NEET Previous Year Questions on Thermodynamics

Q 1 :
In which of the following processes entropy increases?

A. A liquid evaporates to vapour.

B. Temperature of a crystalline solid lowered from 130 K to 0 K.

C. ${2NaHCO}_{3 (s)} \to {Na}_{2} {CO}_{3 (s)} + {CO}_{2 (g)} + H_{2} O_{(g)}$

D. ${Cl}_{2 (g)} \to 2 {Cl}_{(g)}$

Choose the correct answer from the options given below: [2024]

A and C
A, B and D
A, C and D
C and D

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(3)

A. Molecules in gaseous phase have greater freedom of movement, i.e.,have greater randomness than those in liquid phase, hence entropy increases.

B. On decreasing temperature of a solid, entropy decreases.

C. As number of gaseous products increases, randomness increases.

D. The products have larger number of gaseous particles, hence entropy increases.

Q 2 :
For irreversible expansion of an ideal gas under isothermal condition, the correct option is [2021]

$Δ U \neq 0, Δ S_{total} = 0$
$Δ U = 0, Δ S_{total} = 0$
$Δ U \neq 0, Δ S_{total} \neq 0$
$Δ U = 0, Δ S_{total} \neq 0$

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(4)

For reversible and irreversible expansion for an ideal gas under isothermal condition, $Δ U = 0$ , but $Δ S_{total}$ i.e., $Δ S_{sys} + Δ S_{surr}$ is not zero for irreversible process.

Q 3 :
For the reaction, $2 {Cl}_{(g)} \to {Cl}_{2 (g)}$ , the correct option is [2020]

$Δ_{r} H > 0$ and $Δ_{r} S > 0$
$Δ_{r} H > 0$ and $Δ_{r} S < 0$
$Δ_{r} H < 0$ and $Δ_{r} S > 0$
$Δ_{r} H < 0$ and $Δ_{r} S < 0$

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(4)

In the reaction, $2 {Cl}_{(g)} \to {Cl}_{2 (g)}$ , the randomness decreases as 2 moles of ${Cl}_{(g)}$ are converted to 1 mole of ${Cl}_{2 (g)}$ ; thus, $Δ_{r} S < 0$ .
And this is an exothermic reaction, thus, $Δ_{r} H < 0$ .

Q 4 :
In which case change in entropy is negative? [2019]

$2 H_{(g)} \to H_{2 (g)}$
Evaporation of water
Expansion of a gas at constant temperature
Sublimation of solid to gas

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If $Δ n_{g} < 0$ then $Δ S < 0$

Q 5 :
For a given reaction, $Δ H = 35.5$ $kJ$ ${mol}^{- 1}$ and $Δ S = 83.6$ $J$ $K^{- 1} {mol}^{- 1}$ . The reaction is spontaneous at (Assume that $Δ H$ and $Δ S$ do not vary with temperature.) [2017]

$T > 425$ $K$
all temperatures
$T > 298$ $K$
$T < 425$ $K$

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For a spontaneous reaction, $Δ G < 0$ i.e., $Δ H - T Δ S < 0$

$T > \frac{Δ H}{Δ S}$ ; $T > (\frac{35.5 \times 1000}{83.6}) = 424.6 \approx 425$ $K$

$∴$ $T > 425$ $K$

Q 6 :
For a sample of perfect gas when its pressure is changed isothermally from $p_{i}$ to $p_{f}$ , the entropy change is given by [2016]

$Δ S = n R \ln (\frac{p_{f}}{p_{i}})$
$Δ S = n R \ln (\frac{p_{i}}{p_{f}})$
$Δ S = n R T \ln (\frac{p_{f}}{p_{i}})$
$Δ S = R T \ln (\frac{p_{i}}{p_{f}})$

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(2)

For an ideal gas undergoing reversible expansion, when temperature changes from $T_{i}$ to $T_{f}$ and pressure changes from $p_{i}$ to $p_{f}$ ,

$Δ S = n C_{p} \ln \frac{T_{f}}{T_{i}} + n R \ln \frac{p_{i}}{p_{f}}$

For an isothermal process, $T_{i} = T_{f}$ so, $\ln$ $1 = 0$

$∴$ $Δ S = n R \ln \frac{p_{i}}{p_{f}}$

Q 7 :
Consider the following liquid-vapour equilibrium.

$Liquid ⇌ Vapour$

Which of the following relations is correct? [2016]

$\frac{d \ln P}{d T^{2}} = \frac{- Δ H_{v}}{T^{2}}$
$\frac{d \ln P}{d T} = \frac{Δ H_{v}}{R T^{2}}$
$\frac{d \ln G}{d T^{2}} = \frac{Δ H_{v}}{R T^{2}}$
$\frac{d \ln P}{d T} = \frac{- Δ H_{v}}{R T}$

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This is Clausius--Clapeyron equation.

Q 8 :
Which of the following statements is correct for the spontaneous adsorption of a gas? [2014]

$Δ S$ is negative and, therefore $Δ H$ should be highly positive.
$Δ S$ is negative and therefore, $Δ H$ should be highly negative.
$Δ S$ is positive and therefore, $Δ H$ should be negative.
$Δ S$ is positive and therefore, $Δ H$ should also be highly positive.

