.0098o
C and vapour
pressure 4.579 mm Hg is called a triple point. since
three phases co-exist, the system is invariant.

Ice (s) ⇌ Water (l) ⇌ Water vapour (g)
F = C - P + 2 ,
F =1 – 3 + 2 = 0 ( non variant)
If either pressure or temperature is changed,
the three phases would not exist and one of
the phases would disappear

)
TWO- COMPONENT SYSTEM(Pb-Ag)
 A solid liquid equilibrium of an alloy
practically no gas phase and the effect of
pressure is small on this type of
equilibrium.therefore,experiments are usually
conducted under atmospheric pressure .thus
keeping the pressure constant of a system, in
which Vapour phase is not considered, is
known as condensed system.

It will reduce the degrees of freedom of the
system by one and for such a system, the
phase rule becomes:
F = C – P + 1
This is known as the reduced or condensed
phase rule.

Points mentioned in the above diagram
a. Curves OA and OB
b. Area AOB
c. Eutectic point
Point A:
Melting point of pure Ag (9610
C)
Point B:
Melting point of Pure Pb (3270
C )

Curve OA:
Freezing point curve of Ag shows the
effect on freezing point of Ag on
addition of lead in small quantities. the
curve starts from A(9610
C),the melting
point of Ag, where pure Ag co-exists as
solid and liquid
This curve indicates that the melting
point of Ag falls gradually on adding
Pb, along AO, till the lowest point ‘O’
(3030
C) is reached.

At this the following equilibrium will
exist
Solid Ag ⇌ Melt
In the above equilibrium
P = 2; C = 2
Applying reduced phase rule in the
above equilibrium
F=C-P+1
F = 2-2+1 =1
(monovariant system)

Curve OB:
Freezing point curve of Pb represents the
effect on freezing point of Pb on gradual
addition of small amounts of Ag to it.
Point ‘B’ is the melting point of pure lead
(3270
C) along ‘BO’the melting point gradually
falls on the addition of Ag , till the lowest
point ‘O’ is reached, where the solution gets
saturated with respect to Ag and the melting
point of lead does not fall any more.

Along this curve Solid Pb and Liquid Pb
are in equilibrium.
Solid Pb ⇌ Melt
P=2 and C=2
Applying reduced phase rule
F=C-P+1
F = 2-2+1 =1
(monovariant system or univariant)

Point O: Eutectic point
The two curves AO and BO meet
‘O’,where three phases co-exist and
according to condensed phase rule ,the
system will be invariant.
The point ‘O’ represents a fixed
composition (Ag=2.6%; Pb=97.4%) and is
called eutectic composition. No mixture
of lead and silver has a melting point
lower than the eutectic temperature.

At this point the following equilibrium
will exist;
Solid Pb + Solid Ag ⇌ Melt
Where; P=3,C=2
According to reduced phase rule,
F= C-P +1,
F = 2 -3 +1 = 0
(Non variant system)

Pattinson’s Process or Desilverisation of
Argentiferrous lead:
i) If a sample of Argentiferrous lead,containing
less than 2.6% of Ag,is allowed to cool gradually,
lead will separate out and the solution will
become progressively richer in Ag. Till the
percentage 2.6 of Ag is reached.
ii) On further cooling the whole mass will solidify
as such.
Thus the process of raising the relative
proportion of Ag in the alloy is known as
Pattinson’s process.

Eutectic System:
A Binary system consisting of two components,
which are “Completely miscible with one another
in liquid state but on solidification they give rise
to an intimate mixture known as Eutectic System.
Example: A mixture of lead and Silver comprises
of such a system.
Eutectic point:
Two solid substances capable of forming solid
solutions having the properties of lowering each
other’s freezing point. The minimum freezing
point corresponding to eutectic mixture is
eutectic point.

Thermal Analysis
The shape of the freezing point curves
for any system, especially those
involving metals can be determined by
thermal analysis-a method involving a
study of the cooling curves of various
compositions of a system during
solidification.
The form of the cooling curve
indicates the composition of the solid.

Cooling Curve of Pure Metal
When a pure substance in the fused state is
allowed to cool slowly and the temperature
noted at definite times, the graphic
presentation of the rate of cooling will be
continuous curve.
When a freezing point is reached and the
solid makes its appearance, it is indicated by
a break in the continuity of the cooling curve
and the temperature will remain constant.

Cooling curve of a mixture
When a mixture of two solids (say A
and B) in the fused state is allowed to
cool slowly and temperature is noted at
different intervals. Initially the rate of
cooling will be continuous.
When it reaches the point ‘b’ one
substances (either A or B) begins to
solidify out of the melt which is
indicated by a break and the rate of
cooling is different.

Introduction
In electrochemical cell, the
“Chemical energy is converted into
electrical energy”.
In a electrochemical cell, the system does
work by transferring electrical energy through
an electric circuit.

