CHAPTER - 6
Thermo chemistry
Energy. Energy is the capacity to do work.
Types of Energy: Kinetic energy, Potential energy, Radiant energy, Thermal energy and Chemical
energy
Energy changes in a chemical reaction. In a chemical reaction energy changes take place in the
form of Heat and Work.
Thermo chemistry is the study of heat changes in a chemical reaction.
System. That part of the universe, which is under investgation, is called the system.
Surroundings. The rest of the universe outside the system is called surroundings
Types of system
Open system.
A system where mass and energy are exchanged with the surroundings is an open system.
Closed System
A closed system is the one where energy is exchanged with the surroundings but not mass.
Isolated System
A system in which both mass and energy are not exchanged with surroundings is an isolated
system.
Exothermic Process
A process in which heat is liberated to the surroundings is called an exothermic process.
Endothermic Process. A process in which heat is absorbed by the system from the surroundings
Thermo chemistry.
The branch of Chemistry deals with the inter conversion of heat and other forms of energy is called
Thermo chemistry.
State Functions. The properties of the system that depend on the initial and final states of system,
regardless of how that change is accomplished are called state functions.
Example: Pressure, Volume, Energy, and Temperature
Heat and Work are not State Functions
The First Law of Thermodynamics (Law of conservation of Energy)
The law states that energy can neither be created nor destroyed in any physical or chemical change,
though it can be converted from one form to another.
Internal Energy (E)
This is energy associated with the system.
E=q+w
q = heat change between system and surroundings, w = work
For exothermic change ‘q’ is negative and for endothermic change’ q’ is positive.
Work.
Work = Force x Distance
‘W’ is positive, if work is done on the system and it is negative if it is done by the system.
Pressure Volume Work
P = Force / Area
Force = Pressure x Area
W=PxAx l
=Px
V
The PV work is given a negative sign according to the international convention
W=-P
V
Heat Change at constant Volume.
E=q+w
E=q–P
V=0
V
E = qv
Thus the heat change at constant volume is equal to the change in internal energy.
Enthalpy (H)
Enthalpy is defined as the heat content of the system.
H = E + PV
At constant pressure,
H=
E+P
We know that,
H = qp =
V
E=q–P
E+P
V
i.e. q =
E+P
V
V
Thus the change in enthalpy represents the heat change at constant pressure
Enthalpy of Reaction
It is heat liberated or absorbed in a chemical reaction.
H = H products - H reactants
If
H is positive (greater than zero) the reaction is endothermic and if it is negative (less than
zero) the reaction is exothermic.
Thermo chemical equations
A chemical equation where the heat change accompanying the reaction is included is called a
Thermo chemical equation.
Relationship between
H and
E
H=
E+P V
We know that PV = nRT
P
V =
H=
Where,
E+
n RT
nRT
n is the change in number of moles of gaseous substances involved in the reaction.
n = n products – n reactants
Calorimetry:
Heat Capacity (C)
It is the heat required to raise the temperature of a given quantity of the substance by one
degree Celsius.
C=q/
T
Specific Heat. (S)
Heat required raising the temperature of 1 gram of a substance by one degree Celsius.
q
S =
m x
T
Constant Volume Calorimeter
This instrument is used to measure the heat of combustion. A known mass of the compound is
ignited in a closed bomb immersed in water. Heat given out by the combustion is absorbed by
water. It is an isolated system and no heat or mass is exchanged with the surroundings.
q system
= q calorimeter + q rxn = 0
‘q calorimeter’ is the heat absorbed by the calorimeter and ‘q rxn’ is the heat of the reaction.
q rxn = - q cal
q cal = C cal x
T
Constant Pressure Calorimeter
This is used to measure the heat changes of non-combustion reactions. Since Pressure is
constant q rxn =
H
q rxn + q water = 0
q rxn
=
- q water
Standard Enthalpy of Reaction ( H 0)
The heat change accompanying a reaction at standard conditions is called the standard enthalpy of
reaction. Standard condition is 1 atm pressure and 25 0C
Standard Enthalpy of formation (
H f 0)
Heat changes accompanying the formation of 1 mole of a compound from its elements at standard
conditions is called Standard Enthalpy of formation.
Standard enthalpy of a compound is equal to its standard heat of formation.
The standard enthalpy of free elements is zero.
Hess’s law
The enthalpy change accompanying a reaction is constant whether the reaction is carried out in one
or more steps.
Heat of Solution
Heat liberated or absorbed when a certain amount of solute is dissolved in a certain
amount of solvent.
NaCl (s)
Na+aq + Cl –aq
H = 4.0 KJ / mol
In ionic compounds like NaCl , strong electrostatic forces of attraction hold the positive and
negative ions together. When added to a polar solvent like water, these ions interact with the
oppositely charged ends of solvent (water) molecules releasing energy. This interaction results in the
separation of ions. Solvent molecules then surround these ions by a process called Solvation.
If water is the solvent, this process is called Hydration. Hydration prevents the ions from coming
together. The whole phenomenon is called Dissolution.
Lattice Energy (U)
The energy required to completely separate one mole of a solid ionic compound into its gaseous ions
is called Lattice energy.
Na+g +
NaCl (s) + U
Na+g +
Cl -g
Cl -g
Hhydration
U
Na+aq +
NaCl (s)
Cl -aq
H solution
Heat of Dilution
Heat change accompanying the dilution certain amount of a solution at a given
temperature is called heat of dilution.