FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 7

FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 7

Transition Temperature:

“The temperature at which both the crystalline forms of an element or compound are in equilibrium or co-exist is known as transition temperature of that element or compound. or The temperature for an element or compound above or below which only one crystalline form of that elements or compound exists is known as transition temperature of that element or compound. For example transition temp of sulphur (S) is 95.5 degree Celsius, because at 95.5 degree Celsius both rhombic and monoclinic forms of S exist. Or below 95.5oc only rhombic form exists while above 95.5 degree Celsius only monoclinic form exists. i.e. S 95.5 degree Celsius  similarly transition temperature for KNO3 is 128.5 degree Celsius. From the above examples it is clear that both elements and compounds exhibit transition temperature.

Cleavage:

The process of breaking of a crystal at a particular plane into smaller identical crystals is known as cleavage. A crystal can be cleavaged or broken in a particular plane only.

Cleavage Plane:

The plain at which a crystal can be broken down into smaller identical crystals is known as cleavage plane. There are particular planes for breaking down a crystal into similar smaller crystals. It can be cleavaged at any other place.

Isotropy:

Crystalline solids do not show the property of isotropy. This property is shown by Amorphous solids and liquids. Isotropy can be defined as “Substance for which the value of their physical property like refractive index etc does not change with change of angle or direction, are known as isotropic substances and this phenomenon”

Anisotropy:

"Substance from which the value of their properties like refractive index, thermal conductivity, thermal expansion etc change with change of direction are known as anisotropic substances and the phenomenon is known as anisotropy” All the crystalline solids are anisotropic because of the proper arrangement of their particles.

Habit of a Crystal:

The crystals of a particular compound always for in a particular shape. This is known as habit of crystal.

or

The shape in which a crystal.

For example NaCl always forms cubic crystals when it saturated solution is cooled down.

Change of Habit:

In the presence of some impurities a crystal changes its shape and appears in a new shape. This is known as change of habit. For example, it there is some urea ( (NH2)2 CO) in the saturated solution of NaCl and it is cooled, then We get needle like crystals of instead of cubic crystals. Thus NaCl changes it habit. Here urea acts as impurity.

Crystal Lattice:

"The three dimensional arrangement of the particles (ions, atoms, or molecules) of a crystalline solid in space is known as crystal lattice or space lattice.” The arrangement of particles of a crystalline solid is usually three dimensional however sometimes it may also be one dimensional or two dimensional.

Lattice Energy:

The amount of energy required to separate the particles (ions, atoms or molecules) of one mole of a crystal lattice is known as lattice energy. or The amount of energy evolved when one mole of a solid crystal lattice is formed from its ions in gaseous state is known as lattice energy. L.E is denoted by “Ee” and is measured in the unit of KJ/mole or J/mole.

Unit Cell:

The smallest part of a crystal lattice that shows all the characteristic of the crystal lattice is known as unit cell. A crystal lattice is actually a combination of many unit cell. A unit cell has three unit cell lengths which are a, b & c. The angles b/w the unit cell lengths are known as “inter facial angles”.

Crystal System:

On the basis of difference in lengths a , b and c. different crystalline shapes are possible each one of which is known as a crystal system. There are seven crystal systems. Then are explained as follow.

(1) Cubic System:

In cubic crystals all the three sides a, b & c are equal.

i.e. a = b = c

& all angles are 90 degree.

For example KCl, MgO, NaCl etc are all cubic

(2) Teragonal system:

In this system two axes (slides) are of equal length while the third one is either longer or shorter, than the other two sides.

All the three angles are equal to 90 degree Celsius.

i.e.

a = b = c

and all angles are 90 degree. For example BaSO4, urea etc.

Determination of Avogadro No (NA):

Avogadro No is the no of particles (atoms, ions formula units, molecules etc) present in one mole of a substance. This No can be determined by several method one of the most accurate method for determination of Avogadro No. (NA) is based on the study of crystalline solids. To determine the Avogadro No. by this method, we should know:

1. Volume of one mole of a crystalline solid. It can e determined or calculated from the density of the solid.
2. The distance b/w the particles atoms in the crystal lattice of the crystalline solid.

The spacing (distance) b/w the particles of the solid can be measured with the help of x-rays. Following example will clear our idea.

Examples:

The density of LiF is 2.65 g/cm2 of alternate Li and F ions and the distance b/w these ions is 2.01 A. calculate Avogadro No.

Data:

Density of solid (LiF) = 2.65 g/cm3.

Distance b/w Li & F ions = 2.01 A = 2.01*10-8 cm

Avogadro NO (NA) = ?

AS formula mass of LiF = 6.639+18.99 = 25.9374amu.

Volume of one mole of LiF = mass/d = 25.9374/2.65 = 9.78 cm3

Now

Length of one of the cube = 3 9.788 cm3 = 2.139 cm

No of ions along this single edge = 2.139/2.01*10(-8) = 1.064*10(8) ions

Thus

Total No of ions in the cube (one mole LiF) = (1.064*10(8))3

Total No of ions in the cube (li+1 + F-1) = 1.2*10(24) ions

Now No of li+1ions or F-1 ion = 1.2*10(24)/2

Now No of li+1ions or F-1 ion = 6.023*10(23) ions

Thus it is proved that one mole of LiF (5.9374 g/mole) contain 6.023*10(23) Li ions 6.023*10(23)
F-1 ions and 6.023*10(23) LiF formula units which confirms Avogadro No.

Written by: Asad Hussain