Download Chemical Thermodynamic

CBSE CLASS XII Chemistry
Chemical Thermodynamic.
One mark questions with answers
Q1. What is reversible process in thermodynamics?
Ans1. A process which is carried out infinitesimally slowly so that changes occurring
in the direct process can be exactly reversed without disturbing the equilibrium.
Q2. What is the main limitation of the first law of thermodynamics?
Ans2. It cannot predict the spontaneity of a process.
Q3. Define intensive properties?
Ans3. Those properties which depend only on the nature of the substance and not
on the amount of the substance taken are called Intensive properties.
Q4. Why heat changes reported are usually enthalpy changes and not internal
energy changes?
Ans4. This is because most of the processes are carried out in open vessels i.e., at a
constant pressure.
Q5. What does free energy change signify?
Ans5. Free energy change is equal to the useful work done by the system.
Q6. Write mathematical statement of first law of thermodynamics?
Ans6. E = q + w.
Two mark questions with answers
Q1. Describe the Ist law of thermodynamics.
Ans1. This law is a corollary of law of conservation of energy. This law is based on
experience. There is no theoretical proof of this law. Mathematical form of the first
law of thermodynamics is
E = q + w.
Q2. How can you justify that though q and w are not state functions, yet (q + w) is
a state function.
Ans2.
Let the internal energies at A and B be EA and EB. If internal energy is a state
function, the difference in internal energy from A to B or B to A should be same. If
not, suppose the internal energy increases by an amount E while going from A to B.
Now suppose the internal energy decreases by an amount E' while returning from B
to A. Let E > E', that means after returning to A, an amount of energy equal to (E
- E') has been created which is against the law of conservation of energy.
Q3. Briefly explain the term 'enthalpy'. How does it differ from internal energy?
Ans3. Enthalpy is the sum of internal energy and the product of pressure and
volume of the system.
H = E + PV
Internal energy is the energy stored in a substance.
Q4. What is the basic difference between heat of formation and heat of reaction?
Illustrate with suitable examples.
Ans4. Heat of formation is the amount of energy released when one mole of the
substance is formed from its elements under given conditions of temperature and
pressure.
Amount of heat evolved or absorbed in a chemical equation which have completely
reacted, is called heat of reaction.
Q5. What do you understand by free energy and what is its physical significance?
Ans5. It is the thermodynamic property, decrease in whose value during a process is
equal to the useful work done by the system.
G = H - TS.
If G is -ve, the process is spontaneous.
Three mark questions with answers
Q1. State the law of conservation of energy and give a mathematical equation for it.
Mention also two examples to support this law.
Ans1. Energy can neither be created nor destroyed although it may be transformed
from one form to another. Mathematical form of the law is
E = q + w
In the running of an electric motor, electrical energy is converted into mechanical
energy.
Q2. Discuss the significance of the mathematical expression in which the heat
absorbed by a system is related to the internal energy and work done by the system.
Ans2. H = E + PV
Whenever heat is supplied to a system, a part of it is used to do work of expansion
and the rest is used to increase the internal energy of the system.
Q3. Derive the relationship between heat of reaction at constant temperature q p and
heat of reaction at constant volume qv.
Ans3. H = E + PV.
or H = E + PV2 - V1) = E + (PV2 - PV1) ......(1)
For ideal gas PV = nRT
PV1 = n1RT
PV2 = n2RT
Substituting in equation (1), we get
H = E + (n2RT - n1RT)
= E + (n2 - n1)RT.
or H = E + ngRT.
Q4. Define the term 'bond energy'. Why an average value is taken in the polyatomic
molecule?
Ans4. Bond energy is the amount of energy required to break the bonds in one mole
of the substance. For a polyatomic molecule, average value of the bond energy is
taken into consideration because different bond energies are involved with the
breaking or formation of different bonds.
Q5. Explain the entropy concept and discuss the nature of change in entropy for the
following :
(i) H2O(l)  H2O(s)
(ii) MgCO3(s) MgO(s) + CO2(g)
(iii) 2SO2(g) + O2(g)  2SO3(g)
Ans5. Extent of randomness of a system is called entropy.
(i) Entropy will decrease because solids have smaller randomness molecules.
(ii) Entropy will increase because a substance in vapour state is formed.
(iii) Entropy is decreased because the total number of molecules are decreased.
Five mark questions with answers
Q1. Define the following terms:
(i) System.
(ii) State variables.
(iii) State functions.
(iv) Extensive and intensive properties.
(v) Isothermal process.
(vi) Adiabatic process.
Ans1. (i) System : Part of the universe which is under investigation is called
system.
(ii) State variables: Variables like temperature, pressure and volume etc. of a
system which determines the state of a system are called state variables.
(iii) State functions : Properties which depend upon the state of a system are called
state functions.
(iv) Extensive and intensive properties : Properties which depend upon the
quantity of a substance in a system are extensive properties and those which depend
upon the nature of the substance are called intensive properties.
(v) Isothermal process : A process which takes place keeping the temperature of
the system constant is called isothermal process.
(vi) Adiabatic process : A process, which takes place in such a manner that flow of
heat from one to another part of the system does not take place, is called adiabatic
process.
Q2. What are the limitations of first law of thermodynamics? State second law of
thermodynamics in as many ways as you can.
Ans2. First law of thermodynamics has the limitation that it merely indicates that in
any process there is an exact equivalence between various forms of energy. It
provides no information concerning the spontaneity or feasibility of the process.
Various statements of second law of thermodynamics are:
(i) All spontaneous processes (or naturally occurring processes) are
thermodynamically irreversible.
(ii) Without the help of an external agency, a spontaneous process cannot be
reversed.
(iii) The complete conversion of heat into work is impossible without leaving some
effect elsewhere.
(iv) The entropy of the universe is continuously increasing.
Q3. Explain what is meant by Gibb's free energy change. "The sign of free energy is
very important". Justify.
Ans3. Gibb's free energy change which is represented by G is related to enthalpy
and entropy changes by the following equations G = H - TS
(i) If G is -ve, the forward reaction is spontaneous.
(ii) If G is zero, the reaction is in equilibrium.
(iii) If G is +ve, the forward process is non-spontaneous.
Q4. Define entropy. Why is it a state function? Explain the effect of increased
temperature on the entropy of a substance.
Ans4. Entropy is the measure of disorder of a system. Its value depends upon the
state of the system, hence it is a state function. On increasing the temperature, the
molecular motion increases. Hence, entropy will increase with increase of
temperature.
Q5. Derive the relation H = E + nRT.
Ans5. H = E + PV
or H = E + P(V2 - V1)
= E + (PV2 - PV1) ........(i)
For ideal gas PV = nRT
PV1 = n1RT
PV2 = n2RT
Substituting the values of PV1 and PV2 in equation (i)
H = E + (n2RT - n1RT)
or H = E + nRT.