THERMOCHEMISTRY JEE

The process with negative entropy change is

• dissolution of iodine in water

• sublimation of dry ice

• synthesis of ammonia from N₂ and H₂

• dissociation of CaSO_4(s) to CaO_(s) and SO_3(g).

The standard enthalpy of formation of NH₃ is -46 kJ mol^–1. If the enthalpy of formation of H₂ from its atoms is -436 kJ mol^–1 and that of N₂ is -712 kJ mol^–1, the average bond enthalpy of N-H bond in NH₃ is

• -964 kJ mol^–1

• +352 kJ mol^–1

• +1056 kJ mol^–1

• -1102 kJ mol^–1

The enthalpy change for a reaction does not depend upon

• use of different reactants for the same product

• the nature of intermediate reaction steps

• the differences in initial or final temperature of involved substances

• the physical states of reactants and products

Identify the correct statement regarding a spontaneous process

• For a spontaneous process in an isolated system, the change in entropy is positive

• Endothermic processes are never spontaneous

• Exothermic processes are always spontaneous

• Lowering of energy in the reaction process is the only criterion for spontaneity

For a particular reversible reaction at temperature T, ∆H and ∆S were found to be both +ve. If T_e is the temperature at equilibrium, the reaction would be spontaneous when

• T_e > T

• T > T_e

• T_e is 5 times T

• T_e = T

The entropy change involved in the isothermal reversible expansion of 2 moles of an ideal gas from a volume of 10 dm³ to a volume of 100 dm³ at 27°C is

• 32.3 J mol^-1K^-1

• 42.3 J mol^-1K^-1

• 38.3 J mol^-1K^-1

• 35.8 J mol^-1K^-1

A piston filled with 0.04 mol of an ideal gas expands reversibly from 50.0 mL to 375 mL at a constant temperature of 37⁰C. As it does so, it absorbs 208J of heat. The values of q and w for the process will be (R = 8.314J/molK and In 7.5 = 2.01)

• q = + 208J, w = – 208 J

• q = – 208J, w = – 208 J

• q = – 208J, w = + 208 J

• q = + 208J, w = + 208 J

During compression of a spring, the work done is 10 kJ and 2 kJ escaped to the surrounding as heat. The change in internal energy ∆U (in kJ) is

• -8

• 12

• 8

• -12

An ideal gas undergoes isothermal expansion at constant pressure. During the process

• enthalpy increases but entropy decreases

• enthalpy remains constant but entropy increases

• enthalpy decreases but entropy increases

• both enthalpy and entropy remain constant.

Assuming that water vapour is an ideal gas, the internal energy change (U) when 1 mol of water is vaporized at 1 bar pressure and 100°C, (given: molar enthalpy of vapourisation of water at 1 bar and 373 K = 41 kJ mol^–1 and R = 8.3 J mol^–1 K^–1) will be

• 41.00 kJ mol^-1

• 4.100 kJ mol^–1

• 3.7904 kJ mol^–1

• 37.904 kJ mol^–1