TYPES AND PROPERTIES OF SOLIDS - FIRST YEAR CHEMISTRY

Types and Properties of Solids

Types of Solids

     There are four types of crystalline solids depending upon the type of bond present in them.

(i) Ionic Solids

(ii) Covalent Solids

(iii) Molecular Solids

(iv) Metallic Solids

Properties of Ionic Solids

(i) Definition: The crystalline solids in which the particles forming the crystal are positively and negatively charged ions which are held together by strong electrostatic forces of attraction (ionic bond) are called ionic solids. The crystals of NaCl, KBr etc. are ionic solids.

(ii) Physical State: The cations and anions are arranged in a well defined pattern, so they are crystalline solids at room temperature. Under ordinary conditions of temperature and pressure they never exist in the form of liquids or gases.    

(iii) Hardness, Volatility, Melting and Boiling Points: Ionic crystals are very stable compounds. Very high energy is required to separate the cations and anions from each other against the forces of attraction. That's why ionic solids are very hard, have low volatility and high melting and boiling points.

(iv) Nature of Ionic Solids: Ionic solids do not exist as individual neutral independent molecules. The cations and anions attract each other and these forces are non-directional. The close packing of the ions enables them to occupy minimum space. A crystal lattice is developed when the ions arrange themselves systematically in an alternate manner.

(v) Radius Ratio: The structure of ionic crystals depends upon the radius ratio of cations and anions.

Radius ratio = r+/r-

In NaCl; Na⁺ = 95 pm, Cl⁻ = 181 pm

Radius ratio = 95/181 = 0.525

NaCl and CsF have the same geometry because the radius ratio in both the cases is the same.

Properties of Molecular Solids

(i) Definition: Those solid substances in which the particles forming the crystals are polar or non-polar molecules or atoms of a substance are called molecular solids. Ice, sugar, Iodine (I2) and phosphorus (P4) are examples of molecular solids.

(ii) Regular Arrangement: X-rays analysis has shown the regular arrangement of atoms in the constituent molecules of these solids and we get the exact positions of all the atoms.

(iii) Softness: The forces, which hold the molecules together in the molecular crystals, are very weak. So they are soft and easily compressible.

(iv) Volatility, Melting and Boiling Points: They are mostly volatile and have low melting and boiling points.

(v) Conduction, Solubility and Density: They are bad conductors of electricity, have low densities and sometimes transparent to light. Polar molecular crystals are mostly soluble in polar solvents, while non-polar molecular crystals are usually soluble in non-polar solvents.

Notes Prepared By: Prof. Shahbaz Asghar

LIQUID CRYSTALS AND THEIR USES - FIRST YEAR CHEMISTRY

Liquid Crystal and Their Uses

Liquid Crystals

(i) Definition: The turbid liquid phase of a solid that exists in between the melting and clearing temperature is called liquid crystal. 

Crystal ⇋ Liquid Crystal ⇋ Liquid

(ii) Discovery: In 1888, Frederick Reinitzer, an Austrian botanist discovered the liquid crystals. He was studying an organic compound Cholesteryl Benzoate. This compound turns milky liquid at 145⁰C and becomes a clear liquid at 179⁰C. When the substance is cooled, the reverse process occurs. This turbid liquid state was called liquid crystal. 

(iii) Characteristics: Liquid crystals have both properties of liquids and crystals (solids). Liquid like properties include viscosity, surface tension, and fluidity etc. Crystal like properties include optical properties and molecules have some orderly arrangement. We can say that the properties of liquid crystals are intermediate between those of crystals and isotopic liquids. A crystalline solid may be isotopic or anisotropic but liquid crystals are always anisotropic. 

(iv) Types: Those substances which make the liquid crystals are often composed of long rod like molecules. In the normal liquid phase, these molecules are oriented in random directions. In liquid crystalline phase, they develop some ordering of molecules. Depending upon the nature of ordering, liquid crystals can be divided into; Nematic, Smectic and Cholesteric. 

Uses of Liquid Crystals

(i) As Temperature Sensor: Like solid crystals, liquid crystals can diffract light. When one of the wavelengths of white light is reflected from a liquid crystal, it appears coloured. As the temperature changes, the distances between the layers of the molecules of liquid crystals change. Therefore, the reflected light changes accordingly. Thus liquid crystals can be used as temperature sensors. 

(ii) To Find Potential Failure/As Room Thermometers: Liquid crystals are used to find the point of potential failure in electrical circuits. Room thermometers also contain liquid crystals with a suitable temperature range. As the temperature changes, figures show up in different colours. 

