Physical Chemistry Midterm (Due Next Tuesday)
Try to solve all the problems. Show all the steps and the reasoning behind your
approach.
1. In a spacecraft designed to land on the surface of Mars, there was a sampling
device that incorporated a spring of force constant 2.106Newton/meter, attached
to a cam, which alternately compresses and expands it through displacement of 1
cm from equilibrium. Power supplies had to be designed very carefully and
questions arose as to how much power would be necessary to drive the sampler.
a) How much work would have to be done on the spring to compress and extend
it 1000 times?
b) What is the power in watts of an electric motor that accomplishes these 1000
oscillations in 1000seconds?
c) The spring is in contact with a thermally insulated mass of metal of heat
capacity 4.2 kJ/K if its initial temperature is 20C what would be its final
temperature after 1000 oscillations.
2. A 5 gm lump of carbon dioxide is kept in vessel of 100cm3 volume at room
temperature, 298K. Calculate the work done by this system if it is allowed to
expand at 1 atm. pressure.
a. Isothermally against a constant external pressure of 1 atm.
b. Isothermally and reversibly.
c. Find the q, E and H for the paths mentioned in a and b above.
3. A typical sugar cube has a mass of 1.4 gms.
a. What is its enthalpy of combustion?
b. What is the height you can climb assuming that 25 percent of the energy
can be converted to work?
4. Calculate the change in the entropy of a system composed of 14 grams of gaseous
N2 at room temperature if it doubles its volume in following paths.
a. An isothermal reversible expansion
b. An isothermal irreversible expansion
c. An adiabatic reversible expansion
5. A mole of fluorocarbon was allowed to expand twice its volume reversibly and
adiabatically. The temperature of the gas dropped from 298K to 248K.
a. What is the value of it’s molar specific heat?
b. How does this specific heat compare to that for an ideal gas?
c. Hints: Cv=3/2R for ideal gas; Think of Carnot cycle
6. Standard Gibb’s free energy of formation for ammonia from nitrogen and
hydrogen is –33.26kJ/mole and H for the same reaction is –92.38kJ/mole at
298K. Calculate
a. G for the reaction at 500C and 1000C
b. Is the reaction spontaneous at room temperature?
c. Is the formation of Ammonia favored or disfavored by the rise in
temperature?