chemistry - thermodynamics

Class 11 - Chemistry

Thermodynamics

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Questions and Answers


Thermodynamics - Questions
1.

Predict the sign of  âˆ†S for the following reaction  heat
CaCO3 (s) ———> CaO(s) + CO2(g)


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2.

State Hess’s law.


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3.

What is the enthalpy change for an adiabatic process?


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4.

What do you mean by entropy?


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5.

Give a relation between entropy change and heat absorbed or evolved for a reversible reaction occurring at temperature T.


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6.

What is the condition for spontaneity in terms of free energy change?


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7.

What is an adiabatic process?


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8.

Define extensive properties.


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9.

How are internal energy change, free energy change and entropy change are related to one another?


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10.

How is entropy of a substance related to temperature?


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11.

Define intensive properties.


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12.

What is Gibbs Helmholtz equation?


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13.

What are the units of entropy?


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14.

What is a spontaneous change? Give one example.


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15.

When liquid benzene is oxidised at constant pressure at 300 K, the change in enthalpy is -3728 kJ. What is the change in internal energy at the same temperature?


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16.

The enthalpy of formation of methane at constant pressure and 300 K is – 78.84 kJ. What will be the enthalpy of formation at constant volume?


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17.

Calculate the enthalpy change for the reaction: H2(g) + Cl2(g) ————-> 2HCl(g). Given that  bond energies ofH-H, Cl- Cl and H-Cl bonds are 433, 244 and 431 kj mol-1  respectively.


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18.

The bond enthalpy of H2(g) is 436  kj mol-1and that of N2 (g) is 941.3  kj mol-1. Calculate the average bond enthalpy of an N-H bond in ammonia. Given: ∆H (NH3) = -46 kj mol-1


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19.

When two moles of C2H6(g) are burnt, 3129 kj of heat is liberated. Calculate the heat of formation of C2H6(g). ∆fH for  C02(g) and  H20(l) are-393.5 and -286 kj mol-1  respectively.


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22.

1 g of graphite is burnt in a bomb calorimeter in excess of oxygen at 298 K and 1 atmospheric pressure according to the equation C(graphite) + 02 (g) —> C02 (g) During the reaction, temperature rises from 298 K to 299 K. If the heat capacity of the bomb calorimeter is 20.7 kJ/K, what is the enthalpy change for the above reaction at 298 K and 1 atm?


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24.

Define the following:
(i) First law of thermodynamics.
(ii) Standard enthalpy of formation.


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26.

Give reason for the following:
(a)Neither q nor w is a state function but q + w is a state function.
(b)A real crystal has more entropy than an ideal crystal.


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27.

(a)What is a spontaneous process? Mention the conditions for a reaction to be spontaneous at constant temperature and pressure.
(b) Discuss the effect of temperature on the spontaneity of an exothermic reaction.


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28.

Predict in which of the following, entropy increases/decreases.
(i) A liquid crystallizes into a solid
(ii) Temperature of a crystallize solid is raised from OK to 115 K
(iii) 2NaHCO3 (s) ———-> Na2 C03 (s) + C02 (g) + H2O (g)
(iv) H2(g)——>2H(g)


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29.

Why standard entropy of an elementary substance is not zero whereas standard enthalpy of formation is taken as zero?
Under what conditions will the reaction occur, if
(i) both ∆H and ∆S are positive
(ii) both ∆H and ∆S are negative


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30.

(a) What is bond energy? Why is it called enthalpy of atomisation?
(b) Calculate the bond energy of C-H bond, given that the heat of formation of CH4, heat of sublimation of carbon and heat of dissociation of H2  are – 74.8, + 719.6, 435.4 kj mol-1 respectively.


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31.

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


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33.

Many thermodynamically feasible reactions do not occur under ordinary conditions. Why?


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34.

During complete combustion of one mole of butane, 2658 kJ of heat is released. The thermochemical reaction for above change is
ncert-exemplar-problems-class-11-chemistry-chapter-6-thermodynamics-1


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35.

In an adiabatic process, no transfer-of heat takes place between system and surroundings. Choose the correct option for free expansion of an ideal gas under adiabatic condition from the following.

ncert-exemplar-problems-class-11-chemistry-chapter-6-thermodynamics-4


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36.

The pressure-volume work for an ideal gas can be calculated by using the expression
ncert-exemplar-problems-class-11-chemistry-chapter-6-thermodynamics-5

The work can also be calculated from the pV

plot by using the area under curve within the specified limits. When an ideal gas is compressed (a) reversibly or (b) irreversibly from Vi to Vf, choose the correct option.
(a) w (reversible) = w (irreversible)
(b) w (reversible) < w (irreversible)
(c) w (reversible) > w (irreversible)
(d) w (reversible) = w (irreversible) + pex. ∆V


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37.

