The branch of science which deals
with energy changes is known as Thermodynamics .While
branch of science which deals with energy changes during chemical reaction is known as Thermochemistry.
In other words, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route by which the chemical change occurs (provided the initial and final condition are the same).
branch of science which deals with energy changes during chemical reaction is known as Thermochemistry.
Bond breaking absorb energy while bond making release
energy. Total energy of reactants is
always different than total energy of products. The difference of energy is
either absorbed or evolved during a chemical reaction, this is called heat of reaction.
Types of reaction:
1) Exothermic reactions
2) Endothermic reactions
Exothermic
reactions:
Reactions in which heat is evolved are
known as exothermic reactions.
Total energy of products is always less
than total energy of reactants. The evolved heat of reaction is indicated by
–ve sign.
C+O2
→CO2
∆H=-393.7kJ/mol
Endothermic
reactions:
Reactions in which heat
is absorbed during reaction are known as endothermic reactions. Total energy of
products is greater than total energy of reactants, so energy required for
reaction is absorbed from reactants and tempt of system fall below room tempt
and it absorb energy from surrounding, and tempt again rise to room tempt.Heat
absorbed is indicated by +ve sign.
H2+I2→2HI ∆H=+52.96kJ/mol
Importance
and limitation of thermo chemistry:
Thermo chemistry is based upon first law of thermodynamics. Thermo
chemistry tells us about energy of system and heat content of system, which
helps us to explain chemical bonding of system and chemical equilibrium. Heat of reaction is calculated only
for some reactions and these are limitations of thermo chemistry.
Spontaneous
and non spontaneous reactions:
Process which takes place on its own without any external help and move
from non equilibrium to equilibrium state is called spontaneous or natural
process. It is unidirectional, irreversible and real process.
Example; Water flows from higher to lower
level.
Some spontaneous reactions require small
amount of energy to start, but once they are started, they go on by itself
until complete. For example burning of coal and hydrocarbons in air. Coal does
not react with air itself, but it requires spark to start reaction and when
reaction is started, reaction goes to completion by itself. Non spontaneous Reaction: A process
which does not take place on its own is called
non spontaneous reaction. It does not occur in nature and is reverse of
spontaneous process. Some non spontaneous process can be made spontaneous by
continuous supply of heat from external source. For example pumping of
water uphill, Transfer of heat from cold interior part of refrigerator to ho
surroundings. All exothermic reactions are spontaneous because energy is
evolved by products in surrounding; there are also some endothermic reactions,
which are spontaneous.
System:
Any real or imaginary part of universe that
is under study is called system.
Surrounding;
Everything that is not part of system is
called surrounding.
Boundary:
The real or imaginary surface separating
system from surrounding is called boundary.
Example:
Water contained in cup is system, cup is
boundary and everything around it is surrounding.
State:
The condition of system is called state.
Let water in a container having tempt T and
volume V, these are its initial states (T1)
and when water is heated, its conditions changed and this are called its final
state (T2).
∆T= T2-T1
State
function:
It is macroscopic property of system which
has definite values for initial and final states and it is independent of path
through which change take place.
Let initial volume is V1 and its
volume changes to V2 by changing T or P.
∆V=V2-V1,
V is state function since it changes upon
change in T or P while ∆V is independent of path through which
change takes place.
Internal
energy and first law of thermodynamics;
It is sum of all
energies of particles atoms molecules or ions within a system.
It depends upon motion of molecules
translational, vibrational and rotational and their intramolecular and
intermolecular forces. Total energy of system is equal to its kinetic energies
of particles and potetional energy due to binding forces between particles I n
form of covalent bond and van der Waals forces.
Internal energy is state function, it
depends upon its initial and final state of system and not upon path through
which change take place. Internal energy of system cannot be determined. Only
change in internal energy can be determined.
∆E=E2-E1
Work:
Work is form of energy and is defined as
product of force and distance.
Work=Force*distance
It is denoted by w. It is not property of
system. It is path system, it is not state function. System international unit
of work is joule
Sign convention for W;
Work done by system is negative.-w
Work done on system is positive.+w
1erg=10-7
J
Heat;
It is quantity of energy that flows across boundary of system during
change in state, due to difference in T between system and surrounding.
Heat lost by system is negative and heat
gained by system is positive. System international unit of heat is also joule.
1cal=4.184J
First
law of thermodynamics:
Law
of conservation of energy is also called first law of thermodynamics. Energy
can neither be created nor destroyed during reaction but it can only be
converted from one form into another. Energy of system and surrounding is
conserved.