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(2)

Using Gibbs'-Helmholtz equation,

$Δ G = Δ H - T Δ S$

During adsorption of a gas, entropy decreases i.e., $Δ S < 0$

For spontaneous adsorption, $Δ G$ should be negative, which is possible when $Δ H$ is highly negative.

Q 9 :
For the reaction, $X_{2} O_{4 (l)} ⟶ 2 X O_{2 (g)}$

$Δ U = 2.1$ $kcal$ , $Δ S = 20$ cal $K^{- 1}$ at 300 K

Hence, $Δ G$ is [2014]

2.7 kcal
- 2.7 kcal
9.3 kcal
- 9.3 kcal

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(2)

$Δ H = Δ U + Δ n_{g} R T$

Given, $Δ U = 2.1$ kcal, $Δ n_{g} = 2$ ,

$R = 2 \times 10^{- 3}$ kcal, T = 300 K

$∴$ $Δ H = 2.1 + 2 \times 2 \times 10^{- 3} \times 300 = 3.3$ kcal

Again, $Δ G = Δ H - T Δ S$

Given, $Δ S = 20 \times 10^{- 3}$ kcal $K^{- 1}$

On putting the values of $Δ H$ and $Δ S$ in the equation, we get

$Δ G = 3.3 - 300 \times 20 \times 10^{- 3} = 3.3 - 6 \times 10^{3} \times 10^{- 3} = - 2.7$ kcal

Topic Question Set

Q 2 :
For irreversible expansion of an ideal gas under isothermal condition, the correct option is [2021]

Q 3 :
For the reaction, $2 {Cl}_{(g)} \to {Cl}_{2 (g)}$ , the correct option is [2020]

Q 4 :
In which case change in entropy is negative? [2019]

Q 5 :
For a given reaction, $Δ H = 35.5$ $kJ$ ${mol}^{- 1}$ and $Δ S = 83.6$ $J$ $K^{- 1} {mol}^{- 1}$ . The reaction is spontaneous at (Assume that $Δ H$ and $Δ S$ do not vary with temperature.) [2017]

Q 6 :
For a sample of perfect gas when its pressure is changed isothermally from $p_{i}$ to $p_{f}$ , the entropy change is given by [2016]

Q 7 :
Consider the following liquid-vapour equilibrium.

$Liquid ⇌ Vapour$

Which of the following relations is correct? [2016]

Q 8 :
Which of the following statements is correct for the spontaneous adsorption of a gas? [2014]

Q 9 :
For the reaction, $X_{2} O_{4 (l)} ⟶ 2 X O_{2 (g)}$

$Δ U = 2.1$ $kcal$ , $Δ S = 20$ cal $K^{- 1}$ at 300 K

Hence, $Δ G$ is [2014]

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Topic Question Set

Q 1 : In which of the following processes entropy increases? A. A liquid evaporates to vapour. B. Temperature of a crystalline solid lowered from 130 K to 0 K. C. 2NaHCO3(s)→Na2CO3(s)+CO2(g)+H2O(g) D. Cl2(g)→2Cl(g) Choose the correct answer from the options given below: [2024]

Q 2 : For irreversible expansion of an ideal gas under isothermal condition, the correct option is [2021]

Q 3 : For the reaction, 2Cl(g)→Cl2(g), the correct option is [2020]

Q 4 : In which case change in entropy is negative? [2019]

Q 5 : For a given reaction, ΔH=35.5kJmol-1 and ΔS=83.6JK-1mol-1. The reaction is spontaneous at (Assume that ΔH and ΔS do not vary with temperature.) [2017]

Q 6 : For a sample of perfect gas when its pressure is changed isothermally from pi to pf, the entropy change is given by [2016]

Q 7 : Consider the following liquid-vapour equilibrium. Liquid⇌ Vapour Which of the following relations is correct? [2016]

Q 8 : Which of the following statements is correct for the spontaneous adsorption of a gas? [2014]

Q 9 : For the reaction, X2O4(l)⟶2XO2(g) ΔU=2.1 kcal, ΔS=20 cal K-1 at 300 K Hence, ΔG is [2014]

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Q 2 :
For irreversible expansion of an ideal gas under isothermal condition, the correct option is [2021]

Q 3 :
For the reaction, $2 {Cl}_{(g)} \to {Cl}_{2 (g)}$ , the correct option is [2020]

Q 4 :
In which case change in entropy is negative? [2019]

Q 5 :
For a given reaction, $Δ H = 35.5$ $kJ$ ${mol}^{- 1}$ and $Δ S = 83.6$ $J$ $K^{- 1} {mol}^{- 1}$ . The reaction is spontaneous at (Assume that $Δ H$ and $Δ S$ do not vary with temperature.) [2017]

Q 6 :
For a sample of perfect gas when its pressure is changed isothermally from $p_{i}$ to $p_{f}$ , the entropy change is given by [2016]

Q 7 :
Consider the following liquid-vapour equilibrium.

$Liquid ⇌ Vapour$

Which of the following relations is correct? [2016]

Q 8 :
Which of the following statements is correct for the spontaneous adsorption of a gas? [2014]

Q 9 :
For the reaction, $X_{2} O_{4 (l)} ⟶ 2 X O_{2 (g)}$

$Δ U = 2.1$ $kcal$ , $Δ S = 20$ cal $K^{- 1}$ at 300 K

Hence, $Δ G$ is [2014]