The entropy change can be calculated by using the expression ∆S = q rev / T.  When water freezes in a glass beaker, choose the correct statement amongst the following:

When water freezes in a glass beaker, choose the correct statement amongst the following:

(a) ∆S(system) decreases but ∆S(surroundings) remains the same.
(b) ∆S(system) increases but ∆S(surroundings) decreases.
(C) ∆S(system) decreases but ∆S(surroundmgs)  increases.
(d) ∆S(system) decreases but ∆S(surroundings) also decreases.


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38.

Consider the reactions given below. On the basis of these reactions find out which of the algebraic relations given in options (a) to (d) is correct?
(i) C(g) + 4H(g) → CH4(g); ∆rH= kJ mol-1
(ii) C(graphite, s) + 2H2(g) → CH4(g); ∆rH = y kJ mol 1
(a) x = y                                   (b) x = 2y                     (c)x >y         (d)x< y


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39.

The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound
(a) is always negative
(b) is always positive
(c) may be positive or negative
(d) is never negative


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40.

Enthalpy of sublimation of a substance is equal to
(a) enthalpy of fusion + enthalpy of vapourisation
(b) enthalpy of fusion
(c) enthalpy of vapourisation
(d) twice the enthalpy of vapourisation.


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41.

Which of the following is not correct?
(a) ∆G is zero for a reversible reaction.
(b) ∆G is positive for a spontaneous reaction
(c) ∆G is negative tor a spontaneous reaction
(d) ∆G is positive for a non-spontaneous reaction.


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42.

Thermodynamics mainly deals with
(a) interrelation of various forms of energy and their transformation front one from  to another.
(b) energy changes in the processes which depend only on initial and final states of the microscopic system containing a few molecules.
(c) how and at what rate these energy transformations are carried out.
(d) the system in equilibrium state or moving from one equilibrium state to another equilibrium state.


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43.

In an exothermic reaction, heat is evolved, and system loses heat to the surroundings. For such system
(a) qP will be negative                                                              
(b) ∆γHwill be negative
(c) qp will be positive                                                                
(d) ∆γHwill be positive.


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44.

The spontaneity means, having the potential to proceed without the assistance of external agency. The processes which occur spontaneously are
(a) flow of heat from colder to warmer body.
(b) gas in a container contracting into one comer.
(c) gas expanding to fill the available volume.
(d) burning carbon in oxygen to give carbon dioxide.


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45.

For an ideal gas. the work of reversible expansion under isothermal condition 1.0 mol of an ideal gas is expanded isothermally and reversibly to ten times of its original volume, in two separate experiments. The expansion is carried out at 300 K and at 600 K respectively. Choose the correct option.
can be calculated by using expression w = -nRT In Vf / Vi A sample containing
(a) Work done at 600 K is 20 times the work done at 300 K.
(b) Work done at 300 K is twice the work done at 600 K
(c) Work done at 600 K is twice the work done at 300 K.
(d) ∆U= 0 in both cases.


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46.

Consider the following reaction between zinc and oxygen and choose the correct options out of the options given below:
2Zn(s) + 02(g) → 2ZnO(s); ∆H=-693.8 kJ mol-1
(i) The enthalpy of two moles ZnO is less than the total enthalpy of two moles of Zn and one mole of oxygen by 693.8 kJ.
(ii) The enthalpy of two moles of ZnO is more than the total enthalpy of two moles of Zn and one mole of oxygen by 693.8 kJ.
(iii) 8 kJ mol -1 energy is evolved in the reaction.
(iv) 693.8 kJ mol-1 energy is absorbed in the reaction.


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47.

18.0 g of water completely vapourises at 100 °C and 1 bar pressure and the enthalpy change in the process is
40.79 kJ mol-1. What will be the enthalpy change for vapourising two moles of water under the same conditions? What is the standard enthalpy of vapourisation for water?


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48.

One mole of acetone requires less heat to vapourise than 1 mol of water. Which of the two liquids has higher enthalpy of vapourisation?


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49.

Enthalpy is an extensive property. In general, if enthalpy of an overall reaction A→B along one route is ∆rH and ∆rH1, ∆rH2, ∆rH3 …. represent enthalpies of intermediate reactions leading to product B. What will be the relation between ∆rH for overall reaction and ∆rH1, ∆rH2….. etc. for intermediate reactions.


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50.

The enthalpy of atomisation for the reaction CH4(g) → C(g) + 4H(g) is 1665 kJ mol-1. What is the bond energy of C – H bond?


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