BOND ENERGY (BOND ENTHALPY)

 BOND ENERGY (BOND ENTHALPY):

•Definition:

"The energy required to break all the bonds present in one mole of a substance is called bond energy or bond enthalpy".

"It is the energy required to break up an Avogadro number of bonds i.e one mole of bond".

• Unit:

It has unit kJmol-1.

• Information:

The strength of a bond is measured by its bond energy. The larger the bond energy, the stronger is the bond.

• Measurement:

It is determined experimentally by measuring the heat involved in a chemical reaction. It is also called bond enthalpy, as it is measure of enthalpy change at 298K.

•FACTORS:

• Electronegativity difference:

Greater the electronegativity difference,  greater will be the bond energy because of more attraction. So a polar covalent bond is stronger than a non polar covalent bond.

• Atomic character:

Greater the electronegativity difference, greater will be the ionic character and greater will be the bond energy because of more attraction. For example, H-F bond energy is 567 kJmol-1 but H-Cl bond energy is 431 kJmol-1.

• Size of atoms:

Greater size of atoms, smaller Will be the bond energy because of the weak intermolecular forces. For example, the bond energy of H-H bond is 435 kJmol-1 and that Cl-Cl is 243 kJmol-1. It is due to the shorter bond length in H2 molecule and greater bond length in Cl2 molecule.

• Bond length: 

Greater the size of atoms, greater will be bond length and smaller Will be the bond energy because of weak intermolecular forces. For example, bond energy of Cl-Cl is 242 kJmol-1 and that of I-I is 151 kJmol-1.

• Bond order:

It is found that energies of multiple bonds are greater than those of single bonds. But a double bond is not twice as strong as a single bond or a triple bond is not thrice as strong as a single bond. It means that a sigma bond is greater than a pi bond.

VALENCE BOND THEORY (VBT)and Its DRAWBACKS

 VALENCE BOND THEORY (VBT):

 This theory was proposed by Heitler and London (1927) and later on was developed by Pauling. This concept is based on wave-mechanical treatment of molecules and has been successful in explaining the bond energies, bond lengths and shapes of covalent molecules.

• BASIC ASSUMPTION:

According to Valence bond theory, a covalent bond between two atoms is formed when the atoms come so close that a partially filled atomic of one atom overlap with a partially filled atomic orbital of the other. The two overlapping atomic orbitals retain their identity.

~POSTULATES OF VBT:

• Overlapping:

A bond formed between two atoms is formed by the overlap of half filled atomic orbitals of two atoms.

• Identity:

The two overlapping atomic orbitals retain their identity.

• Opposite spin:

The two overlapping orbitals must have electrons of opposite spin.

• Unpaired electrons:

The number of bonds formed by an atom is equal to the number of unpaired electrons present in the Valence shell of the atom.

• Single and multiple bonds:

If only two orbitals overlap, a single bond is formed. If additional orbitals of atoms overlap then multiple bonds are formed.

• Symmetry:

In order to form a bond, the overlapping orbitals must have same symmetry with respect to the bond axis.

• Energy:

In overlapping of orbitals energy is released. The greater the overlap between the orbitals, the greater is the energy released and the stronger will be the bond formed.

"BOND AXIS"

" A line joining the nuclei of two bonded atoms is called bond axis".

•>DRAWBACKS OF VALENCE BOND THEORY:

It fails to explain;

1. The formation of coordinate covalent bond.

2. The formation of odd electron molecules or ions.

3. The para magnetic behaviour of oxygen molecule.

KINETIC MOLECULAR THEORY

 KINETIC MOLECULAR THEORY:

The Kinetic Molecular Theory was first proposed by Daniel Bernoulli, a Swiss mathematician. The Kinetic Molecular Theory of gases was elaborated and extended by a number of well known physicist such as James Maxwell (1859) and Boltzmann in (1870). In 1857, Clausius derived the Kinetic equation and deduced all the gas laws from it.

• POSTULATES OF KINETIC MOLECULAR THEORY:

The Kinetic Molecular Theory of gases is based upon following postulates.

• Molecules:

Gases are considered to be composed to minute discreet particles called molecules.

• Mass and size of molecules:

The molecules of a gas are thought to be of the Same mass and size but are different from gas to gas.

• Random motion:

The gas molecules move randomly in the space available and collide with each other and with the walls of container.