(iii) Medical Diagnosis: Liquid crystals are used to locate veins, arteries, infections and tumors. The reason is that these parts of the body are warmer than the surrounding tissues. Specialists can use the techniques of skin thermography to detect the blockages in veins and arteries. When a layer of liquid crystal is painted on the surface of a breast, a tumor shows up as a hot area which is coloured blue. This technique has been successful in the early diagnosis of breast cancer. 

(iv) Electrical Devices: Liquid crystals are used in the display of electrical devices such as digital watches, calculators and laptop computers. These devices operate due to the fact that temperature, pressure and electromagnetic fields easily affect the weak bonds, which hold molecules together in liquid crystals. 

(v) Solvents in Chromatography: In chromatographic separation, liquids crystals are used as solvents. 

(vi) Oscillograph and TV Displays: Oscillograph and TV displays also use liquid crystal screens. 

Notes Prepared By: Prof. Shahbaz Asghar

PLASMA, ITS FORMATION AND TYPES - FIRST YEAR CHEMISTRY

Plasma, Its Formation and Types

Plasma

• Plasma is a hot ionized gas consisting of approximately equal numbers of positively charged ions and negatively charged electrons. 
• Plasma is often called "fourth state of matter", the other three being solid, liquid and gas. 
• Plasma was identified by the English scientist William Crookes in 1879. 
• Plasma is estimated to constitute more than 99 percent of the visible universe. 
• Although, naturally occurring plasma is rare on earth, there are many man-made examples.  
• It occurs only in lightning discharges and in artificial devices like fluorescent lights, neon signs etc. 
• It is everywhere in our space environment. All the stars that shine are all plasma. 

 How is Plasma Formed?

     When more heat is supplied, the atoms are molecules may be ionized. An electron may gain enough energy to escape its atom. This atom loses one electron and develops a net positive charge. It becomes and ion. In a sufficiently heated gas, ionization happens many times, creating clouds of free electrons and ions. However, all the atoms are not necessarily ionized, and some of them may remain completely intact with no net charge. This ionized gas mixture, consisting of ions, electrons and neutral atoms is called plasma. 

     It means that plasma is a distinct state of matter containing a significant number of electrically charged particles, a number sufficient to affect its electrical properties and behaviour. 

Types of Plasma

     There are two types of plasma, artificial plasma and natural plasma.

(i) Artificial Plasma: Artificial plasma can be created by ionization of a gas, as in neon signs. Plasma at low temperatures is hard to maintain outside a vacuum, low temperature plasma reacts rapidly with any molecule it encounters. This aspect makes this material, very useful and hard to use. 

(ii) Natural Plasma: Natural plasma exits only at very high temperatures, or low temperature vacuums. Natural plasma does not breakdown or react rapidly, but is extremely hot (over 20,000⁰C minimum). Their energy is so high that they vaporize any material they touch. 

Notes Prepared By: Prof. Shahbaz Asghar

VOLUME AND PRESSURE CORRECTION BY VAN DER WAALS'S EQUATION - FIRST YEAR CHEMISTRY

Volume and Pressure Correction By Van der Waals's Equation

     Van der Waals pointed out that both volume and pressure factors in ideal gas equation needed correction to make it applicable to the real gases.

Volume Correction

(i) Compression of a Gas: When a gas is compressed, the molecules are pushed so close together that the repulsive forces operate between them. When pressure is increased further, it is opposed by the molecules themselves. Actually, the molecules have definite volume, no doubt, very small as compared to the vessel, but it is not negligible.

(ii) Van der Waals' Postulate: Van der Waals postulated that the actual volume of molecules can no longer be neglected in a highly compressed gas. If the effective volume of the molecules per mole of a gas is represented by b, then the volume available to gas molecules is the volume of the vessel minus the volume of gas molecules.

Vfree = Vvessel - b (where Vfree is the volume available to gas molecules)

(iii) Excluded Volume "b": The volume of a gas which is occupied by 1 mole of gas molecules in highly compressed state, but not in the liquid state, is called excluded volume or effective volume or incompressible  volume (b). It is a constant and characteristic of a gas. It value depends upon the size of the molecules. It is also a Van der Waals constant. It is not equal to the actual volume of gas molecules. In fact, it is four times the actual volume of molecules.

b = 4Vm (where Vm is actual volume of one mole of gas molecules. 

Pressure Correction

(i) Attraction Between Molecules: A molecule in the interior of a gas is attracted by other molecules on all sides, so these attractive forces are cancelled out. However, when a molecule strikes the wall of a container, it experiences a force of attraction towards the other molecules in the gas. The decreases the force of its impact on the wall.

