THERMOCHEMISTRY
Thermo Chemical Reaction:
"These
chemical reactions, which are accompanied with the energy changes during the
conversion of the reactants into the products as known as "Thermo chemical
Reactions"
There are two types of thermo
chemical reaction, which are as follows
1. Exothermic reaction
2. Endothermic reaction
1. Exothermic
Reaction:
"An
exothermic reaction is one during which heat is liberated or released to the
surrounding"
example
1. C + O2 CO2 + Heat
2. CH4
+ 2O2 CO2 + 2H2O + Heat
2. Endothermic
Reaction:
"An endothermic reaction is one
during which heat is absorbed from the surroundings".
example:
1. C
+ 2S + Heat CS2
2. H2
+ I2 + Heat 2HI
Thermodynamic & Its Terms:
"The branch of science, which
deals with the study of energy changes in a system based on the principle of
conservation of energy, is known as "THERMODYNAMIC".
In the study of the thermodynamics,
it is convenient to identity the terms system, surrounding and state.
1. System:
"The part of universe, which is
under study or anything, which is under consideration is called
"SYSTEM".
The
term system has a very wide application it may range from a small quantity
e.g. a cup of water up to the entire universe
In the study of thermal
decomposition of CaCO3, the sample of this compound is a system similarly;
the liquid whose boiling point is being determined is a system.
2. Surrounding:
"The environments in which a
system is examined and which is not the part of the system are called "SURROULNDING".
e.g. In the study of thermal
decomposition of CaCO3, the beaker, the source of heat, table etc.
all constitute the surrounding of the system.
3. State:
"A system can be described by
its properties e.g. volume, pressure, temperature etc. A system is said to be
in a definite "state" when each of its properties has a definite
value and thus the system is completely defined".
The description of the system before
it undergoes a change is known as the INITIAL STATE and the description of a
system after the change is known as the FINAL CHANGE. Change in the system is
therefore described by comparing the final and initial state of the system e.g.
a system composed of a flask containing water of temperature is heated to
temperature T2, the change in temperature, therefore
T = T2 - T1
Similarly
the change in volume will be
V = V2 - V1
Macroscopic Properties:
The properties of a system in bulk,
which are easily measurable as known as the "MACROSCOPIC PROPERTIES"
These properties may be divided into two main group which are
1. Intensive Properties
2. Extensive Properties
(A) Intensive Properties:
"Those properties of a system,
which are independent of the amount of material, concerned."
For Example:
Density,
Pressure, Temperature, Viscosity, Surface, Tension, Melting and Boiling points.
(B) Extensive Properties:
"The properties of a system
that depends upon the amount of the substance are called “EXTENSIVE
PROPERTIES"
For Example
Mass,
Volume, Mole, Enthalpy, Entropy, Internal energy etc.
FIRST LAW OF THERMODYNAMICS:
This law is also known as “LAW OF CONSERVATION OF ENERGY” and may
be defined as:
"Energy can
neither be created nor destroyed although it may
change from one form to another"
OR
"The total energy
of the system and its surrounding must remains constant although it may change
from one form to another"
Explanation
Suppose a cylinder of a gas fitted
with a frictionless and weightless position is present at constant atmospheric
pressure. Let "q" is the quantity of heat supplied to the system this
heat energy.
(i)
Increases the internal energy of the system,
(ii)
Perform the work of expansion by moving the piston.
Let
the internal energy at initial state =
E1
&
the internal energy at final state ` = E2
\ The increase in internal energy =E2- E1=DE
&
the work done by the system = W
There
for according to the definition of Law,
q = DE + W
OR DE = q - W
This
is the mathematical expression of first law of thermodynamic.
