Ncert Solutions Chemistry Class 11th

40. (i) First law of thermodynamics: It states that energy can neither be created nor be destroyed. The energy of an isolated system is constant.
?u = q + w

(ii) The standard enthalpy change for the formation of one mole of a compound from its elements in their most stable states of aggregation (also known as reference state is called Standard Molar Enthalpy of Formation. Its symbol is Δ_f H^?, where the subscript ' f ' indicates that one mole of the compound in question has been formed in its standard state from its elements in their most stable states of aggregation.

39. In thermodynamics, a distinction is made between extensive properties and intensive properties. An extensive property is a property whose value depends on the

quantity or size of matter present in the system. For example, mass, volume, internal energy, enthalpy, heat capacity, etc. are extensive properties.

Those properties which do not depend on the quantity or size of matter present are known as intensive properties. For example, temperature, density, pressure etc. are intensive properties.

38. Since, there is an increase in the number of moles on the product side so, entropy increases and ?S is positive.

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Molar enthalpy change for graphite (ΔH)

= enthalpy change for 1 g x molar mass of C = -20.7*12 = -2.48 x 102 kJ mol-1

Since the sign of ΔH = -ve, it is an exothermic reaction.

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Given that, Cv = heat capacity at constant volume,

Cp = heat capacity at constant pressure

 Difference between Cp and Cv is equal to gas constant (R).

.'. Cp – Cv = nR                                (where, n = no. of moles)

= 10 x 8.314 = 83.14J

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For water, molar heat capacity = 18 x Specific heat or Cp = 18 x c

But, specific heat,

C = 4.18 J g^-1 K^-1 Heat capacity,

Cp = 18 x 4.18 JK^-1  = 75.24 JK^-1

36. Graphite has greater entropy since it is loosely packed. At absolute zero the entropy of a substance is zero.

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During free expansion, external pressure is zero, so Work done, w = -pextΔV

= -0(5 – 1) = 0

Since the gas is expanding isothermally, therefore, q = 0

ΔU = q + w =0+0=0