﻿ Engineering Entrance Exam Question and Answers | Thermodynamics - Zigya

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# Thermodynamics

#### Multiple Choice Questions

1.

Which of the following is incorrect regarding the first law of thermodynamics?

• It introduces the concept of internal energy

• It introduces the concept of entropy

• It is not applicable of any cyclic process

• It is a restatement of principle of conversation of energy.

B.

It introduces the concept of entropy

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2.

Which one of the following statements is NOT true about the effect of an increase in temperature on the distribution of molecular speeds in a gas?

• The most probable speed increases

• The fraction of the molecules with the most probable speed increases

• The distribution becomes broader

• The area under the distribution curve remains the same as under the lower temperature

B.

The fraction of the molecules with the most probable speed increases

Most probable velocity increase and a fraction of a molecule possessing most probable velocity decreases.

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3.

Two thermally insulated vessels 1 and 2 are filled with air at temperatures (T1, T2), volume (V1, V2) and pressure (P1, P2) respectively. If the valve joining two vessels is opened, the temperature inside the vessel at equilibrium will be

• T1 + T2

• T1 + T2/2

C.

There will be no change in number of moles if the vessels are joined by the valve. Therefore, from gas equation

Now, according to Boyle's law (pressure = constant) P1 V1 + P2 V2 = P(V1 + V2 )

1103 Views

4.

If the bond dissociation energies of XY, X2 and Y2 (all diatomic molecules) are in the ratio of 1:1:0.5 and ∆f H for the formation of XY is -200 kJ mole-1. The bond dissociation energy of X2 will be

• 100 kJ mol-1

• 800 kJ mol-1

• 300 kJ mol-1

• 400 kJ mol-1

B.

800 kJ mol-1

XY→ X(g) + Y(g) ; ∆H +a kJ/ mole ............(i)
X2 → 2X; ∆H = +a kJ/mole........(ii)
Y2 → 2Y; ∆H =+0.5a kJ/mole.......(iii)

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5.

Consider the reaction: N2 +3H2 → 2NH3 carried out at constant temperature and pressure. If ∆H and ∆U are the enthalpy and internal energy changes for the reaction, which of the following expressions is true?

• ∆H = 0

• ∆H = ∆U

• ∆H < ∆U

• ∆H >∆U

C.

∆H < ∆U

∆H = ∆U + ∆nRT
∆n = -2
∆H = ∆U - 2RT
∆H < ∆U

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6.

The temperature-entropy diagram of a reversible engine cycle is given in the figure. Its efficiency is

• 1/2

• 1/4

• 1/3

• 2/3

C.

1/3

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7.

Which of the following statements is correct for any thermodynamic system?

• The internal energy changes in all processes.

• Internal energy and entropy are state functions.

• The change in entropy can never be zero.

• The work done in an adiabatic process is always zero.

B.

Internal energy and entropy are state functions.

In thermodynamic system, entropy and internal energy are state functions.

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8.

A system goes from A to B via two processes I and II as shown in the figure. If ∆U1 and ∆U2 are the changes in internal energies in the processes I and II respectively, the

• ∆U1 = ∆U2

• relation between ∆U1 and ∆U2 can not be determined

• ∆U2 > ∆U1

• ∆U2 < ∆U1

A.

∆U1 = ∆U2

Internal energy is state function

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9.

The enthalpies of combustion of carbon and carbon monoxide are -393.5 and -283 kJ mol-1 respectively. The enthalpy of formation of carbon monoxide per mole is

• 110.5 kJ

• -110.5 kJ

• -676.5 kJ

• 676.5 kJ

B.

-110.5 kJ

This equation (III) also represents formation of ore mole of CO and thus enthalpy change is the heat formation of CO (g)
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10.

The standard enthalpy of formation (∆fHo)  at 298 K for methane, CH4(g), is –74.8 kJ mol–1. The additional information required to determine the average energy for C – H bond formation would be

• the dissociation energy of H2 and enthalpy of sublimation of carbon

• latent heat of vapourization of methane

• the first four ionization energies of carbon and electron gain enthalpy of hydrogen

• the dissociation energy of hydrogen molecule, H2

A.

the dissociation energy of H2 and enthalpy of sublimation of carbon

304 Views