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States of Matter
Introduction to Gaseous State
A given substance may occur in solid, liquid or gaseous phase depending upon the relative value of two tendencies namely Mutual Attraction (MA) and Escaping Tendency (ET)
Out of the three states of matter, the simplest one is the gaseous state.
The state is characterized by sensitivity of volume change with change of pressure and temperature. It is due to large distance between molecules as compared to their own dimensions. There exist weak Vander Waal's forces, molecules move independent of each other with speed about 400 ms–1.
Gases show maximum equality in their behaviour irrespective of their nature.
2. Measurable properties of gases
Def. The gases do possess mass. The mass of gas is generally used in the form of number of moles which is related as
(i) no. of moles
Two other useful formulae to calculate number of moles of gas are-
(ii) number of moles
(iii) no. of moles
When container contain more than one gas then molecular mass of mixture is termed as effective molecular mass (EMM) which is intermediate between molecular mass (EMM) which is intermediate between molecular masses of all the gases present in the container.
Effective molecular mass
Def. Volume of gas is nothing but volume of the container in which it is present.
Relation between different units of volume
Def. Degree of hotness or coldness of a body is measured by temperature
C – Celcius scale
K – Kelvin scale
F – Fahrenheit scale
Note: In all the problems of gaseous state (i.e in all gas law equations), temperature must be expressed in kelvin scale. i.e. ,
Def. Force acting per unit area
CGS : dyne/cm2
MKS : Newton/m2
Relation : 1 N/m2=10 dyne/cm2
Units of pressure:
1 atm = 76 cm of Hg
=760mm of Hg
=10.33 meters of H2o
Def. Mass per unit volume
Relation: 1 kg/m3=10-3 g/cm3
Density of gases
Absolute density Relative density
(Mass per unit volume) (Relative to hydrogen turned as vapour density)
Pressure, no. of moles
Note: Mass, volume and no. of moles are extensive
Properties that depend on mass hence then all
directly additive in nature.
Note: Density, pressure and temperature are intensive properties they does not depend on mass hence they are non-additive in nature.
3. The Gas Laws
(i) Boyle’s Law:
It relates the volume and the pressure of a given mass of a gas at constant temperature. Boyle’s law states that, “at constant temperature, the volume of a sample of a gas varies inversely with the pressure”.
(When temperature and number of moles are kept constant)
The proportionality can be changed into an equality by introducing a constant k, i.e. ,
Boyle’s law can be verified by any one of the following three ways graphically.
Alternatively, Boyle’s law can be stated as follows:
“Temperature remaining constant, the product of pressure and volume of a given mass of a given mass of a gas is constant”.
The value of the constant depends upon the amount of a gas and the temperature.
Mathematically, it can be written as.
(ii) Charles’ Law:
It relates the volume and temperature of a given mass of a gas at constant pressure.
For each degree change of temperature, the volume of a sample of a gas changes by the fraction of its volume 0℃.
Let the volume of a given amount of gas be at 0℃ . The temperature is increased by t℃ and the new volume becomes Thus,
Have definite volume
Have indefinite volume
Their shape is indefinite volume
Their shape is indefinite
Intermolecular attraction. Hence their shape as well as volume is fixed
Their force of attraction is intermediate between solids and gases. So shape is variable but volume is fixed.
They have very weak force of attraction. Hence molecules are very-very loosely connected with other and hence have neither shape nor volume
The outstanding macroscopic properties of gases are compressibility and fluidity. In contrast, the most noticeable macroscopic features of crystalline solids are rigidity, incompressibility and characteristic geometry. We shall find that the explanation of these macroscopic properties in terms of the atomic theory involves the idea of lattice: a permanent ordered arrangement of atoms held together by forces of considerable magnitude.
Thus the extremes of molecular behaviour occur in gases and solids. In the former we have molecular chaos and vanishing intermolecular forces, and in the latter we have an ordered arrangement in which the interatomic forces are large.
Types of Solids
(a) Crystalline solids: In a single crystal the regularity of arrangement of the pattern extends throughout the solid and all points are completely equivalent.
(b) Amorphous solids: An amorphous solid differs from a crystalline substance in being without any shape of its own and has a completely random particle arrangements, i.e. no regular arrangement. Example: Glass, Plastic
Classification of crystalline
Molecular solids: Those solids which consists of small molecules are called molecular solids.
(1)Non - polar molecular solids: (electronegativity sequence (F > O > N . Cl > Br > S > C . H)
The solids which have zero dipole moment are called non-polar molecular solids. The molecules are held together by weak Vander waal’s forces. Hence they are either gas or liquids at room temperature.
They are poor conductor of electricity due to their non-polar nature.
(2)Polar molecular solids:
Those solids which have non-zero dipole moments are called polar molecular solids. Polar molecular solids have dipole - dipole interaction, which is slightly stronger than vander waal’s force and hence they have larger melting and boiling points than the non - polar molecular solids.
Example: Solid CO2, Solid NH3
They are generally liquids or gases at room temperature.
(3) Hydrogen bonded molecular solids:
Those molecular solids which are bonded with each other by hydrogen bonds are called hydrogen bonded molecular solids.
They are non-conductor of electricity
Generally they are liquid at room temperature or soft solids.
IONIC SOLIDS: All those solids whose constituent particles are ions are called ionic solids,
Example: NaCl, CsBr, AgBr, CsCl
All those solids which are bonded by metallic bonds are called metallic solids. Inner core electrons are immobile Metallic solids show a great electrical conductivity due to availability of large number of free electrons whose movements constitute electric current. Metallic solids are malleable and ductile. Metallic solids have lustre. Metallic solids have good thermal conductivity.
COVALENT SOLIDS OR NETWORKED SOLIDS:
Whenever electric field is applied, electrons move between the layers. Graphite is a good conductor of electricity due to availability of free electrons. Networked solids are hard and brittle. Carbon - carbon bond has got partial double bond character in graphite. Two layers in graphite are attached with each other by weak vander waal force. Graphite can be used as a lubricant at high temperatures.
What is a crystal?
A crystalline solid consist of a large number of small units, called crystals, each of which possesses a definite geometric shape bounded by plane faces. The crystals of a given substance produced under a definite set of conditions are always of the same shape.
In this topic we would be studying certain properties of a solid which depend only on the constituents of the solid and the pattern of arrangement of these constituents. To study these properties, it would be convenient to take up as small amount of the solid as possible because this would ensure that we deal with only the minimum number of atoms or ions. This smallest amount of the solid whose properties resemble the properties of the entire solid irrespective of the amount taken is called a unit cell. It is the smallest repeating unit of the solid. Any amount of the solid can be constructed by simply putting as many unit cells as required.
Bravais (1848) showed from geometrical considerations that there are only seven shapes in which unit cells can exist. These are:
Moreover he also showed that there are basically four types of unit cells depending on the manner in which they are arranged in a given shape. These are: Primitive, Body Centered, Face Centered and End Centered. He also went on to postulate that out of the possible twenty eight unit cells (i.e. seven shapes * four types in each shape = 28 possible unit cells), only fourteen actually would exist. These he postulated based only on symmetry considerations. These fourteen unit cells that actually exist are called Bravais Lattices.
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