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Chemistry MCQs for NEET — Practice Questions with Answers

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The spontaneous flow of heat is always

From low to high pressure

From high to high pressure

Unidirectional from lower temperature to higher temperature

Unidirectional from the higher to lower temperature

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Explanation

Heat is always flow from the higher to lower temperature.

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Mixing of non-reacting gases is generally accompanied by

Decrease in entropy

Increase in entropy

Change in enthalpy

Change in free energy

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Explanation

Mixing of non-reacting gases increases randomness and so increase entropy.

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An irreversible process occuring isothermally in an isolated system leads to

Zero entropy

An increase in the total entropy of the system

A decrease in the total entropy of the system

None of these

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Explanation

Entropy of the system increases as the process occur irreversibly and isothermally in an isolated system.

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The entropy values (in JK^-1 mol^-1) of H_2(g) = 130.6, Cl_2(g) = 223.0 and HCl_(g) = 186.7 at 298 K and 1 atm pressure. Then entropy change for the reaction $H_{2 (g)} + {Cl}_{2 (g)} \to 2 {HCl}_{(g)}$ is [BHU 2005]

+ 540.3

+ 727.3

– 166.9

+ 19.8

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Explanation

$Δ S^{o} = 2 S_{H C l}^{o} - (S_{H_{2}}^{o} + S_{C l_{2}}^{o})$

$= 2 \times 186 .7 - (130 .6 + 223 .0) = 19 .8 J K^{- 1} m o l^{- 1}$

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The ΔS for the vaporisation of 1 mol of water is 88.3 J/mole K. The value of ΔS for the condensation of 1 mol of vapour will be

88.3 J/mol K

(88.3)² J/mol K

– 88.3 J/mol K

$\frac{1}{88 .3}$ J/mol K

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Explanation

For reverse reaction sign will be change.

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The occurrence of a reaction is impossible if [AIIMS 1982, 91; MP PET 1997; MP PMT 1999; DPMT 2002]

ΔH is +ve ; ΔS is also + ve but ΔH < TΔS

ΔH is – ve ; ΔS is also – ve but ΔH > TΔS

ΔH is – ve ; ΔS is + ve

ΔH is + ve ; ΔS is – ve

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Explanation

+ve ΔH and –ve ΔS both oppose the reaction.

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When enthalpy and entropy change for a chemical reaction are –2.5 × 10³ cal and 7.4 cal deg^–1 respectively. Predict the reaction at 298 K is [AFMC 1998; MH CET 1999; CBSE PMT 2000]

Spontaneous

Reversible

Irreversible

Non-spontaneous

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Explanation

When ΔH = –ve, ΔS = +ve and ΔG = –ve than reaction is spontaneous.

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Which of the following is true for the reaction $H_{2} O (l) ⇌_{}^{} H_{2} O (g)$ at 100°C and 1 atmosphere [KCET 1991; AIIMS 1996]

ΔE = 0

ΔH = 0

ΔH = ΔE

ΔH = TΔS

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Explanation

At equilibrium, ΔG = 0

Hence 0 = ΔH – TΔS or ΔH = TΔS.

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The enthalpy of vapourization water is 386 kJ. What is the entropy of water [BHU 1997]

0.5 kJ

1.03 kJ

1.5 kJ

22.05 kJ

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Explanation

$∆ S = ∆ H_{v a p \div} T ∆ S = 386 \times 1000 \div 373 = 1.03 K J$

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Identify the correct statement regarding entropy [CBSE PMT 1998; BHU 2001]

At 0°C, the entropy of a perfectly crystalline substance is taken to be zero

At absolute zero of temperature, the entropy of a perfectly crystalline substance is +ve

At absolute zero of temperature, the entropy of all crystalline substances is taken to be zero

At absolute zero of temperature, the entropy of a perfectly crystalline substance is taken to be zero

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Explanation

This is the statement of third law of thermodynamics.

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