State the second law of thermodynamics.

The second law of thermodynamics can be expressed in terms of entropy. It can be stated as “the entropy of an isolated system increases during a spontaneous process”. The total entropy change ( ∆ S total ) for the system and the surroundings of a spontaneous process is given by, ∆ S total = ∆ S system + ∆ S surrounding > 0 When a system is in equilibrium, the entropy is maximum and the change in entropy, ∆ S = 0 . Hence, entropy of a spontaneous process increases till it reaches a maximum and at equilibrium the change in entropy is zero.

The work done in ergs for the reversible expansion of one mole of an ideal gas from a volume of 10 litres to 20 litres at 25 ° C is

298 × 10 7 × 8 . 31 × 2 . 303 log 2 .

2 . 303 × 298 × 0 . 082 log 2 .

2 . 303 × 298 × 0 . 082 log 0 . 5 .

8 . 31 × 10 7 × 298 × 2 . 303 log 0 . 5 .

For a reaction taking place in a container in equilibrium with its surroundings, the effect of temperature on its equilibrium constant K in terms of change in entropy is described by:

with increase in temperature, the value of K for exothermic reaction decreases because favourable change in entropy of the surroundings decreases.

with increase in temperature, the value of K for exothermic reaction decreases because the entropy change of the system is positive.

with increase in temperature, the value of K for endothermic reaction increases because the entropy change of the system is negative.

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The enthalpy of combustion of propane $(C_{3} H_{8})$ gas in terms of given data is :

Bond energy $(kJ {mol}^{- 1})$

$ε_{C - H}$ $ε_{O = O}$ $ε_{C = O}$ $ε_{O - H}$ $ε_{C - C}$

$+ x_{1}$ $+ x_{2}$ $+ x_{3}$ $+ x_{4}$ $+ x_{5}$

Resonance energy of ${CO}_{2}$ is $- z kJ {mol}^{- 1}$ and ${ΔH}_{vaporization} [H_{2} O (ℓ)]$ is $y kJ {mol}^{- 1}$ .

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Important Questions on Thermodynamics

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 23

x_{1}, x_{2}

and

x_{3}

are enthalpies of

H - H, O = O

and

O - H

bonds respectively, and

x_{4}

is the enthalpy of vaporization of water, estimate the standard enthalpy of combustion of hydrogen.

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 24

${NH}_{3 (g)} + 3 C_{2 (g)} ⇌ {NCl}_{3 (g)} + 3 {HCl}_{(g)}; - {ΔH}_{1}$

$N_{2 (g)} + 3 H_{2 (g)} ⇌ 2 {NH}_{3 (g)}; - {ΔH}_{2}$

$H_{2 (g)}, C_{2 (g)} ⇌ 2 {HCl}_{(g)}; {ΔH}_{3}$

The heat of formation of ${NCl}_{3 (g)}$ in tents of ${ΔH}_{1}, {ΔH}_{2}$ and ${ΔH}_{3}$ is:

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 26

Ethanol can undergo decomposition to form two sets of products.

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If the molar ratio of $C_{2} H_{4}$ to ${CH}_{3} CHO$ is $8 : 1$ in a set of product gases, then the energy involved in the decomposition of $1$ mole of ethanol is:

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 27

Find the enthalpy of

S - S

bond from the following data.

$(i) C_{2} H_{5} - S - C_{2} H_{5} (g) {ΔH}_{f} ° = - 147.2 kJ / mol$

$(ii) C_{2} H_{5} - S - S - C_{2} H_{5} (g) {ΔH}_{f} ° = - 201.9 kJ / mol$

$(iii) S (g) {ΔH}_{f} ° = 222.8 kJ / mol$

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 28

From given following equations and $ΔH °$ values at $298 K,$ determine the enthalpy of reaction (in $kJ) : -$

$C_{2} H_{4} (g) + 6 F_{2} (g) ⟶ 2 {CF}_{4} (g) + 4 HF (g); {ΔH}^{o} =$ ?

$H_{2} (g) + F_{2} (g) ⟶ 2 HF (g); {ΔH}_{1} ° = - 537 kJ$

$C_{(s)} + 2 F_{2} (g) ⟶ {CF}_{4} (g); {ΔH}_{2} ° = - 680 kJ$

$2 C_{(s)} + 2 H_{2} (g) ⟶ C_{2} H_{4} (g); {ΔH}_{3}^{0} = 52 kJ$

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 29

Animals operate under conditions of constant pressure and most of the process that maintain life are isothermal (in a broad sense). How much energy is available for sustaining this type of muscular and nervous activity from the combustion of

1

mol of glucose molecules under standard conditions at

37 ° C

(blood temperature) ? The entropy change is

+ 182.4 {JK}^{- 1}

for the reaction stated above.

{ΔH}_{combustion} [glucose] = - 2808 kJ

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 31

Calculate the free energy change at

298 K

for the reaction;

{Br}_{2} (l) + {Cl}_{2} (g) ⟶ 2 BrCl (g) .

For the reaction

ΔH ° = 29.3 kJ &

the entropies of

{Br}_{2} (l), {Cl}_{2} (g) & BrCl (g)

298 K

are

152.3, 223.0, 239.7 J

{mol}^{- 1} K^{- 1}

respectively.

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Alpha Question Bank for Engineering: Chemistry>Chapter 4 - Thermodynamics>Exercise-4>Q 32

One gram sample of oxygen undergoes free expansion in insulated container from

0.75 L

3.0 L

298 K .

Calculate

Δ S,

q, w, ΔH

and

Δ E

$ε_{C - H}$	$ε_{O = O}$	$ε_{C = O}$	$ε_{O - H}$	$ε_{C - C}$
$+ x_{1}$	$+ x_{2}$	$+ x_{3}$	$+ x_{4}$	$+ x_{5}$

The enthalpy of combustion of propane C3H8 gas in terms of given data is : Bond energy kJ mol-1 εC-H εO=O εC=O εO-H εC-C +x1 +x2 +x3 +x4 +x5 Resonance energy of CO2 is -z kJ mol-1 and ΔHvaporization H2Oℓ is y kJ mol-1.

Important Questions on Thermodynamics

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