GENERAL CHEMISTRY – IV 18BCH43C
• Unit – I
• The nature of Cohesive forces in liquids and solids.
• Amorphous and Crystalline Solids – Differences – Isotropy – Anisotropy.
• The Symmetry in Crystals-Bravais lattices. , Laws of Rational indices – Miller indices,
• X-ray studies of crystal -Bragg’s Equation . Imperfections in Crystals – Point defects – Stoichiometric-Schotky and Frenkel defects and Non-stoichiometric defects-metal excess defects (colour centre)and metal deficiency defects.
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The nature of Cohesive forces in liquids and solids.
In liquids -Cohesion is the intermolecular attraction between like molecules (in mercury)and Adhesion is an attraction between unlike molecules (water and glass)
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Strong intermolecular forces, High surface tension
Viscosity is a measure of a fluid’s resistance to flow. Strong intermolecular forces, High Viscosity
Classification of solids: crystalline solids and amorphous solids.
A crystalline solid is a substance whose constituent particles possess regular orderly arrangement e.g. Sodium chloride, sucrose, diamond etc.
An amorphous solid is a substance whose constituent particles do not possess a regular orderly arrangement e.g. glass, plastics, rubber, starch, and proteins.
Though amorphous solids do not possess long range regularity, in some cases they may possess small regions of orderly arrangement. These crystalline parts of an otherwise amorphous solid are known as crystallites.
Amorphous substances are also, sometimes, referred to as super cooled liquids because they posses disorderly arrangement like liquids. In fact many amorphous solids such as glass are capable flowing. Careful examination of the window panes of very old houses reveals that the panes are thicker at the bottom than at the top because the glass has flown under constant influence of gravity.
Uses of Amorphous SolidsAmorphous solids such as glass and plastics are very important materials and are widely used in construction, house ware, laboratory ware etc.
Amorphous silica is likely to be the best material for converting sunlight into electricity (photovoltaic). - amorphous solid is rubber which is used in making tyres shoes soles etc.
Surface tension
Distinction between Crystalline and Amorphous Solids
S.No Crystalline solids Amorphous solids
1 The internal arrangement of particles is regular so they possess definite and regular geometry
1.The internal arrangement of particles is irregular. Thus they do not have any definite geometry.
2 They have sharp melting points They do not have sharp melting points
3 There is regularity in the external form when crystals are formed
There is no regularity in the external form when amorphous solids are formed
4 Crystalline solids give a regular cut when cut with a sharp – edged knife
Amorphous solids give irregular cut.
5 They have characteristic heat of fusion. They do not have characteristic heat of fusion.
6 Crystalline solids are rigid and their shape is not distorted by mild distorting forces
Amorphous solid are not very rigid.
These can be distorted by bending or compressing forces.
7 Crystalline solids are regarded as true solids
Amorphous solids are regarded as super cooled liquids or pseudo solids 8 Crystalline solids are anisotropic. This
implies that physical properties such as refractive index, conductivity, thermal expansion etc are different in different directions. This is due to orderly arrangement of particles
Amorphous solids are isotropic in nature. This implies that various physical properties are same in all the directions. This is because of random arrangement of particles.
Classification of Crystalline Solids based on Different Binding Forces
Crystalline solids can be classified into different categories depending upon the type of constituent particles and the nature of attractive forces operating between them.
Atomic Solids
In these solids the constituent particles are atoms. These closely packed atoms are held up by London dispersion forces. Some examples are crystals of noble gases. Such solids are very
soft, possess very low melting points and poor conductors of heat and e lectricity.
Molecular Solids
In these solids, the constituent particles which pack up together are molecules of the substance. These molecules may be non – polar (dipole moment = 0) such as etc. or they may be polar (dipole moment > 0) like etc.
In case of non – polar molecules, the attractive forces operating between the molecules are Vander Waal forces (also called dispersion forces). The example of such solids are : dry ice (Solid, iodine (crystals).
In case of polar molecules, the attractive forces operating between the molecules in solid state are dipole – dipole forces. The examples of such solids are : solid, solid HCl. However, in some solids with polar molecules, the interparticle forces are hydrogen bonds. The examples of such solids are ice; solid hydrogen fluoride (HF); solid ammonia, etc.
Characteristics of Molecular Solids
Some of the general characteristics of molecular solids are :
They are generally soft.
Their melting points are low to moderately high. The melting points of solids with non – polar molecules are relatively low whereas solids with polar molecules have moderately high melting points.
They are generally bad conductors of heat and electricity.
They have generally low density.
Ionic Solids
In ionic solids, the constituent particles are ions of opposite charges. Each ion is surrounded by a definite number of ions of opposite charge.and the interparticle forces in ionic solids are ionic bonds operating between the ions of opposite charges
The number of ions that surround a particular ion of opposite charge its called co – ordination number of the ion. For example, in sodium chloride crystal each sodium ion is surrounded by six chloride ions. Hence coordination number of is 6. At the same time each chloride ion is surrounded by six ions. Therefore the co – ordination number of ion is also 6. However, in calcium fluoride crystal each ion is surrounded by eight fluoride ions and each calcium ion is surrounded by four ions.Thus, in crystal some examples of ionic solids are : sodium chloride (NaCl) ; ceasium chloride (CsCl), zinc sulphide (ZnS), calcium fluoride, etc.
Characteristics of Ionic Solids
They are hard, brittle and have low volatility.
They have high melting points.
They are poor conductors of electricity in solid state, however they become good conductors of electricity in molten state or in dissolved state.
