Download Thermodynamics

Unit-7
Thermodynamics
Introduction:
The term ‘Thermo’ means heat and ‘dynamics’ means flow or movement.. So
thermodynamics is concerned with the flow of heat.
The different forms of the energy are interconvertible from one form to another.
For example, chemical energy can be converted in to electric energy and electrical energy
can be converted into chemical energy and so on. During the process of transformation,
the changes are accompanied by different physical and chemical processes.
The study of flow of heat or any other form of energy accompanying the physical
or chemical transformations is called thermodynamics.
System, Boundary and surroundings:
Surroundings
System
Boundary
Fig 1. Thermodynamic system illustrated
A system is that part of universe which is under thermodynamic study and the rest
of universe is surroundings.
The real or imaginary surface separating the system form surroundings is called
boundary.
Nature of heat and work:
When a change in the state of a system occurs energy is transferred to the
surroundings or from the surroundings. This energy may be transferred as heat or
mechanical work.
Sign convention of heat:
The symbol of heat is “q”.
If heat flows from the surroundings into the system, the energy of the system is
raised. So it is considered as positive, ‘+q’
If heat flows from the system into the surroundings, the energy of the system is
lowered. So it is considered as negative, ‘-q’
Heat
Absorbed
Heat
released
q is (+)
q is (-)
System
Fig 2.Sign convention for heat flow in a thermodynamic system.
Sign Convention of work:
The symbol of work is “w”
If work is done on a system (known as work of compression) the energy of the
system is increased.
So it is considered as positive, +w.
If work is done by the system (known as work of expansion) the energy of the
system is decreased.
So it is considered as negative –w.
Summary :
Heat enters the system ;
q is +ve
Heat leaves the system
q is –ve
Work is done on the system ;
w is +ve
Work is done by the system
w is –ve
Thermochemistry
Internal Energy (I.E) :
It is the energy associated with the given mass of a substance.
Internal energy is represented by the symbol “U”.
The absolute value of internal energy can not be measured. When a system
changes from initial state (reactant) to the final state (product) there is change in internal
energy (∆U). This can be calculated from the values of initial and final state.
∆U = U2-U1 = Uproduct - Ureactant
The change in internal energy (∆U) is the heat change in a chemical reaction at
constant volume because no external work is performed.
Enthalpy:
In the laboratory most of the chemical reactions are carried out at constant
pressure (atmospheric pressure) rather than at constant volume. In order to study the heat
changes for reactions taking place at constant pressure and at constant temperature a new
term ‘enthalpy’ was introduced.
It is represented by the symbol ‘H’ and is also called heat content.
The enthalpy of a system defined as the sum of the internal energy and the
product of pressure and volume.
Thus,
H = U + PV
Where U = Internal energy
P = Pressure
V = Volume.
Just like internal energy, absolute value of enthalpy (H) can not be measured.
However a change in enthalpy (∆H) can be measured accurately and is given by
∆H = Hproduct - Hreactant
∆H = HP – HR
Exothermic Reaction:
A reaction in which heat is evolved or liberated when reactants are converted into
products is called exothermic reaction.
Consider the combustion reaction between carbon (graphite) and oxygen (air) to form
gaseous carbon dioxide. In this reaction heat is released. The reaction is represented as ,
C(s) + O2(g)
CO2(g)
+ heat
In the above reaction the mixture of carbon, Oxygen and carbon dioxide constitutes the
system and the rest is the surroundings. The heat energy is transferred from the system to
the surroundings.
Several chemical reactions are exothermic in nature.
Ex:
C(s) + O2(g)
CO2(g)
+ 393.5kJ
CO2(g)
∆H = -393.5kJ
For exothermic reaction.
HP < HR so ∆H is –ve
So the above reactions can be written as
C(s) + O2(g)
Few more examples are,
N2(g) + 3H2(g)
2 NH3(g) ∆H = -91.17kJ
H2(g) + ½ O2(g)
H2O(2) ∆H = -285.8 kJ
Endothermic Reaction:
A reaction in which heat is absorbed when reactants are converted into products is called
endothermic reaction.
Consider the reaction between gaseous nitrogen. N2(g) and gaseous oxygen O2(g)
to form gaseous nitric oxide NO(g). This reaction is carried out at high temperature.
So during the reaction heat is absorbed, by the system from the surroundings.
The reaction is represented as,
N2(g) + O2(g) + heat
2NO(g)
Such reactions in which heat is absorbed by the system are called endothermic
reactions.
For endothermic reaction,
HP>HR
so ∆H is +ve
The above reaction can be written as
Note:
N2(g) + O2(g)
2NO(g) ∆H = +180.8 kJ
In an exothermic reaction, the energy of the products is less than that of reactants
and in an endothermic reactions the energy of the products is greater than that of the
reactants. This is shown the figure.
Fig 3.Energy changes in exothermic and endothermic reactions
CO2 (g)
Progress of reaction Æ
Heat
Given
Out
2NO(g)
Energy Æ
(kJ)
Energy Æ
(kJ)
C (s) + O2(g)
N2(g)+O2(g)
Heat
Taken
In
Progress of Reaction Æ
Sign of ∆H
Energy
Terms Used
Sign
Released
Absorbed
Exothermic
Endothermic
-ve
+ve
Thermochemical Equation:
A thermochemical equation is a balanced chemical equation which shows the
quantity of heat liberated or absorbed during the reaction and physical states of reactants
and products.
In a thermochemical equation, the quantity of heat liberated or absorbed is
represented by writing the value of ‘∆H’ (heat change) at the right side of the equation.
The sign of ‘∆H ’ indicates whether the reaction is exothermic or endothermic.
Example:
1. H2(g) + ½ O2(g)
H2O(l) ∆H = -285.8 kJ
It tells that 285.8 kJ of heat is liberated when 1 mole of hydrogen gas combines with half
mole of oxygen gas to form 1 mole of liquid water.
2. N2(g) + O2(g)
2NO(g) ∆H = +180.8 kJ
It tells that 180.8kJ of heat is absorbed when 1 mole of nitrogen gas reacts with 1 mole of
oxygen gas to form 2 mole of nitric oxide gas .
Exercises:
1. Define thermodynamics.
2. What is a system?
3. If work is done on a system, what happens to the energy of the system?
4. What happens to the energy of a system during the work of expansion?
5. What happens to the energy of a system when heat enters and leaves the
system?
6. What is internal energy?
7. How is enthalpy related to internal energy?
8. What is an exothermic reaction? Give an example.
9. What is an endothermic reaction? Give an example.
10. Define a thermochemical equation.
11. Write the thermochemical equation for the combustion of carbon.
12. The value of ∆H is +ve for an endothermic reaction. Why?
13. The value of ∆H is -ve for an exothermic reaction. Give reason.
14. Mention the sign of ∆H, if energy is released during a chemical reaction.
15. Mention the sign of ∆H, if energy is absorbed during a chemical reaction.
16. Explain the following thermochemical equation.
H2(g) + ½ O2(g)
H2O(l) ∆H = -285.8 kJ
17. Write the thermochemical equation for the formation of nitric oxide.
18. Mention the sign of work ‘w’ during the work of expansion.
19. Why is the sign of work ‘w’ positive during the work of compression?
20. The sign of ‘q’ is negative, when heat leaves the system. Give reason.