PRACTICE EXAM QUESTIONS: These are actual questions from previous exams to help you study for the upcoming first exam. 

 

1) For FOUR of FIVE of the following give a short (3-4 sentences) explanation or definition, including equations, uses, applications, examples, etc.

Zeroth Law of Thermodynamics; State Function; Standard Enthalpy of Formation; Maxwell Distribution; Kinetic Theory of Gases.

 

2) 37.2 g of Xe is at 250 oC in a vessel of 5.00 dm3. For Xe, a = 4.250 atmL2/mol2 and b = 5.105 x 10-2 L/mol. i) Calculate the pressure assuming the Xe behaves ideally. ii) Calculate the pressure assuming the Xe behaves as a van der Waals gas. iii) Explain what (in an approximate way) the a and b parameters in the van der Waals equation of state represent.

 

3)  For the following problem, assume combustion takes place with oxygen gas and that carbon dioxide gas and liquid water are the only products of combustion, and that you can look up the standard enthlpies of formation of these products. a) The enthalpy of combustion of cyclopropane gas is -2091 kJ/mol.  What is the standard enthalpy of formation of cyclopropane gas? b) Propene gas has a standard enthalpy of formation of +20.42 kJ/mol.  What is the standard enthalpy of combustion for propene gas? c) What is the standard enthalpy for the isomerization reaction: cyclopropane (g) ----> propene (g)

 

4) For the following problem, assume a vessel at 300 K containing 1.00 g each of He, Ar, and Ne, and that all gases behave ideally.  a) If the partial pressure of Ar is 250 torr, what are the total pressure and volume? b) Now suppose the vessel has a piston at one end and expands isothermally to twice its final volume in a) against a constant external pressure equal to the total pressure.  What are q, w, DU and DH for the expansion? c) Now suppose the same isothermal expansion happens as in b) but reversibly to a final pressure equal to the total pressure.  What are q, w, DU and DH for the expansion?

 

5) MULTIPLE CHOICE NO PARTIAL CREDIT. Circle the correct answer for each.

i) Compared to the ideal gas pressure, the van der Waals pressure of a gas will:

a)      Always be greater than ideal                              b)   Always be less than ideal.

c)   Always be equal to ideal                                    d)   Depend on the nature of the gas.

 

ii) In an adiabatic expansion of an ideal gas, which of the following is always true?

a)      The temperature of the gas will rise.                  

b)      The work done by the gas on the surroundings is equal to the decrease in the internal energy of the gas.

c)      No work is done by the gas on the surroundings.

d)      The work done by the gas on the surroundings is equal to the increase in the internal energy of the gas.

 

iii) The standard enthalpy of fusion of silver at 1234 K is 11.30 kJ/mol. How much energy is required to completely melt 90.0 g of silver at 1234 K?

a) +11.30 kJ                  b) +9.43 kJ                    c) +13.5 kJ                    d) 9.43 kJ

 

iv) DU for the following reaction was measured at temperature T in a constant volume calorimeter.           A (s) + 2B (g) 2C (s) + d (g) DHo for the reaction is given approximately by

a) DUo RT                 b) DUo                          c) DUo + RT                 d) DUo + 2RT

 

v) A vessel of volume 22.4 L contains 3.0 mol O2 and 4.5 mol N2 at 273.15 K. The partial pressures in atmospheres of O2 and N2 respectively, are

a) 0.4 and 0.6                b) 7.5 and 7.5                c) 3.0 and 4.5                d) 4.5 and 3.0

 

Answers:

 

1) See text book

 

2) i) P = 2.43 atm  ii) 2.43 atm (OK, 2.4253 atm)  iii) a is intermolecular force correction (as real gas molecules do interact with each other) and b is a volume correction (as real gas molecules are not negligibly small).

 

3) Can look up the standard enthalpy of formation for CO2 and H2O.

a) +53 kJ/mol  b) -2058 kJ/mol  c) -33 kJ/mol

 

4) a) P = 4.26 atm, V = 1.87 L 

b) w = -809 J, q = +809 J, Delta U = Delta q = 0 J

c) w = -561 J, q = +561 J, Delta U = Delta q = 0 J

 

5 i) answer is d (subtracting a makes pressure smaller, dividing by decreased volume (less nb) makes pressure larger, so it depends on the a and b terms and hence the gas)

 

5 ii) answer is b (adiabatic so q is zero, Delta U is work, and decreases as work done by system in expansion)

 

5 iii) answer is b) (moles times enthalpy give 9.43 kJ, positive as must add energy to system (silver) to melt it)

 

5 iv) answer is a) (H = U + PV, so Delta H = Delta U + Delta n(gas)RT and Delta n(gas) is -1)

 

5 v) answer is c (lots of ways to calculate this from ideal gas law)