Enthalpy Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. As per the law of thermodynamics, during an interaction, energy can change from one form to another, but the total amount of energy

1. Decreases
2. Increases
3. Becomes Zero Always
4. Remains Constant

Ans. The correct answer is d. Remains constant

Explanation: When energy can transform from one form to another, the total energy remains constant.

Ques. How is enthalpy represented?

1. H = Energy – Mass
2. H = Energy + Mass
3. H = Energy/Mass
4. H = Energy × Mass

Ans. The correct answer is c. H = Energy/Mass

Explanation: Enthalpy is represented as H = Energy/Mass.

Ques. The SI unit of enthalpy is

1. Radians
2. Watt
3. Kilogram
4. Joule

Ans. The correct answer is d. Joule

Explanation: Enthalpy refers to the energy that is stored in a thermodynamic system. Therefore, its SI unit is Joule.

Ques. Enthalpy of vaporization is expressed in units of

1. Joules per mole
2. Radians per mole
3. Watts per second
4. Kilowatts

Ans. The correct answer is a. Joules per mole

Explanation: The amount of energy required for converting an amount of a liquid substance into a gas is called the enthalpy of vaporization, often known as the heat of vaporization or heat of evaporation. It is expressed in Joules per mole.

Ques. Which among the following is a secondary dimension?

1. Time
2. Mass
3. Velocity
4. Length

Ans. The correct answer is c. Velocity

Explanation: Length, Mass, and Time are the primary dimensions whereas velocity is the secondary dimension.

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Short Answers Questions [2 Marks Questions]

Ques. Define enthalpy.

Ans. Enthalpy is the total internal energy. It is the energy needed to establish a system with pressure P and volume V. Enthalpy, thus, is the system's overall energy content. Understanding various thermodynamic systems is now simpler and easier by means of the idea of enthalpy.

Ques. What is the formula to find enthalpy?

Ans. The formula of enthalpy is given by

H = U + PV

Where

• H = Enthalpy of the system
• U = Internal energy of the system
• P = Pressure of the system
• V = Volume of the system

Ques. What is the difference between enthalpy and entropy?

Ans. The value of entropy represents the randomness of molecules, whereas enthalpy is the sum of internal energy and energy flow. 

Only under controlled conditions does the value of entropy hold true, but the value of enthalpy holds true under all situations.

Ques. Define the internal energy of a system.

Ans. Internal energy is the sum of the kinetic energies and potential energies of all the constituents molecules of the system. It depends on the state of the system.

Also read:

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Long Answers Questions [3 Marks Questions]

Ques. For a gaseous system find the change in internal energy if the heat applied to the system is 50 J and work done by the system is 16 J.

Ans. Given

• Heat is applied to the system, ΔQ = 50 J
• Work done by the system, ΔW = 16 J

Let ΔU be the change in internal energy of the system, then according to the first law of thermodynamics

ΔQ = ΔU + ΔW

⇒ ΔU = ΔQ - ΔW

⇒ ΔU = 50 - 16

⇒ ΔU = 34 J

Ques. The latent heat of vaporization of water is 2240 J/g. If the work done in the process of expansion of 1 g is 168 J, then what is the increase in internal energy?

Ans. Given

• The latent heat of vaporization, ΔH = 2240 J/g
• Work done in the process of expansion of 1 g is, ΔW = 168 J

Let ΔU be the change in internal energy of the system, then according to the first law of thermodynamics

ΔQ = ΔU + ΔW

⇒ ΔU = ΔQ - ΔW

⇒ ΔU = 2240 - 168

⇒ ΔU = 2072 J

Ques. The specific heat of hydrogen gas at constant pressure is CP = 3.4 x 103 cal/kg℃ and at constant volume is CV = 2.4 x 103 cal/kg℃. If one-kilogram hydrogen gas is heated from 10 ℃ to 20 ℃ at constant pressure, then what is the external work done on the gas to maintain it at constant pressure?

Ans. From the first law of thermodynamics,

ΔQ = ΔU + ΔW

Work done under constant pressure, (ΔW)_P = (ΔQ)_P - ΔU

Where (ΔQ)_P = nC_Pdt is the heat required to raise the temperature at constant pressure.

