Engineering Chemistry MCQ (Uses and Limitations of Phase Rule)
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Uses and Limitations of Phase Rule:
1. For the T-C relation in phenol, there happens a regular sharpening in
concentration of phenol. What will happen if the equilibrium conditions are varied
at a particular point?
a) The curve remains unchanged
b) The curve decreases
c) The curve increases
d) The curve inverts
Explanation: We know that both the equilibrium points are achieved at the same
concentration and hence there is a regular increase. If the equilibrium conditions
are varied, then the steadiness vanishes and hence the curve will show an abnormal
increase in its concentration.
2. In the general phase diagram of Ferric chloride – water system, what causes reduction in water content in ferric chloride at each step?
a) Reduction in moisture content
b) Reduction in concentration
c) Reduction in equilibrium shift
d) Increase in temperature
Explanation: Since for each transition from one phase to another for a component,
the moisture content decreases due to increase in temperature. Hence the water
concentration in ferric chloride decreases as the temperature increases.
3. The mechanism of diffusion of small atomic radius compared to the parent material would be described by __________________
a) Substitutional diffusion
b) Interstitial diffusion
c) Vacancy diffusion
Explanation: During equilibrium process, metastable curve appears and due to
variation in equilibrium composition, the curve disappears. To fix this equilibrium
back to form, there takes place diffusion between interstitial atoms called interstitial diffusion.
4. What happens during a non-steady state diffusion?
a) The composition behavior does not change with time
b) The composition behavior changes with time
c) The diffusion can be described by Mond’s Second Law
d) Zero composition
Explanation: Only during a steady state reaction (invariant reaction), the composite
and other parameters like concentration, temperature, pressure, solubility etc can
be determined. Hence under non- equilibrium conditions, all the above parameters
are negligible and hence there is zero composition.
5. The driving force of the diffusion process in phase diagram is __________________
a) The slope in Gibbs free energy
b) The slope in solubility
c) The slope in temperature
d) The slope in pressure
Explanation: Since the driving force (parameter defining force) is determined only by
a small change in chemical composition with respect to temperature, the slope in
the chemical composition will determine the driving force.
6. In the mixture of propanol and iso propanol, identify the component at the meeting point of these two components.
Explanation: When two different components are made to show phase relation,
together, then the component which is formed at the eutectic region is a mixture of
their metastable components. Thus here it is Propane-2-ol.
7. What are the limits of phase rule?
a) Cannot determine the presence of more the one phase in a system
b) Cannot determine solubility
c) Cannot determine composition
d) Can determine temperature
Explanation: Since phase rule focusses only on one equilibrium state of the system,
the presence of even tiny traces of multiple phases cannot be identified using phase
rule which is the main drawback of this rule. This drawback is solved by Lever rule.
8. In the mixture of liquid and vapor compositions, identify the type of component present in between the two components (at equilibrium point).
a) Regular solution
b) Equilibrium solution
c) Transition solution
d) Steady state solution
Explanation: Since phase rule cannot determine multiple phases in a single system,
regular solution is not obtained. Similarly, equilibrium solution is also not obtained (according to Weiss law). Thus a steady solution is obtained.
9. Calculate the concentration from the given data.
T = 20 degrees
P = 2 atm
Explanation: According to Weiss law (which suits only for phase components),
concentration is given by temperature/pressure. Thus in this case, concentration is equal to 20/2=10.
10. What is the efficiency of phase rule over Lever rule?
Explanation: Since, phase rule works only under equilibrium conditions and also
only a few parameters like composition, solubility, temperature can be determined,
it has limitations over lever rule. Another point is it fails to explain the T-C relation.