Thermodynamics Test

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Thermodynamics Test - 82

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Question 1
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$$\Delta U = q + w$$, is mathematical expression for

A
first law of thermodynamics

B
second law of thermodynamics

C
third law of thrmodynamics

D
zeroth law of thermodynamics

Solution

First law of thermodynamics is represented mathematically as $$\Delta U = q + w ;$$ where, $$\Delta U $$ is change in internal energy, q is the heat absorbed or released and w is work done.
Question 2
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A reaction proceeds through two paths I and II to convert $$X \rightarrow Z$$
What is the correct relationship between $$Q, Q_{1}$$ and $$Q_{2}$$?

A
$$Q = Q_{1} \times Q_{2}$$

B
$$Q = Q_{1} + Q_{2}$$

C
$$Q = Q_{1} - Q_{2}$$

D
$$Q = Q_{1} / Q_{2}$$

Solution

Hess's Law of Constant Heat Summation (or just Hess's Law) states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes.

So, $$Q = Q_{1}+Q_{2}$$
Question 3
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A gaseous system is initially characterised by 500 mL volume and 1 atm pressure at 298 K. This system is allowed to do work as
(i) In isobaric conditions it expands to 800 mL resulting a decrease in pressure and temperature to 0.6 atm and 273 K respectively.
(ii) In adiabatic conditions it is allowed to expand upto 800 mL and results a decrease in pressure and temperature to 0.6 atm and 273 K respectively.

If Glibbs energy change in (i) $$\Delta G_{a}$$ and in is $$\Delta G_{b}$$ then what will be the ratio of $$\dfrac{\Delta G_{a}}{\Delta G_{b}}?$$

A
$$0$$

B
$$1$$

C
between $$0-1$$

D
$$> 1$$

Solution

$$\dfrac{\Delta G_{a}}{\Delta G_{b}}=1$$ as Gibbs energy is a state function and initial and final states are same in (i) and (ii)

Option B is correct.
Question 4
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Read the following statements regarding the spontaneity of a process and mark the appropriate choice.
(i) When enthalpy factor is absent then randomness factor decides spontaneity of a process.
(ii) When randomness factor is absent then enthalpy factor decides spontaneity of a process.
(iii) When both the factors take place simultaneously the magnitude of both factors decides spontaneity of a process.

A
Statements (i) and (ii) are correct and (iii) is incorrect.

B
Statements (iii) and (i) are correct and (ii) is incorrect.

C
Statements (i), (ii) and (iii) are correct.

D
Statements (i), (ii) and (iii) are incorrect.

Solution

We know,

$$\Delta G=\Delta H-T\Delta S$$

Now,

1. $$\Delta G=\Delta H$$ when $$\Delta S=0.$$

2. $$\Delta G=T\Delta S$$ when $$\Delta H=0.$$

3. $$\Delta G=\Delta H-T\Delta S$$ when $$\Delta H\neq0\neq \Delta S$$
Question 5
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Which thermochemical process is shown by the following figure?

A
Standard enthalpy of a reaction

B
Born - Haber cycle of lattice enthalpy

C
Hess's law of constant heat summation

D
Standard enthalpy of a solution .

Solution

The thermochemical process shown by the figure is: Hess's law of constant heat summation.

1. $$ X \overset{\Delta H}{\rightarrow} Y $$

2. $$X\overset{\Delta H_{1}}{\rightarrow} P\overset{\Delta H_{2}}{\rightarrow} Q\overset{\Delta H_{3}}{\rightarrow} Y $$

$$\Delta H = \Delta H_{1} + \Delta H_{2} + \Delta H_{3} $$
Question 6
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At dynamic equilibrium, the reactions on both sides occur at the same rate and the mass on both sides of the equilibrium does not undergo any change. This condition can be achieved only when the value of $$\Delta G$$ is:

A
$$-1$$

B
$$+1$$

C
$$+2$$

D
$$0$$

Solution
Question 7
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Which of the following processes is a non-spontaneous process?

A
Dissolution of salt or sugar in water.

B
Mixing of diferent gases through diffusion.

C
Precipitation of copper when zinc rod is dipped in aqueous solution of copper sulphate.

D
Flow of heat from a cold body to a hot body in contact.

Solution

Heat cannot flow from a cold body to a hot body. Heat from a cold body to a hot body in contact spontaneously for the conducting material.
Question 8
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Which of the following statements is not correct?

A
For a spontaneous process, $$\Delta G$$ must be negative.

B
Enthalpy, entropy, free energy etc. are state variables.

C
A spontaneous process is reversible in nature.

D
Total of all possible kinds of energy of a system is called its internal energy.

Solution

Spontaneous processes are irreversible because they can be reversed only by taking a different path to get back to their original state. A reversible process can take the same path to return to its original state. An irreversible process must take a different path to get back to its original state.
Question 9
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For the reaction given below the values of standard Gibbs free energy of formation at $$298 K$$ are given. What is the nature of the reaction?

$$I_{2} + H_{2}S \rightarrow 2HI + S$$

$$\Delta G^{o}_{f} (HI) = 1.8\ kJ mol^{-1}, \Delta G^{o}_{f} (H_{2}S) = 33.8\ kJ mol^{-1}$$

A
Non- spontaneous in forward direction.

B
Spontaneous in forward direction.

C
Spontaneous in backward direction.

D
Non- spontaneous in both forward and backward directions.

Solution

$$I_{2}+H_{2}S\rightarrow 2HI+S$$

$$\Delta G^{o}=\sum G^{o}_{f\ (products)}-\sum G^{o}_{f\ (Reactants)}$$

$$\Delta G^{o}=(2\times 1.8+0)-(0+33.8)=-30.2\ kJ$$

Hence, the reaction is spontaneous in the forward direction.
Question 10
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Which of the following conditions will always lead to non-spontaneous change?

A
$$\Delta H$$ and $$\Delta S$$ both +ve

B
$$\Delta H$$ is -ve and $$\Delta S$$ is +ve

C
$$\Delta H$$ and $$\Delta S$$ both -ve

D
$$\Delta H$$ is +ve and $$\Delta S$$ is -ve

Solution