CBSE NCERT Solutions for Class 11 Chemistry Chapter 6...CBSE NCERT Solutions for Class 11 Chemistry Chapter 6 Back of Chapter Questions 1. Choose the correct answer. A thermodynamic

Class–XI–CBSE-Chemistry Thermodynamics

CBSE NCERT Solutions for Class 11 Chemistry Chapter 6

Back of Chapter Questions

1. Choose the correct answer. A thermodynamic state function is a quantity

(i) used to determine heat changes

(ii) whose value is independent of path

(iii) used to determine pressure-volume work

(iv) whose value depends on temperature only

Solution:

A thermodynamic state function is a quantity whose value is independent of a path. Functions like

etc. depend only on the state of a system and not on the path.

2. For the process to occur under adiabatic conditions, the correct condition is:

(i)

(ii)

(iii)

(iv)

Solution:

A system is said to be under adiabatic conditions if there is no exchange of heat between the

system and its surroundings. Hence, under adiabatic conditions,

3. The enthalpies of all elements in their standard states are:

(i) unity

(ii) zero

(iii)

(iv) different for each element


Solution:

The enthalpy of all elements in their standard state is taken to be zero.

4. of combustion of methane is The value of is

(i)

(ii)

(iii)

(iv)

Solution:

given

Since and

Therefore, alternative is correct.

5. The enthalpy of combustion of methane, graphite and dihydrogen at are,

and respectively. Enthalpy of formation

of will be

(i)

(ii)

(iii)

(iv)

Solution:

According to the question,


(i)

(ii)

(iii)

Thus, the desired equation is the one that represents the formation of i.e.,

Enthalpy of formation of

6. A reaction, is found to have a positive entropy change. The reaction will be

(i) possible at high temperature

(ii) possible only at low temperature

(iii) not possible at any temperature

(v) possible at any temperature

Solution:

Given : positive

negative (since heat is evolved)

For a reaction to be spontaneous at constant pressure and temperature , should be negative.

According to the question, for the given reaction,

negative

Therefore, the reaction is spontaneous at any temperature. Hence, alternative is correct.


7. In a process, of heat is absorbed by a system and of work is done by the system. What

is the change in internal energy for the process?

Solution:

Given: heat is absorbed by a system=701 J

work is done by the system=394 J

According to the first law of thermodynamics,

Formula : change in internal energy for a process (

(Since heat is absorbed than sign of q is positive )

(Since work is done by the system so sign of w is negative )

Substituting the values in a formula we get

Hence, the change in internal energy for the given process is

8. The reaction of cyanamide, with dioxygen was carried out in a bomb calorimeter,

and was found to be at Calculate the enthalpy change for the

reaction at

Solution:

Formula : Enthalpy change for a reaction

Where,

change in internal energy =

change in the number of moles = (moles of product – moles of reactant )

For the given reaction,


moles

moles

And,

We know : =

Substituting the values in the expression of

9. Calculate the number of of heat necessary to raise the temperature of of aluminium

from to Molar heat capacity of is

Solution:

molar mass of aluminium = 27

From the expression of heat Where,

molar heat capacity=

mass of substance =

change in temperature=( )= 20

Moles of Al= weight/molecular weight =60/27

Substituting the values in the expression of


10. Calculate the enthalpy change on freezing of of water at to ice at

at

Solution:

Given : of water=1.0

Total enthalpy change involved in the transformation is the sum of the following changes:

(a) Energy change involved in the transformation of of water at to of

water at

(b) Energy change involved in the transformation of of water at to of ice at

(c) Energy change involved in the transformation of of ice at to of ice at

Hence, the enthalpy change involved in the transformation is

11. Enthalpy of combustion of carbon to is Calculate the heat released upon

formation of of from carbon and dioxygen gas.

Solution:


Given: Enthalpy of combustion of carbon to =

Weight of

Formation of from carbon and dioxygen gas can be represented as:

Heat released on formation of

The heat released on formation of

12. Enthalpies of formation of and are and

respectively. Find the value of for the reaction:

Solution:

Given: Enthalpies of formation of

=

Enthalpies of formation of =

for a reaction is defined as the difference between value of products and value of

reactants.


Substituting the values of for and from given data, we get:


Hence, the value of for the reaction is

13. Given

What is the standard enthalpy of formation of gas?

Solution:

Given :

Standard enthalpy of formation of a compound is the change in enthalpy that takes place during

the formation of of a substance in its standard form from its constituent elements in their

standard state.

Re-writing the given equation for of

Standard enthalpy of formation of

14. Calculate the standard enthalpy of formation of from the following data:

Solution:


The reaction that takes place during the formation of can be written as:

…(i)

(ii)

(iii)

(iv)

The reaction (i) can be obtained from the given reactions by following the algebraic calculations

as:

15. Calculate the enthalpy change for the process

and calculate the bond enthalpy of in

where is enthalpy of atomization

Solution:

The chemical equations implying to the given values of enthalpies are:

(i)

(ii)

(iii)

(iv)


The enthalpy change for the given process can be calculated using the

following algebraic calculations as:

in

so Bond enthalpy of bond in

16. For an isolated system, what will be

Solution:

Since

i.e. The Changes in internal energy (∆U) for an isolated system is zero So; it does not exchange

any energy in any form with the surroundings.

But , The entropy tends to increase in case, will be positive, i.e. greater than zero

of spontaneous reaction.

17. For the reaction at

and

At what temperature will the reaction become spontaneous, considering and to be constant

over the temperature range?

Solution:

From the expression,

Assuming the reaction at equilibrium, for the reaction would be:


For the reaction to be spontaneous, must be negative. Hence, for the given reaction to be

spontaneous, should be greater than

18. For the reaction,

what are the signs of and

Solution:

The given reaction ; the formation of chlorine molecule from chlorine atoms.

Here, bond formation is taking place. Therefore, energy is being released. Hence, is negative.

Also, two moles of atoms have more randomness than one mole of a molecule. Since spontaneity

is decreased, is negative for the given reaction.

19. For the reaction

and

Calculate for the reaction, and predict whether the reaction may occur spontaneously.

Solution:



mole

Substituting the value of in the expression of

=12977.572 J

Substituting the values of and in the expression of

Since for the reaction is positive so the reaction is non- spontaneously.

20. The equilibrium constant for a reaction is What will be the value of

Solution:

At equilibrium = 0 and Q =

Hance

for the reaction,


21. Comment on the thermodynamic stability of given

Solution:

The positive value of indicates that heat is absorbed during the formation of This

means that has higher energy than the reactants ( and ). Hence, is unstable.

The negative value of indicates that heat is evolved during the formation of from

and The product, is stabilized with minimum energy.

Hence, unstable changes to unstable

22. Calculate the entropy change in surroundings when of is formed under standard

conditions.

Solution:

It is given that of heat is evolved on the formation of of Thus, an

equal amount of heat will be absorbed by the surroundings.

Entropy change of the surroundings

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CBSE NCERT Solutions for Class 11 Chemistry Chapter 6...CBSE NCERT Solutions for Class 11 Chemistry Chapter 6 Back of Chapter Questions 1. Choose the correct answer. A thermodynamic