Veermoon 2

CBSE
Class 11 Chemistry
Important Questions
Chapter 6
Thermodynamics
2 Marks Questions
1.Change in internal energy is a state function while work is not, why?

Ans. The change in internal energy during a process depends only upon the initial and final
state of the system. Therefore it is a state function. But the work is related the path followed.
Therefore, it is not a state function.
2.With the help of first law of thermodynamics and H = U + pv, prove = qp
Ans.The enthalpy is defined as

H = U + pv
For a change in the stales of system,
=
= …………(i)
The first law of thermodynamics states that –
= …………………….(ii)
From (i) and (ii),
=
When the pressure is constant,
3.Why is the difference between and not significant for solids or liquids?
Material downloaded from myCBSEguide.com. 1 / 6

Ans. The difference between and is not usually significant for systems consisting of
only solids and / or liquids because they do not suffer any significant volume changes upon
heating.
4.What is an extensive and intensive property?

Ans. Extensive property is a property whose value depends on the quantity or size of matter
present in the system.
Intensive property is a property which do not depend upon the quantity or size of matter
present.
5.Show that for an ideal gas Cp- Cv = R

Ans.When a gas is heated under constant pressure, the heat is required for raising the
temperature of the gas and also for doing mechanical work against the external pressure
during expansion.
At constant volume, the heat capacity, C is written as Cv and at constant pressure this is
denoted by Cp.
we write heat q
at constant volume as qv = Cv
at constant pressure as qp =
The difference between Cp and Cv can be derived for an ideal gas as :
For a mole of an ideal gas,
On putting the values of
Cp = Cv +R
6.Show that for an ideal gas, the molar heat capacity under constant volume conditions
is equal to 3/2 R.

Ans.For an ideal gas, from kinetic theory of gases, the average kinetic energy per mole (Ek)
of the gas at any temperature Tk is given by
At (T+1)k, the kinetic energy per mole (Ek1) is Ek1 =
Therefore increase in the average kinetic energy of the gas for 10C (or 1K) rise in
temperature is
by definition is to the molar heat capacity of a gas at constant volume, Cv.
7.A 1.25g sample of octane (C18 H18) is burnt in excess of oxygen in a bomb calorimeter.
The temperature of the calorimeter rises from 294.05 to 300.78K. If heat capacity of the
calorimeter is 8.93 KJ/K. find the heat transferred to calorimeter.
Ans .Mass of octane,
M = 1.250g.
= 0.00125.
Heat capacity, c = 8.93 kJ/k
Rise in temp,
= 6.73K
Heat transferred to calorimeter
= 0.00125 x 8.93 x 6.73

= 0.075 kJ
8.Calculate the heat of combustion of ethylene (gas) to from CO2 (gas) and H2O (gas) at
298k and 1 atmospheric pressure. The heats of formation of CO2, H2O and C2H4 are –
393.7, - 241.8, + 52.3 kJ per mole respectively.

Ans. C2H4 (g) + 302(g) 2CO2(g) + 2H2O (g)

reactants
= [2 x (CO2) + 2 x ] –
= 2 x[(-393.7)m+2x (-241.8)] – [(523.0) + 0)]
= [-787.4 – 483.6 ] -53.3

= - 1323.3 kJ.
9.Give two examples of reactions which are driven by enthalpy change.

Ans. Examples of reactions driven by enthalpy change:
The process which is highly exothermic, i.e. enthalpy change is negative and has large value
but entropy change is negative is said to be driven by enthalpy change, eg.
(i)
(ii)
10.Will the heat released in the following two reactions be equal? Give reasons in
support of your answer.
(i)H2 (g) +
Ans. No, the heats released in the two reactions are not equal. The heat released in any
reaction depends upon the reactants, products and their physical states. Here in reaction (i),
the water produced is in the gaseous state whereas in reaction (ii) liquid is formed. As we
know, that when water vapors condensed to from water, heat equal to the latent heat of
vaporization is released. Thus, more heat is released in reaction (ii).

11.What is the relation between the enthalpy of reaction and bond enthalpy?
Ans .A chemical reaction involves the breaking of bonds in reactants and formation of new
bonds in products. The heat of reaction (enthalpy change) depends on the values of the heat
needed to break the bond formation .Thus
(Heat of reaction = (Heat needed to break the bonds in reactants – Heat liberated to from
bonds in products).
= Bond energy in (to break the bonds) - Bond energy out (to form the bonds)
= Bond energy of reactants – Bond energy of products.
12.The reaction C (graphite) + O2 (g) CO2(g) + 393.5 kJ mol-1 represents the formation
of CO2 and also combustion of carbon. Write the values of the two processes.
Ans.(i) The standard enthalpy of formation of CO2 is -393.5 kJ per mole of CO2.
That is
(ii) The standard enthalpy of combustion of carbon is - 393.5 kJ per mole of carbon i.e.
13.Explain how is enthalpy related to spontaneity of a reaction?

Ans.Majority of the exothermic reactions are spontaneous because there is decrease in
energy.
Burning of a substance is a spontaneous process.
C(s) +O2(g)
Neutralisation of an acid with a base is a spontaneous reaction.
Many spontaneous reactions proceed with the absorption of heat. Conversion of water into
water vapour is an endothermic spontaneous change. Therefore change in enthalpy is not
the only criterion for deciding the spontaneity of a reaction.
14.The are given + 61.17kJ mol-1 and +
132 Jk-1mol-1 respectively. Above what temperature will the reaction be spontaneous?
Ans. The reaction

2Ag2O (s)
Will be spontaneous when is negative.

Since
Shows that would be –ve when,
Or T >
The process will be spontaneous above a temperature of .

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CBSE

1.Change in internal energy is a state function while work is not, why?

2.With the help of first law of thermodynamics and H = U + pv, prove = qp

Ans.The enthalpy is defined as

Material downloaded from myCBSEguide.com. 1 / 6

4.What is an extensive and intensive property?

5.Show that for an ideal gas Cp- Cv = R

On putting the values of

Material downloaded from myCBSEguide.com. 2 / 6

of the gas at any temperature Tk is given by

At (T+1)k, the kinetic energy per mole (Ek1) is Ek1 =

by definition is to the molar heat capacity of a gas at constant volume, Cv.

= 0.00125 x 8.93 x 6.73

393.7, - 241.8, + 52.3 kJ per mole respectively.

Material downloaded from myCBSEguide.com. 3 / 6

= 2 x[(-393.7)m+2x (-241.8)] – [(523.0) + 0)]

= [-787.4 – 483.6 ] -53.3

9.Give two examples of reactions which are driven by enthalpy change.

Material downloaded from myCBSEguide.com. 4 / 6

13.Explain how is enthalpy related to spontaneity of a reaction?

Neutralisation of an acid with a base is a spontaneous reaction.

14.The are given + 61.17kJ mol-1 and +

Material downloaded from myCBSEguide.com. 5 / 6

Will be spontaneous when is negative.

Shows that would be –ve when,

The process will be spontaneous above a temperature of .

Material downloaded from myCBSEguide.com. 6 / 6

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