Questions carrying 2 marks
Q.1.
Differentiate between lyophillic and lyophobic colloids.
Q.2.
Why is adsorption always exothermic in nature?
Q.3.
What is demulsification? Name two demulsifying agent.
Q.4.
What do you understand by the term activity and selectivity?
Q.5.
Give reasons for the following
a) Cottrell’s smoke precipitator
is fitted at the mouth of the chimney used in factories.
b) Colloidal gold is used for
intermuscular injection.
Q.6. Why is it essential to wash the precipitate with water before estimating it quantitatively?
Q.7. Comment on the statement that “colloid is not a substance but state of
a substance.”
Q.8. Why is the ester hydrolysis slow in the beginning and becomes faster after some time?
Answer Keys:
1. a)
Lyophillic sols are easily prepared by directly mixing with the liquid dispersion medium but lyophobic
sols cannot be prepared directly by mixing with liquid.
b) lyophillic sols are stable and are not easily coagulated but
lyophobic sols can be easily precipitated by the addition of suitable
electrolyte.
2. When
a gas is adsorbed on the surface of a solid, its entropy decreases and ΔS
becomes negative. Now ΔG = ΔH- T ΔS
and for the process to be spontaneous, free energy change must
be negative.
As TΔS is negative i.e. -T ΔS is positive and for
free energy change to be negative enthalpy change should be negative hence
reaction should be exothermic always.
3. The process of separation of constituent liquid of an
emulsion is called demulsification. It can be done by either centrifuging or boiling.
4. Activity
of a catalyst means its capacity to increase the speed of a chemical reaction
Selectivity of a catalyst means its ability to direct a particular reaction to
form particular products.
5.
a) It neutralise the charge on carbon particles which
get precipitated and thus gases entering into chimney are free from carbon
particles.
b) This is
done because Gold particles have large surface area and easily assimilated to
the blood which is colloidal.
6.
Some amount of the electrolytes mixed to form the
precipitate remains adsorbed on the surface of the particles of the precipitate. Hence, it is essential to wash the precipitate with water to
remove the sticking electrolytes or any other impurity before estimating it
quantitatively.
7. The
given statement is true. This is because the same substance may exist as colloid
under certain conditions and as a crystalloid under certain conditions for e.g.
NaCl in water behave as crystalloid but in benzene as a colloid. It is the size
of solute particle which matters i.e. ,the state in which the substance exists.
If it lies in the range of 1nm to 1000nm its a colloid.
8.
RCOOR’ + H2O <==> RCOOH + R’OH
The acid
produced in above reaction act as auto catalyst for the reaction. Hence the
reaction becomes faster after some time.
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