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Monday, 28 September 2015

FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 3

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.


Written by: Asad Hussain

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