A Cell is a single unit device which converts
chemical energy into electric energy.
A cell consists of two half cells
a. Anode and
b. Cathode

Primary battery
In which the cell reaction is not reversible,
Thus, when the reactants have for the most
part been converted to products, no more
electricity is produced , and the battery
become ‘dead’
Example: Alkaline battery

Alkaline battery
It is an improved form of dry cell, in which NH4Cl
is replaced by KOH as the electrolyte.
The alkaline electrolyte of potassium hydroxide
remains, as there are equal amounts of OH−
consumed and produced.
Mn is reduced from +4 oxidation state to +3
oxidation state.

Cell reaction
(At Anode)
Zn (s) + 2OH−
→ Zn (OH)2 (s) + 2e−
(At Cathode)
2MnO2 (s) + H2O (l) + 2e−
→ Mn2O3 (s) + 2OH−
Net reaction:
Zn (s) + 2MnO2(s) + H2O (l) → Zn (OH)2 (s) +
Mn2O3 (s)

Uses:
Alkaline batteries find used
1.In Camera exposure controls,
2.In calculators and
3.In watches etc.

Secondary battery –LEAD-ACID BATTERY
It is the rechargeable battery .A storage cell is one
that can operate both as voltaic cell and as an
electrical cell.
The typical example for secondary battery is Lead-
acid storage cell.
When it acts as a voltaic cell, it supplies electrical
energy and becomes run down.

Electro chemical parts:
Anode : Lead metal (Pb)
Cathode : Lead dioxide (PbO2)
Electrolyte : Dilute. Sulphuric acid

Construction and Working
1. A lead – acid storage cell consists of a number
of voltaic cells (3 to 6) connected in series to
get 6 to 12 V battery. In each cell, a number of
Pb plates, used as anodes are connected in
parallel and a number of PbO2 plates, used as
cathodes are connected in parallel.

Plates are separated by insulators like rubber
or glass fibre.
The entire combination is immersed in 20%
dil. H2SO4.
Cell notation:
Pb | PbSO4 || H2SO4 || PbSO4 | PbO2 | Pb
The diagram is given;

Advantages of Lead–Acid batteries
It is made easily.
It produces very high current.
The self discharging rate is low.
It works effectively even at low
temperatures

Nickel-cadmium battery
Nickel-Cadmium and lithium
batteries are rechargeable one.
Electrochemical reaction of both
batteries can be reversed.
 Discharging and charging reactions
observed in both batteries.

Construction of Nickel-Cadmium
battery
This battery is otherwise called as
NICAD-battery.
The battery consists of
Cadmium anode and NiO(OH)(s) as
cathode.
It has an electrolyte of KOH.
It is represented as:
Cd/Cd (OH)2 // KOH / NiO2 / Ni

At anode:
Cadmium is oxidized to Cd2+
and
further it combines with OH-
ions to
form Cd(OH)2
Cd(s) + 2(OH)-
⇌ Cd(OH)2 + 2e-
In the above reaction forward
reaction leads discharging and the
backward reaction leads charging.

At cathode:
NiO2 is reduced to Ni2+
ions
which further combine with OH-
ions to form Ni(OH)2
2NiO(OH) (s) + 2H2O + 2e-
⇌
2Ni(OH)2 + 2(OH)-

Cell reaction:
Cd(s) + 2NiO(OH) (s) + 2H2O ⇌
Cd(OH)2 (s) + Ni(OH)2 (s)
In the above reaction
forward reaction leads discharging
and
the backward reaction leads
charging.

Advantages
1. It is smaller and lighter and portable.
2. Cell voltage is fairly constant about
1.4V.
3. Like a dry cell, it can be packed in a
sealed container.
Uses:
In calculators and
electronic flash unit, transistors,

Lithium Battery
Lithium battery is a secondary
battery where solid polymer gel is
used as an electrolyte
Electrochemical parts:
Anode : Lithium
Cathode : TiS2
Electrolyte: Solid polymer

Construction:
1. The Lithium battery consists of a
lithium anode and a TiS2 cathode.
2. A solid electrolyte generally a
polymer is packed in between the
electrodes.
3. The electrolyte (Polymer) permits
the passage of ions but not that of
electrons.

Cell reaction is as follows:
At anode : Li Li+ + e
→ -
At cathode: TiS2 +e- TiS
→ 2
-
Net cell reaction:
Li + TiS2 Li
→ +
+ TiS2
-
Li+
+ TiS2
-
Li + TiS
→ 2
The cell is rechargeable and produces a
cell voltage of 3.0 V.

Advantages:
1. It is smaller in size.
2. It can be made into variety of shapes
3. It produces high voltage.
4. Lithium acts as a electropositive
metal suitable foranode.
Disadvantages:
Lithium battery is more expensive than
other batteries.

Lithium is the future of a cell due to
the following reasons;
1. Its cell voltage is high, 3.0 V.
2. Since Li is a light-weight metal, only
7g (1 mole) material is required to
produce one

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