They are generally soluble in polar solvents like water.
Covalent SolidsIn these types of solids the constituent particles are atoms of same or different elements connected to each other by covalent bond network.
For example, in diamond only carbon atoms constitute the covalent network while carborundum covalent bond network is constituted by silicon and carbon atoms. the interparticle forces operating in these solids are covalent bonds. Some examples of covalent solids are :
Diamond, silicon carbide, aluminium nitrite etc.
Characteristics of Covalent Solids
They are very hard. Diamond is the hardest naturally occurring substance.
They have very high melting points.-They are poor conductors of heat and electricity.
They have high heats of fusion.
Metallic Solids
In this solids the constituent particles are metal atoms. The interparticle forces in these solids are metallic bonds. In the metallic crystals the metal atoms occupy the fixed positions but their valence electrons are mobile.
The close packed assembly of metal kernels (part of metal atom without valence elec trons) remain immersed in the sea of mobile valence electrons. The attractive force between the kernels and mobile valence electrons is termed as metallic bond.
Characteristics of Metallic Solids
The common characteristics of metallic solids are as follows:
They generally range from soft to very hard.
They are malleable and ductile.
They are good conductors of heat and electricity.
They possess bright lustre.
They have high melting and boiling points.
They have moderate heats of fusion.
Isotropic minerals have the same chemical bonding in every direction, and consequently have the same refractive index in every direction. ... Anisotropic minerals have different chemical bonds in different directions and consequently have different refractive indices in different directions.
CRYSTAL DEFECTS
What ae the reasons for defects in crystals ?
Crystal defect, imperfection in the regular geometrical arrangement of the atoms in
a crystalline solid. These imperfections result from deformation of the solid, rapid cooling from high temperature, or high-energy radiation (X-rays or neutrons) striking the solid.
There are mainly two types of defects:-
1. Point defect:-When the deviation or irregularities exists from ideal
arrangement around a point or an atom in a crystalline substance the defect is called the point defect.
2. Line defect:- When the deviation from the ideal arrangement exists in the
entire row of lattice points the defect is called as line defect.
Types of the point defects:-
1.
Stoichiometric defects 2. Non stoichiometric defects 3. Impurity defects
Stoichiometric defect:-
If imperfection in the crystal are such that the ratio between cation and onions remains same. Stoichiometry of substance do not disturbed defect is called stoichiometric defect these defects are of the following types.
1. Vacancy defect:-
When is in a crystalline substance, some of the lattice sites are vacant the crystal is said to have vacancy defect it results in decrease of density of substance.
2. Interstitial defect:-
When some extra constituent particles are present in the interstitial site the crystal is said to be have interstitial defect.
This defect increases density of the crystal
These above types of defects are shown only by non – ionic solids.
3. Schottky defect:-
If in an ionic crystal of the type A+B- equal number of cations and anions are missing from the lattice site. So that electrical neutrality is remained is called Schottky Defect.
Compounds exhibiting schottky defect are NaCl, KCl
Which compounds have small difference in size of cation and anions show defect.
Frenkel defect:- If an ion is missing, from its lattice site and is occupies the interstitial site, electrical neutrality as well as Stoichiometry of the compound are maintained this type of defect is called Frenkel Defect. It is also called dislocation defect.
Example:-
(AgCl, AgBr, AgI, ZnS) shows this defect which have a large difference in size of
cations and anions.
Non stoichiometric defects:-
If imperfections in the crystal id due to ratio f the cations and anions becomes different from that indicated by ideal chemical formula.
The defects are called non-stoichiometric defects.
There are two types non-stochimetric defects:- i. Metal excess defects:-example in NaCl
A negative ion Cl
-ion may be missing from its lattice site, leaving a hole which is occupied by an electron, there by maintain the electrical neutrality.
The interstitial sites containing the electron thus trapped in the anion vacancies are called the F – centers. They are responsible for imparting colour to the crystals.
Example:- When NaCl is heated in an atmosphere of Na vapours. The excess
of Na atoms deposition the surface of NaCl crystal Cl- ions then diffuse to the surface where the combine with Na+ ions which becomes due to losing electrons.
These electrons loses by Na atom are diffuse back into the crystal and occupy the vacant site created by Cl- ions and imparts Yellow colour to NaCl crystal
Probable questions
Why does NaCl appears yellow colour?
Yellow colour of NaCl
. By presence of extra cation in interstitial sites:-
• Metal excess may also be caused by an extra cation occupying the interstial site. For example when ZnO is heated it loses oxygen and turns yellow due to following ZnO → Zn+2 + (1/2)O2 + 2e-
The excess interstitial sites and the electrons in neighbouring interstitial sites.
Questions:Why does Zinc salts appear yellow while hot and colourless when cold?
Metal deficiency defect:- This defect occurs when the metal shows the
variable valancy. Due to metal deficiency the compounds obtained are
non -stoichiometric. For example it is difficult to prepare ferrous oxide with ideal composition because Iron (Fe) exists as both Fe+2 and Fe+3 ions thus we obtain Fe
0.95O or Fe
xO where ( x = 0.93 to 0.96)
What is colour centre ? or What is F-centre in crystals?
Color centers can occur naturally in compounds (particularly metallic oxides)
because when heated to high temperature the ions become excited and are
displaced from their normal crystallographic positions, leaving behind some electrons in the vacated spaces.
F-center, Farbe center or color center is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more unpaired electrons. Electrons in such a vacancy tend to absorb light in the visible spectrum and becomes coloured,
Example NaCl.( explanation given above in metal excess)