NATIONAL OPEN UNIVERSITY OF NIGERIA
PLOT 91, CADASTRAL ZONE, NNAMDI AZIKIWE EXPRESSWAY, JABI – ABUJA
FACULTY OF SCIENCES
DEPARTMENT OF PURE & APPLIED SCIENCES
JUNE 2018 EXAMINATION QUESTIONS
CHM 301-PHYSICAL CHEMISTRY III
INSTRUCTION: Answer question one (1) and any other four (4) questions.
Duration 3 hours
(Take the values for F= 96,500 coulombs, k= 1.38066 x 10-23;R= 0.0821 L atmmol-1K-1=8.314JK-1mol-1=62.396mmHgLK-1mol-1 =1.987calK-1mol-1; = 3.142; and use when required)
QUESTION 1
(a) Give an isothermal irreversible process, drive an expression for total work done when a gas in
(b) a cylinder expand from V1 to V2 against a constant external pressure. (7marks)
(a) List the colligative properties and write the corresponding equations and define the terms.
(4 marks)
(c) An organic compound W on analysis, gave the following percentage composition. C= 30.5 %, H=1.7% and Br =67.8%. [C=12; H=1; Br=80]. Calculate the emperical formular of W (2 marks)
(d) A solution made by dissolving 4.0g of sample W in 50.0g of benzene freezes at 3.74oC. The freezing point of pure benzene is 5.48oC. [ Kf of benzene =5.12 deg molality1]
Calculate
The molality of the solution (2 ½ marks)
The number of moles of W (2 marks)
Molar mass of W (2 marks)
Molecular formula of W (2 ½ marks)
QUESTION 2
(a) The equilibrium constant for the reaction
is 18.5 at 925 K and 9.25 at 1000 K respectively. Calculate
(i) the standard enthalpy of the reaction (3 marks)
(ii) ∆rGo at 925 K (3 marks)
(iii) ∆rSo at 925 K (3 marks)
(b) Calculate the entropy change when 2.0 mol of a perfect gas A and 3.0 mol of a perfect gas B mix spontaneously. (3 marks)
QUESTION 3
(a) State the third law of thermodynamics (2 marks)
(b) Hg2Cl2(s) + H2 (1atm) ⇌ 2Hg(l) + 2H+ (a=1) + 2Cl- (a=1) is E0298.15 = +0.2676 volt and ) at constant pressure is -3.09 x10-4 volt/deg. where T is the Celsius temperature. Given that 2 moles of electrons are involved in the cell reaction, calculate ΔG0, ΔH0, ΔS0 for the cell at 25oC. (6 marks)
(c) Giving your reasons, state the conditions in which the reactions will occur spontaneously
i) N2(g) + 3H2(g) → 2NH3(g) (The reaction is exothermic) (2 marks)
ii) O2(g) → 2O(g) (The reaction is endothermic) (2 marks)
QUESTION 4
(a)Differentiate between a state and path function. (2 marks)
(b) A diatomic gas assumed ideal, initially at 23.7 L 0.9 bar and 308K expands to 38.2 L. calculate:
i) Number of moles present (1 mark)
work done:
ii) Isothermally and reversibly (2 marks)
iii) Under isobaric conditions (2 marks)
iv) Adiabatically (5 marks)
QUESTION 5
(a) (i) State the Carnot theorem (3 marks)
(ii) What are the features used by carnot to analyse the functioning of an engine
(5 marks)
(b) Define the term Entropy (2 marks)
(c) Calculate the change of entropy when of heat is transferred reversibly and isothermally to a system at 600 K. (2 marks)
QUESTION 6
Define the following terms as applied to chemical thermodynamics Internal energy (ii) heat (iii) work (3 marks)
(b) Methane gas, CH4 originally at , undergoes a reversible adiabatic expansion that
doubles its volume. Assuming the gas is ideal calculate the following
(i) The final temperature. (2 marks)
(ii) The maximum work done for 0.5 moles of the gas (1 mark)
(c) The vapour pressure of propanol (C3H8O) is 375 torr at 38.8 oC, but fell to 372.1 torr when 8.69 g of an involatile organic compound Y is dissolved in 50 g of the propanol. Calculate
The mole fraction of solute and solvent (2 marks)
the number of moles of compound Y (1 ½ marks)
The molar mass of compound Y (1 mark)
(d) Calculate the change in the chemical potential of a perfect gas when it expands isothermally at a temperature of 20.0°C so that its volume doubles. (1½ marks)
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