FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 3
(4) Ion – Dipole Interactions:
The forces of attraction between ions and polar molecules
are known ad ion-dipole interactions. Or “The forces of attraction between the
+ve pole of a polar molecule (dipole) with an anion (-ve ion) or – ve pole of a
dipole with a caution (+ve ion), are known as “ion-dipole interactions.” For
example mot of the ionic compounds dissolves in water (polar solvent) because
of these interactions. Let us take the example of dissolution of NaCl (ionic
compound) in water. When NaCl is put into water the Na+ and Cl- ions are
separated from each other due to hydration energy offered by H2O molecules. The
Na+ ions are surrounded by H2O molecules (dipoles) in such a way that the –ve
poles of H2O molecules come close to Na+ ion are generated which keep Na+ ions
away from Cl- ions.
On the other hand Cl- ions are also surrounded by H2O
molecules (dipoles) in such a way that their +ve poles come close to Cl- ion
and thus ion-dipole interactions are established over here too. Thus both Na+
and Cl- ions get separated and NaCl remains dissolved in water.
(5) Dipole Induced Dipole Interactions:
The forces of attraction between a dipole and an induced
dipole are known as dipole induced dipole interactions.
Such forces are also known as “Debye forces”.
Such forces operate when a non-polar substance is dissolved
in a polar substance i.e. we can say that when non-polar molecule come in
contact with polar molecules (dipoles) dipole induced dipole interactions are
established.
Actually the –ve and +ve poles of the dipoles (polar
molecules) repel or attract the valence electrons of the nearby non-polar
molecules and thus produce induced dipoles. Then these dipoles and induced
dipoles attract each other establishing dipole induced dipole interaction among
them.
Evaporation:
The spontaneous escape of the molecules of a liquid from
liquid to vapour (gaseous) phase is known as evaporation,”
Explanation:
The molecules of liquid which are at its surface have
highest K.E and thus undergo more intense jumps as compare to lower molecules.
During these high jumping some (few) of the surface molecules overcome the
intermolecular attractive forces and go(escape) from liquid into vapour phase.
Evaporation occurs at all temperatures. However with
increases of temperature the rate of evaporation increases. If a liquid is
placed in a container which is made of a conducting material then as long as
evaporation occurs the surroundings of the container get cold. Thus we say that
evaporation of a liquid causes cooling.
Reason:
It is because of the fact that during evaporation the
highest K.E molecules escape from the liquid due to which the average K.E of
the molecules of the liquid decreases we can say that temperature of the liquid
falls. Thus heat will start flowing from the nearby surroundings of the
container, into the liquid and thus the temperature of the surroundings of the
container that evaporation of a liquid causes cooling.
Rate of evaporation:
The no of molecules of a liquid going from liquid phase into
vapour phase per unit time is known as rate of evaporation.
Factors Affecting Evaporation:
Following are the important factors which affect the
evaporation of a liquid.
(i) Surface Area:
Greater the surface area of a liquid greater will be the
number of highest K.E molecules and hence large no of molecule will have the
opportunity to escape into vapour phase. Thus we can say that liquids with
larger surface are a have higher rate of evaporation and vice versa.
(ii) Inter Molecular attractive force:
Stronger the inter molecular attractive force among the
molecules of a liquid lower will be its rate of evaporation and vice versa. For
example when water (H2O) and hydrochloric and (HCl) are placed in two container
at same temperature then the rate of evaporation of water will be lower than
that of HCl it is because of the fact that the intermolecular attractive forces
in water molecules are stronger than those of HCl.
(iii) Temperature:
The rate of evaporation of a liquid increase with increase
of temperature and vice versa. It is because of the fact that with increase of
temperature the K.E of the molecules increases due to which the intermolecular
attractive forces decrease and thus rate of evaporation increases. For example
when same volumes of water are taken in two containers. Let the temperature of
one is 25oc and the other at 50oc. then the evaporation of water at 50oc will
be higher than the other one.
Condensation:
Condensation is the reverse of evaporation. Thus
condensation may defined as “The process in which the molecules of a liquid
come back from vapour phase into liquid phase”
Rate of Condensation:
The no of molecule of a liquid coming from vapour into liquid
phase per unit time is known as rate of condensation.
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