Results of Midterm 2

1
Results of Midterm 2
of students
10
20
30
40
50
60
70
80
90
100
0
points
Grade Points
A 85-100
B 75-84
B 60-74
C 50-59
C 30-49
D,F lt30
Midterm 1
2
Problem 1 (heat engine)
(20) The heat engine uses an ideal gas as its
working substance. 1-2 is an isochoric process,
2-3 adiabatic, 1-3 isothermal. T1 and T2 are
given. Find ?Q for each process and the
efficiency of the heat engine in terms of T1 and
T2 (?12/f, CV(f/2)NkB). (10) Which way of
increasing the efficiency of the Carnot heat
engine is better to increase the temperature of
the hot reservoir by ?T (?T ltltTH, TC) or to
decrease the temperature of the cold reservoir by
the same ?T? Explain.
1 2
2 3
3 1
3
Problem 2 vdW
(20) Two insulated tanks with volumes V1 and V2
are connected by a valve. Each tank contains one
mole of the same monatomic van der Waals gas (the
constants a and b are known). Initially, when the
valve was closed, both gases were at the same
temperature Ti. The valve is open and the system
reaches its equilibrium state. Find the
equilibrium temperature Tf. Is Tf higher or lower
than Ti? For which value of V1 /V2 you expect Tf
to be equal to Ti?
the temperature will decrease
if
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Problem 3 (phase transformations)
(25) The pressure of the saturated water vapor at
T 0.010C is 0.006 bar (these T and P correspond
to the triple point of water). The latent heat
of the solid-liquid transformation at 0.010C is
335 kJ/kg, the latent heat of the liquid-gas
transformation is 2500 kJ/kg. Find the pressure
of the saturated water vapor at T -10C.
Along the solid-gas phase equilibrium curve
- the latent heat of sublimation at the triple
point
- we neglected the volume of solid and expressed
the gas volume using the ideal gas law. n the
number of moles in 1 kg.
5
Problem 4 (chem. equilibrium)
(25) Consider the following reaction at T 298K
and P 1bar H2CO3 (aq) ? HCO3-- (aq) H
(aq) (a) Using the data in the Table on p.404,
calculate ?G for this reaction, and the
equilibrium constant K. (b) If the initial amount
of H2CO3 is 1 mole, what will be the amounts of
each reactant and product at equilibrium?
To obtain the value of ?G for the reaction,
subtract ?G of the reactants from ?G of the
products
The mass action law
Thus, in equilibrium,