Phase Equilibria Question Paper

Phase Equilibria

Course:Bachelor Of Science

Institution: Kenyatta University question papers

Exam Year:2009

KENYATTA UNIVERSITY
UNIVERSITY EXAMINATIONS 2009/2010
FIRST SEMESTER EXAMINATION FOR THE DEGREE OF BACHELOR OF
SCIENCE
SCH 403: PHASE EQUILIBRIA

DATE: TUESDAY, 22ND DECEMBER 2009

TIME: 8.00 A.M. - 10.00 A.M.

INSTRUCTIONS: Answer ALL questions.

DATA: T = 273K, R = 8.314

1.
(a)
Define the following terms:
(i)
Phase

(ii)
Degree
of
freedom
(iii)
Components
(iv)
Azeotropic
mixture

(6 marks)

(b)
Distinguish
between

(i)
Coltical temperature and consulate temperature
(ii)
Triple
point and Eutectic point

(iii)
True and untrue equilibrium.

(4 1 marks)
2

(c)
What is the mole fraction of glycine in aqueous solution of molality
0.14
molKg-1.

(2 marks)

Page 1 of 5

2.
The phase diagram of water may be given as under.

(a)
Curve OC is inclined towards the pressure axis.
Using
Claussius-Clapeyron
equation, explain how this affects the
melting
point
of
ice.
(4
marks)

(b)
Explain clearly how curve A
O ' comes about and what it represents.

(2 marks)

(c)
(i)
Comment on the vapour pressures of curve A O
' and AO .

(ii)
Apply phase rule at point O and comment on the result obtained.

(3 marks)

(d)
Give the salient features of the system for curve OC and OA.
(1 mark)

3.
A certain phase system consisted of a substance, which when heated very fast

melted at 114oC and boiled at 444oC.
(a)
Name
the
substance
involved.
(1
mark)

(b)
Draw the phase diagram of the system described above and indicate
only
the
phase
changes
involved.
(4
marks)

Page 2 of 5

(c)
How
many

(i)
Equilibrium curves does the system have?
Apply
phase
rule
to
any.
(2
marks)

(ii)
Triple points does the system have?

(2 marks)
Apply
phase
rule
to
any.
(d)
What
observation
could
you
make
if the liquid of this substance

was cooled very fast?

(1 mark)

4.
The following temperature composition data were obtained for a mixture of

Octane (O) and Toluene (T) and 760 mmHg, where X is the mole fraction in

the liquid and Y the mole fraction in the vapour at equilibrium.

ToC 110.9 112.0 114.0 115.0 117.3 119 120.0 123.0
(Liquid) XT 0.9 0.79 0.61 0.52 0.41 0.30 0.20 0.09
(Vapour) YT 0.92 0.83 0.69 0.62 0.52 0.41 0.29 0.16

The boiling points are 110.6oC for Toluene (T) and 125.6oC for Octane (O).

(a)
Plot the temperature-composition diagram of the mixture.
(4 marks)

(b)
What is the composition of the vapour in equilibrium with the
liquid
of
composition.
(i)
XT
=
0.25
XO
=
0.25

(2 marks)

(c)
In your diagram, mark a position “b” of the liquid mixture of

composition XT = 0.4 and temperature 115oC.

(i)
What will be the boiling point of this mixture if heated?
(1 mark)

(ii)
What will be the composition of vapours and distillate obtained

from this mixture?

(2 marks)

(iii)
If the same distillate was repeatedly redistilled for 100 operations,

give the composition of the final distillate and its boiling point.

(1 mark)
Page 3 of 5

5.
Using clapeyron equation, show that claussius clapeyron equation.
P
Hv T
T
2
?
?
- ?

i.e. log
=
?
? 2
1 ??
P
230R
T T
1
? 1 2 ?

(a)
Using clapeyron equation, develop the log form of claussius clapeyron
equation.
P

i.e. log 2 = ........................

(5 marks)
P1

(b)
Calculate the heat of vapourisation of acetone if the vapour pressure

of acetone at 0oC is 53.46 mmHg and at 30oC.

(c)
It is impossible to heat a solid above its melting point, but it is possible

to super cool some liquids.
Comment
on
this
statement.
(2
marks)

6.
(a)
Draw a well-labelled LEAD-SILVER SYSTEM using the following data.

Lead m.p - 327oC

Silver m.p - 961oC

Eutectic temp at 2.6% silver - 303oC

(4 marks)

(b)
Apply phase rule

(i)
On the freezing point curve of silver

(ii)
at the Eutectic point

(3 marks)

(c)
Mark a point x in the melt area which ahs 327oC temperature with

a composition of 1.5 percent silver. Draw and describe a cooling

curve for the solution or melt at x.

(3 marks)

Page 4 of 5

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