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HSC Board Mahrashtra Chapter 4 Chemical Thermodynamics VSA

1➤ Write the expression to calculate maximum work done when I mole of an ideal gas expands isothermally and reversibly from V₁ to V₂. V₂

=> Ans: W-2.303 RT logio - V₁ where, Wax Maximum work done, R= Gas constant, T= Temperature in Kelvin, V₂ and V₁- Final and initial volume respectively.

2➤ Write the mathematical relation between AH and AU during formation of one mole of CO₂ under standard conditions.

=> Ans: Formation of one mole of CO₂ under standard conditions: COCO An, (moles of product gases) - (moles of reactant gases) An-1-1=0 Now, AH AU + An,RT AH AU+0x RT AH = AU

3➤ Standard enthalpy of formation of water is -286 kJ mol'. Calculate the enthalpy change for formation of 0.018 kg of water.

=> Ans: Mass of H₂O=0.018 kg 18 g Mass of H₂O. Number of moles of H₂O = I mol Molar mass of H₂O 18 g mol The thermochemical equation is, 18 g ¹0H₂0AH° = -286 kJ mol Enthalpy change for formation of 1 mole H₂O = -286 kJ

4➤ Write the expression of first law of thermodynamics for an isothermal process.

=> Ans: The mathematical expression of first law of thermodynamics for an isothermal process is: W=-Qor-W=Q

5➤ What is the sign convention when work is done on the system by the surrounding?

=> Ans: When work is done on the system by the surroundings, work positive value (i.e., +W).

6➤ Write the expression showing relation between enthalpy change and internal energy change for gaseous phase reaction.

=> Ans: AH AU + An, RT where, AH is change in enthalpy, AU is change in internal energy. An, is difference between the number of moles of products and those of reactants.

7➤ Calculate enthalpy of formation of HCI if bond enthalpies of H₂, Cl₂ and HCI are 434 kJ mol, 242 kJ mol¹ and 431 kJ mol-¹ respectively.

=> Ans: AHEAH (reactant bonds) - EAH (product bonds) H₂ + Cl2HCl AH = [1 mol x 434 kJ mol + 1 mol x 242 kJ mol™¹] - [2 mol x 431 kJ mol"] =-186 kJ Hag+Cl-2HC A,H=186 kJ For enthalpy of formation of HCI, the reaction is κΈ€+ 2 2 -186 kJ A,H= 2 mol =-93 kJ mol

8➤ What is an isolated system?

=> Ans. Isolated system: It is a system which can neither exchange energy nor matter with its surroundings, e.g. hot water filled in a thermally insulated closed vessel like thermos flask.

9➤ What is an intensive property? Give an example.

=> Ans. Intensive property: It is a property of a system whose magnitude is independent of the amount of matter present in the system. For example, viscosity.

10➤ Explain an adiabatic process.

=> Ans. Adiabatic process: It is a process in which there is no exchange of heat energy between the system and its surroundings. Hence, q=0. (a) An adiabatic process is carried out in an isolated system. (b) In this process, temperature and internal energy of a system change, AT 0, AU 0.

11➤ Classify the following properties as intensive or extensive : (i) Density, (ii) Energy.

=> Ans. Intensive property: (i) Density Extensive property: (ii) Energy.

12➤ What is the enthalpy of atomisation? Give an example

=> Ans. Enthalpy of atomisation (AatmH): It is the enthalpy change accompanying the dissociation of one mole of gaseous substance into its atoms at constant temperature and pressure. For example, CH)C(g) +4H(g) Aatom H=1660 kJ mol-¹.

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