(ii) Pressure on the Wall of Container: Consider a molecule "A" which is unable to create pressure on the wall due to the pressure of attractive forces due to "B" type molecule. Let the observed pressure on the wall of the container is "P". The pressure is less than the actual pressure Pi by an amount P'. So,

P = Pi - P'

Where Pi is the true kinetic pressure, if the forces of attraction would have been absent. P' is the amount of pressure lessened due to attractive forces. Ideal pressure Pi is;

Pi = P + P'     ..... (i)

It is suggested that a part of the pressure "P" for one mole of a gas used up against inter-molecular attractions should decrease as volume increases.  

(iii) Value of P': The value of P' in terms of a constant "a" which accounts for the attractive forces and the volume V of vessel can be written as:

P' = a/V² ..... (ii) 

Greater the attractive forces among the gas molecules, smaller the volume of vessel, greater the value of lessened pressure P'. "a" is a coefficient of attraction or attraction per unit volume. It has a constant value for a particular real gas. 

(iv) Value of Pi: Putting the value of P' from equation (ii) into (i)

Pi = P + a/V²

(v) For One Mole of a Gas:

(vi) For "n" Moles of a Gas

     This is called van der Waals's equation. "a" and "b" are called van der Waals's constants. 

(vii) Common and SI Units of "a" and "b"

Notes Prepared By: Prof. Shahbaz Asghar

KINETIC MOLECULAR THEORY OF GASES (KMT) - FIRST YEAR CHEMISTRY

Kinetic Molecular Theory of Gases (KMT)

Definition

     A set of postulates that describe the nature and behaviour of an ideal gas is called kinetic molecular theory of gases.

Fundamental Postulates

1. Every gas consists of a large number of very small particles called molecules. Gases like He, Ne, Ar have mono-atomic molecules. 

2. The molecules of a gas move haphazardly, colliding among themselves and with the walls of the container and change their directions. 

3. The pressure exerted by a gas is due to the collision of its molecules with the walls of a container. The collisions among the molecules are perfectly elastic.

4. The molecules of a gas are widely separated from one another and there are sufficient empty spaces among them. 

5. The molecules of a gas have no forces of attraction for each other. 

6. The actual volume of molecules of a gas is negligible as compared to the volume of the gas. 

7. The motion imparted to the molecules by gravity is negligible as compared to the effect of the continued collisions between them. 

8. The average kinetic energy of the gas molecules varies directly as the absolute temperature of the gas. 

Notes Prepared By: Prof. Shahbaz Asghar

KINETIC MOLECULAR THEORY OF GASES (KMT) - FIRST YEAR CHEMISTRY

Kinetic Molecular Theory of Gases (KMT)

Definition

     A set of postulates that describe the nature and behaviour of an ideal gas is called kinetic molecular theory of gases.

Fundamental Postulates

1. Every gas consists of a large number of very small particles called molecules. Gases like He, Ne, Ar have mono-atomic molecules. 

2. The molecules of a gas move haphazardly, colliding among themselves and with the walls of the container and change their directions. 

3. The pressure exerted by a gas is due to the collision of its molecules with the walls of a container. The collisions among the molecules are perfectly elastic.

4. The molecules of a gas are widely separated from one another and there are sufficient empty spaces among them. 

5. The molecules of a gas have no forces of attraction for each other. 

6. The actual volume of molecules of a gas is negligible as compared to the volume of the gas. 

7. The motion imparted to the molecules by gravity is negligible as compared to the effect of the continued collisions between them. 

8. The average kinetic energy of the gas molecules varies directly as the absolute temperature of the gas. 

Notes Prepared By: Prof. Shahbaz Asghar

GRAHAM'S LAW OF DIFFUSION OF GASES - FIRST YEAR CHEMISTRY

Graham's Law of Diffusion of Gases

Statement

    The rate of diffusion or effusion of a gas is inversely proportional to the square root of its density at constant temperature and pressure.

Mathematical Form

        Let us have two gases 1 and 2, having rates of diffusion as r₁ and r₂ and densities as d₁ and d₂ respectively.

        According to Graham's law,

Demonstration of Graham's Law

     This law can be verified in laboratory by noting the rates of diffusion of two gases in a glass tube, when they are allowed to move from opposite ends.

     Two cotton plugs soaked in HCl and NH3 solutions are introduced in the open ends of 100 cm long tube simultaneously. HCl molecules travel a distance of 40.5 cm and NH3 molecules cover 59.5 cm. They produce dense white fumes of NH4Cl at point of junction. So, 

                 1.46 = 1.46

Hence, the law is verified. 

Notes Prepared By: Prof. Shahbaz Asghar