Deck 3: The Second Law of Thermodynamics

Question 1

Calculate the change in the entropy of a large system when 500 J of energy is transferred from the system to the surroundings as heat in an exothermic process at a temperature of 298 K.

A) +1.68 J K^-1
B) -1.68 J K^-1
C) +500 J K^-1
D) -500 J K^-1

Answer 1

B

Question 2

Calculate the change in the molar entropy of a perfect gas when it is compressed isothermally to 10% of its initial volume.

A) -19.1 J K^-1 mol^-1
B) -16.6 J K^-1 mol^-1
C) +0.813 J K^-1 mol^-1
D) -8.31 J K^-1 mol^-1

Answer 2

A

Question 3

Calculate the change in the standard molar entropy of sulfur trioxide gas, SO₃, when it is cooled from a temperature of 100 °C to 10 °C at a constant pressure of 1 bar. The standard molar constant-pressure heat capacity of sulfur trioxide is 50.1 J K^-1 mol^-1.

A) +7.81 J K^-1 mol^-1
B) -13.8 J K^-1 mol^-1
C) -115 J K^-1 mol^-1
D) -6.01 J K^-1 mol^-1

Answer 3

B

Question 4

Calculate the standard molar entropy of vaporization of phosphorus trichloride, PCl₃, at its boiling temperature, 74 °C, given that the standard molar enthalpy of vaporization of phosphorus trichloride is 30.5 kJ mol^-1.

A) +2.43 J K^-1 mol^-1
B) +109 J K^-1 mol^-1
C) +412 J K^-1 mol^-1
D) +78.9 J K^-1 mol^-1

Question 5

The boiling temperature of cyclohexane, C₆H₁₂, is 354 K. Use Trouton's rule to estimate the enthalpy of vaporization of cyclohexane.

A) 6.89 kJ mol^-1
B) 18.7 kJ mol^-1
C) 85 kJ mol^-1
D) 30.1 kJ mol^-1

Question 6

Calculate the entropy of vaporization of ethanol, C₂H₅OH, at 298 K from the following data. The constant-pressure molar heat capacities of ethanol as liquid and vapour are 111.46 and 65.44 J K^-1 mol^-1 respectively. The enthalpy of vaporization of ethanol at its boiling temperature, 352 K, is 38.56 kJ mol^-1.

A) +117.21 J K^-1 mol^-1
B) +139.01 J K^-1 mol^-1
C) +109.55 J K^-1 mol^-1
D) +29.46 J K^-1 mol^-1

Question 7

At low temperatures, the heat capacity of some metals has been found to follow a modified version of the Debye T³ law, varying with temperature according to the relationship
C_p.m=aT³+bT
With the additional, linear term arising from the electrons. Derive an expression for the molar entropy of such a metal at a low temperature T.

A) S(T)=⅓aT²
B) S(T)=aT²+bT
C) S(T)=⅓aT²+bT
D) bT

Question 8

A hypothetical system consists of four molecules with a total energy 4Calculate the total number of ways of distributing four quanta of energy  amongst the four molecules.

A) 4
B) 70
C) 35
D) 12

Question 9

Use the following data to calculate the standard reaction entropy at 298 K for the thermal decomposition of ammonium chloride, NH₄Cl
NH₄Cl(s)  NH₃(g) + HCl(g)
S_m^o / J mol^-1
NH₄Cl(s) 94.85
NH₃(g) 192.77
HCl(g) 186.90

A) +474.52 J K^-1 mol^-1
B) 284.82 J K^-1 mol^-1
C) -100.72 J K^-1 mol^-1
D) +88.98 J K^-1 mol^-1

Question 10

For hydrogen sulfide, H₂S, the standard enthalpy of formation is -20.2 kJ mol^-1 and the standard entropy of formation is +43.0 J mol^-1 at 298 K. Calculate the standard Gibbs energy of formation at this temperature.

A) -33.0 kJ mol^-1
B) +12.8 kJ mol^-1
C) -20.5 kJ mol^-1
D) -7.4 kJ mol^-1