Class 11 Chemistry Thermodynamics Exam Questions

LEVEL-1

1. Can the absolute value of internal energy can be determined? Why or why not?

Ans: No, because it is the sum of different types of energies some of which cannot be determined.

2. Separate out the following into extensive and intensive: Volume, Temperature, Pressure, Boiling point, Free energy

Ans: Volume and free energy are extensive, other are intensive.

3. Under what condition ΔH becomes equal to ΔU?

Ans: ΔH=ΔU during a process which is carried out in a closed vessel (ΔV=0) or number of moles of gaseous products = number of moles of gaseous reactants or the reaction does not involve any gaseous reactant or products.

4. Water can be lifted into the water tank at the top of the house with the help of a pump. Then why is not considered to be spontaneous?

Ans: A spontaneous process should continue taking place by itself after initiation. But this is not so in the given case because water will go up so long as the pump is working.

5. For the process to occur under adiabatic conditions, the correct conditions is: (i) ΔT=0 (ii) Δp=0 (iii) q=0 (iv) w=0

Ans: (iii)

6. A reaction A + B→ C+ D +q 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

(iv) possible at any temperature

Ans: Here ΔH = - ve and ΔS= +ve. ΔG=ΔH-TΔS. For the reaction to be spotneous, ΔG should be –ve which will be so at any temperature i.e., option is (iv)

7. In a process, 701 J of heat is absorbed by a system and 394 J of work is done by the system. What is the change in the internal energy of the process?

Ans: Internal energy of the system increases by 307 J

8. Why endothermic reactions are favoured at high temperature?

Ans: If temperature is high, then TΔS will be much greater than ΔH in magnitude so that ΔG is highly negative.

9. A real crystal has more entropy than an ideal crystal.

Ans: A real crystal has some disorder due to presence of defects whereas ideal crystal has no disorder.

LEVEL-2

1. State the first law thermodynamics and derive a mathematical expression for it.

Ans: Definition and expression for the same i.e., ΔU= q+w where ΔU is change in internal energy, q is the heat given to the system and w is the work done on the system.

5. what is bond energy? Why is it called enthalpy of atomization?

Ans: Amount of energy required to dissociate one mole of bonds present between the atoms in gaseous molecules. As the molecules dissociate completely into atoms in the gaseous state, therefore bond energy of diatomic molecule is called enthalpy of atomization.

6. Why standard entropy of an elementary substance is not zero whereas standard enthalpy of formation is taken as zero?

Ans: Entropy is zero only at absolute zero. Enthalpy of formation is the heat change involved in the formation of one mole of the substance from its elements. An element formed from itself means no heat change,

2. Justify the statement: An exothermic reaction is always thermodynamically spontaneous.

Ans: As the reaction of the type accompanied by decrease of randomness, the heat released is absorbed by the surroundings so that the entropy of the surroundings increases to such an extent that ΔS_total is positive.

3. Justify the statement: The entropy of a substance increases on going from liquid to vapour state at any temperature.

Ans: The molecules in the vapour state have greater freedom of movement and hence greater randomness than those in the liquid state. Hence, entropy increases in going from liquid to vapour state.