Also, ΔU = (ΔQ)_V = nC_Vdt is the heat required to raise the temperature at constant volume.

⇒ (ΔW)_P = nC_Pdt - nC_Vdt = n(C_P - C_V)dt

⇒ (ΔW)_P = 1 x (3.4 x 10³ - 2.4 x 10³) x 10 = 10⁴ calorie

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Very Long Answers Questions [5 Marks Questions]

Ques. A girl is running along a beach and she does 4.3 x 10⁵ J of work and gives off 3.8 x 10⁵ J of heat.

• What is the change in her internal energy?
• If she is not running but walking, she gives off 1.2 x 10⁵ J of heat and her internal energy decreases by 2.6 x 10⁵ J. How much work has she done while walking?

Ans. Consider the girl to be a system.

She does work on the surroundings i.e. work done by the system on surrounding

ΔW = + 4.3 x 10⁵ J

She releases heat to the surroundings i.e. ΔQ is negative

ΔQ = - 3.8 x 10⁵ J

• Using the first law of thermodynamics

ΔQ = ΔU + ΔW

Therefore, change in internal energy, ΔU = (-3.8 x 10⁵) - 4.3 x 10⁵ J

⇒ ΔU = -8.1 x 10⁵ J

• While walking,

She gives off heat to surroundings, ΔQ = -1.2 x 10⁵ J

Her internal energy decreased, ΔU = -2.6 x 10⁵ J

Using the first law of thermodynamics,

ΔQ = ΔU + ΔW

ΔW = ΔQ - ΔU

⇒ ΔW = -1.2 x 10⁵ - (- 2.6 x 10⁵)

⇒ ΔW = 1.4 x 10⁵ J

Ques. Heat is supplied at a constant pressure to a diatomic gas. What will be part of this heat which goes to increase its internal energy?

Ans. From the first law of thermodynamics, we gave

dQ = dU + dW

Where

• dQ = Change in heat energy
• dU = Change in internal energy
• dW = Work done

Let the part of heat which increase internal energy will be dU/dQ

Now at constant pressure heat supplied is given by

dQ = nC_Pdt and dU = nC_Vdt

Where

• n = number of moles
• C_P = Specific heat at constant pressure
• C_V = Specific heat at constant volume

Therefore, dU/dQ = nC_Vdt / nC_Pdt

⇒ dU/dQ = C_V / C_P

But, C_P/C_V = ℽ

⇒ dU/dQ = 1/ℽ

But ℽ = 1 + 2/f

Where f is the degree of freedom

⇒ dU/dQ = 1/(1 + 2/f) = f / (2 +f)

For diatomic gas, f = 5

⇒ dU/dQ = 5 / (2 +5) = 5/7

Ques. 70 calories of heat are required to raise the temperature of 2 moles of an ideal diatomic gas at constant pressure from 30 ℃ to 35 ℃. What will be the amount of heat required in calories to raise the temperature of the same gas through the same range i.e. 30 ℃ to 35 ℃ at constant volume?

Ans. The heat required to raise the temperature at constant pressure is given by

(ΔQ)_P = nC_Pdt …(i)

The heat required to raise the temperature at constant volume is given by

(ΔQ)_V = nC_Vdt …(ii)

Where

• n = number of moles
• C_P = Specific heat at constant pressure
• C_V = Specific heat at constant volume

Dividing equation (i) by equation (ii), we get

(ΔQ)_P / (ΔQ)_V = nC_Pdt / nC_Vdt

⇒ (ΔQ)_P / (ΔQ)_V = C_P / C_V = ℽ

But, C_P/C_V = ℽ

⇒ dU/dQ = 1/ℽ

But ℽ = 1 + 2/f

Where f is the degree of freedom

⇒ (ΔQ)_P / (ΔQ)_V = 1 + 2/f

For diatomic gas, f = 5

⇒ (ΔQ)_P / (ΔQ)_V = 1+ 2/5 = 7/5

Given, (ΔQ)_P = 70 calories

⇒ 70 / (ΔQ)_V = 7/5

⇒ (ΔQ)_V = 50 calories

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