∆E=E2-E1=q+w
Heat
changes at constant volume:
At constant volume let heat supplied to
system is qv ,then first law of
thermodynamics is :
∆E=qv+w
Since
w=-P∆V
∆E=qv-P∆V
Since volume is constant, therefore ∆V=0,
∆E=qv
It means that heat exchanged at constant
volume is equal to change of internal energy of system and no work is done.
Heat change at constant pressure or
Enthalpy:
Enthalpy of a system is sum of its
internal energy and product of pressure and volume (PV).
H=E+PV
It
is state function. Its unit is joule.
∆H=qp
It means heat exchange at constant volume
is equal to change in enthalpy of system.
Standard state:
The natural physical state of
substance at 25 degree centigrade and 1 atm pressure is known as standard state
of substance.
For example, standard state of CO2 is
gas, H2O is liquid and Fe is solid.
1) Enthalpy of reaction ∆Hr:
The standard enthalpy
change which occur when the number of moles of substance as indicated by
chemical reaction react together to give product under standard conditions.
2) Enthalpy of formation ∆Hf:
The standard change of
enthalpy which occur when 1 mole of compound is formed from given reactants at
standard condition.
3)
Enthalpy
of atomization ∆Hat:
The enthalpy change
which occurs when 1 mole of gaseous atoms are formed from given elements at
standard conditions.
4)
Enthalpy
of neutralization:
The enthalpy change
which occurs when 1 mole of hydrogen ions from acid react with one mole of
hydroxide ions from base to form one mole of water is called enthalpy of neutralizations.
5)
Enthalpy
of combustion:
The enthalpy
change which occurs when one mole of given substance burn completely in excess
of oxygen under standard conditions.
6)
Enthalpy
of solution :
The enthalpy
change which occur when one mole of given substance when dissolved in solvent
that further dilution result in no detectable heat change is called Enthalpy of
solution.
·
Measurement
of enthalpies:
1) Bomb Calorimeter:
A
bomb calorimeter is a type of constant-volume calorimeter used in measuring the
heat of combustion of a particular reaction. Bomb calorimeters have to
withstand the large pressure within the calorimeter as the reaction is being
measured. Electrical energy is used to ignite the fuel; as the fuel is burning,
it will heat up the surrounding air, which expands and escapes through a tube
that leads the air out of the calorimeter. When the air is escaping through the
copper tube it will also heat up the water outside the tube. The temperature of
the water allows for calculating calorie content of the fuel.
q=m*s*∆T
q=c*∆T
2): Glass
Calorimeter:
IT consists of glass,
insulated with wool, and also consists of stirrer and thermometer to note
temperature. Stoichiometric amount of substance is placed in it, tempt is noted
before and after reaction, ∆T give
change in tempt and enthalpy measurement is given by:
q=m*s*∆T
Ø
HESS'S law :
Hess's law or Hess law
states that the enthalpy or heat change accompanying a chemical reaction is
independent of the pathway between the initial and final states.In other words, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route by which the chemical change occurs (provided the initial and final condition are the same).
Addition of chemical equations leads
to a net or overall equation. If enthalpy change is known for each equation,
the result will be the enthalpy change for the net equation. If the net
enthalpy change is negative (ΔHnet < 0), the reaction is exothermic
and is more likely to be spontaneous; positive ΔH values corresponding to
endothermic reactions. Entropy also plays an important role in determining
spontaneity, as some reactions with a positive enthalpy change are nevertheless
spontaneous.
Hess's Law states that enthalpy
changes are additive. Thus the ΔH for a single reaction
Example:
a) Cgraphite+O2 → CO2
(g) ;(ΔH = -393.5 kJ) (direct step)
b) Cgraphite+1/2 O2 → CO (g) ;
(ΔH = -110.5 kJ)
c) CO (g)+1/2 O2 → CO2 (g); (ΔH = -
283.02 kJ)
→In the reactions b) and c), the
total ΔH = -393.5 kJ which is equal to ΔH in a)
The difference in the value of ΔH is
0.02 kJ which is due to measurement errors.
Application of Hess’s Law:
Ø
The Born
Haber Cycle :
The
sum of energy changes for closed cyclic process is zero, if initial and final
states are same. It is used to determine lattice energy of ionic crystals. The
amount of energy released when one mole of gaseous ions of opposite charges
combine to form crystal ionic compound.
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