Chemistry · Chemical thermodynamics · chapter 5 of 9

Class 11 Chemistry Chapter 5: Thermodynamics MCQs with answers

10 multiple-choice questions on Thermodynamics, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
A sealed glass ampoule containing a reaction mixture is dipped in a water bath at 60 °C. Taking the ampoule and its contents as the system, it is
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Answer: (b) a closed system

Glass conducts heat, so energy passes between the bath and the mixture, but the seal stops any matter leaving or entering: a closed system.

Question 2
Which set contains only intensive properties?
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Answer: (d) molar volume, pressure, temperature

Pressure and temperature do not depend on the amount of matter, and a molar property such as molar volume is an extensive property divided by n, so it is independent of the amount. Enthalpy, heat capacity and internal energy are extensive.

Question 3
A gas in a cylinder is compressed by pushing the piston in, and during the compression it gives out heat to the surroundings. By the IUPAC sign convention
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Answer: (a) w is positive and q is negative

Work done on the system is positive; heat leaving the system for the surroundings is negative.

Question 4
An ideal gas expands isothermally from one bulb into a second, evacuated bulb when the stopcock between them is opened. Which of the following is not zero for this change?
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Answer: (c) ΔS of the gas

Expansion into vacuum is free expansion: pex = 0, so w = 0; Joule found q = 0, so ΔU = q + w = 0. The expansion is spontaneous and irreversible, so ΔStotal > 0; the surroundings receive no heat (ΔSsurr = 0), so the whole increase is in the gas, which has spread into a larger volume: ΔS of the gas > 0.

Question 5
A system is taken from state 1 to state 2 by two different routes. Which of the following can have different values for the two routes?
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Answer: (c) w

ΔU, ΔH and q + w depend only on the initial and final states. q and w separately depend on how the change is carried out.

Question 6
Which of the following processes has a negative ΔH?
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Answer: (d) freezing of water at 273 K

Melting, sublimation and vaporisation absorb heat (positive ΔH). Freezing is the reverse of melting and gives the same heat out to the surroundings, so ΔH is negative.

Question 7
Which of the following has a standard enthalpy of formation equal to zero at 298 K?
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Answer: (b) S(rhombic)

ΔfH⊖ is zero only for an element in its reference state, its most stable form at 25 °C and 1 bar. For sulphur that is rhombic sulphur; C(diamond) is +1.89 and Br₂(g) is +30.91 kJ mol⁻¹ (bromine's reference state is the liquid), and oxygen's reference state is O₂(g), not O₃.

Question 8
The entropy of which of the following approaches zero as the temperature approaches absolute zero?
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Answer: (a) a pure crystalline solid

The third law is confined to pure crystalline solids, which are perfectly ordered at 0 K; the entropy of solutions and supercooled liquids is not zero at absolute zero.

Question 9
A 200 g block of a metal absorbs 3.6 kJ of heat and its temperature rises by 20 K. The specific heat of the metal is
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Answer: (c) 0.90 J g⁻¹ K⁻¹

q = c × m × ΔT, so c = 3600 J / (200 g × 20 K) = 0.90 J g⁻¹ K⁻¹.

Question 10
On the enthalpy diagram of a reaction, the level of the products lies 50 kJ mol⁻¹ above the level of the reactants. The reaction
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Answer: (b) is endothermic, with ΔrH = +50 kJ mol⁻¹

Hp above Hr means ΔrH = Hp − Hr = +50 kJ mol⁻¹: heat is absorbed from the surroundings, an endothermic reaction.

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