Work done by the gas:
Suppose,
A cylinder fitted
weightless frictionless piston which the area of cross section is A, and piston
covers a small distance “∆h”
\
Work = Force x Displacement
\
W= F x ∆h
But 
Þ 
Þ F = PA
\ W
= P x A ∆h
But
∆h = h2 –h1
Change
of volume = A ∆h
W = P DV
Hence
first law of thermodynamic may also be written as
Q = DE + W
Þ
q = DE
+ PDV
Or
DE = q
- PDV
i) At Constant Volume:
When a system has constant volume,
the piston of the cylinder does not move or fixed, then the quantity of heat
supplied will be qv
Therefore in this case
DV = O
& q = qv
The
first law of thermodynamic becomes
q = DE + PDV
qv = DE + O
Þ
qv
= DE
i.e.
A constant volume, the heat absorbed by the system is completely utilized
increase the internal energy of the system.
ii) At Constant Pressure:
When the system is at constant
pressure, then quantity of heat supplied to this system is qp. It
increase the internal energy change of the system and performs some work if
expansion. Hence we have
qp = DE + p DV
But
DE
= E2- E1
& DV
= V2- V1
qp = E2-
E1 + P (V2- V1)
Þ qp = (E2 + PV2) – (E1
+ PV1)
According
to definition, E+ PV is the enthalpy of the system, which is denoted by H.
E2 + PV2
= H2
E1 + PV1
= H1
qp
= H2- H1
Þ qp = DH
This expression shows that at
constant pressure the heat absorbed by the system is simply equal to the change
in enthalpy.
Enthalpy:
Enthalpy is also called heat constant.
For all exothermic reaction DH = -ve & for all endothermic reaction DH =
+ve.
A
summary of exothermic and endothermic reactions explained n terms of heat
constant is diagrammatized as follows:
Thermochemistry:
"The
branch of science which deals with the measurement or calculation of heat
changes in a chemical reaction is called THERMOCHEMISTRY"
Usually heat is measured in the
units of kilo joules (KJ) and
Kilocalorie (K,
cal)
Hess's Law Of Constant Heat Of
Summation:
According to
this law,
"The
resultant heat change in a chemical reaction is the same whether the reaction
takes place in one or several stages"
OR
"The
enthalpy change of system depends upon its initial and final states only, no
matter by which method the change is brought about"
Explanation
Consider
a chemical reaction in which reaction in which reactant "A" changes
to the product "D" in a single step with
DH as the heat change
→ A D` DH
It may proceed
through different inter mediate stages i.e. "A" , "B" then
change "C" changes to "D" with heat change as DH3
A → B DH1
B → C DH2
C → D DH3
It
follows from Hess's law that enthalpy changes accompanying changes from A to D
According
to Hess's Law
DH = DH1 + DH2
+ DH3
It
is the mathematical expression of Hess's Law.
VERIFICATION:
To verify this law, we take the
example of washing soda. This compound is prepared by tow methods. i.e.
(a) One- step method
(b) Tow- steps method.
(A) One Step Method:
In this method one mole of CO2
is reacted with two moles of NaOH to give one mole of sodium carbonate along
with 90 K/mole energy released.
CO2 + 2NaOH Na2CO3 + H2O DH = 90 KJ/mol
(B) Two Step Method:
In this method firstly one mole of
CO2 is reacted with one mole of NaOH to give a mole of sodium
bicarbonate along with 49KJ/mole energy.
i.e.
CO2 + NaOH NaHCO3 DH = -49 KJ/mol
In the second step NaHCO3 is reacted
with second mole of NaOH to form sodium carbonate and water along with -41KJ/mole
energy
NaHCO3
+ NaOH Na2CO3 +H2O DH2
= -41 KJ/mol
According
the two step equations we have
CO2
+ NaOH NaHCO3
DH = -49 KJ/mol
NaHCO3
+ NaOH Na2CO3 + H2O
DH = -41 KJ/mol
CO2 + 2NaOH Na2CO3 + H2O
+ 90 KJ/mole.
i.e. DH1 + DH2=
-49 + (-41) = -90KJ/mole = DH
Þ DH = DH1 + DH2
Hence
the Hess's Law if verified.
Heat Of Formation:
"It is the quantity of heat
i.e. enthalpy change which is evolved or absorbed when one mole of the
substance if formed from its elements"
Generally
heat of formation is represented by, DHF
e.g. C
+ O2 CO2 DHf = - 393.7 KJ/mol
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