• Elastic collision:

When molecules collide with each other, the collisions are perfectly elastic i.e the total kinetic energy remains constant.

• Pressure:

The pressure of gases is due to the elastic collisions of gas molecules with the walls of container.

• Intermolecular distances:

At relatively low pressure, the average distance between the molecules are large as compared with molecular diameters.

• Intermolecular forces:

There are no attractive or repulsive forces between the two.

• Actual volume:

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

• Kinetic energy:

The average kinetic energy of a molecule is directly proportional to the absolute temperature.

TRENDS IN ATOMIC RADIUS

 TRENDS IN ATOMIC RADIUS:

•Definition: "The distance of outermost electrons from the nucleus of an atom while considering it spherical is called atomic radius"

Trend:

Along the period:

•The size of atoms and ions decreases from left to right along with period. In a given period, the number of shells in all the elements remains the same. 

Reason:

Reason is that the value of effective nuclear nuclear charge increases from left to right due to increase in number of protons. The increased effective nuclear charge pulls the electron cloud of atom closer to the nucleus. Thus, the size of the atoms and ions goes on decreasing from left to right.

•In any period, the alkali metals are present at the extreme left of the periodic table. Hence, they have largest size in any period.

•In any period, the halogens are present at the extreme right of the periodic table (excluding noble gases). Hence, they have smallest size in any period.

Along the group:

Atomic radius increase when we move from top to bottom.

Reason :Reason is that when we move from top to bottom no of shell increases.

Example: Sodium is larger in size then lithium because lithium has 2 shells while sodium has three shells.

 Structure of lithium

                       Structure of sodium 

IONIZATION ENERGY

IONIZATION ENERGY:

•Definition:

"The first ionization energy is  the energy required to remove the most loosely held electron from 1 mole of gaseous atoms to produce 1 mole of gaseous ions(+1).

   A(g)----------> A+(g) + 1e

•FACTORS AFFECTING IONIZATION ENERGY:

The first ionization energy depends upon following factors;

1. Atomic size is inversely proportional to ionization energy.

2. Nuclear charge is also inversely proportional to ionization energy.

3. Shielding/screening effect: It is also inversely proportional to ionization energy.

4. Nature of orbital: It is difficult to remove electron from s orbital then p orbital .The order for ionization energy in different orbital is;

   s>p>d>f

TRENDS:

Along the period:

•Generally, the ionization energy of the elements increases from left to right in periodic table due to successive increase in the nuclear Charge and decrease in atomic size.

•However, some irregular trends in period 3 are present.e.g. ionization energy of Mg and P are higher than those of Al and S respectively.

Along the group:

Generally, the ionization energy of the elements decrease down the group in periodic table due to successive increase in the no of shells and increase  in atomic size.

Differences between sigma and pi bond

 Sigma bond:

1.It is a type of bond formed by head on collision of orbitals.

2.It is a strong bond.

3.In sigma bond elctron density is on the line joining two nuclie.

4.Ist bond is always sigma bond .

5.Only one sigma bond is possible btween two atoms.

6.Example Its example is hydrogen molecule.H-----H

Pi bond:

1.It is a type of bond formed by side ways overlap .

2.It is weak bond.

3.In pi bond electron density is above or below the bond axis.

4.More then one pi bonds are possible between two nuclie.

5.2nd and 3rd bond is pi bond .

6.Example :Its example is oxygen molecule

O::::O

HYDROPHILIC AND HYDROPHOBIC MOLECULES

HYDROPHILIC AND HYDROPHOBIC MOLECULES:

•HYDROPHILIC MOLECULES:

"Compounds that are miscible with water are known as hydrophilic molecules."

.Hydro means"water" and philic means"loving"

.So hydrophilic means water loving

.Such molecules form interaction with water molecules such as hydrogen bond. 

.For example, molecules of sodium chloride, glucose, methanol, acetone,etc. are called hydrophilic molecules.

•HYDROPHOBIC MOLECULES:

Molecules that do not dissolve in water are called hydrophobic molecules.

Hydro means "water" and phobic means " repellent". 

So hydrophobic molecules are water repellent. 

Such molecules do not show interaction with water molecule. 

These molecules are like benzene, cyclo pentanes, oils, etc. that doesn't dissolve in water because these molecules are non-polar in nature.

DIFFERENCE BETWEEN TEMPERATURE AND HEAT

 DIFFERENCE BETWEEN TEMPERATURE AND HEAT:

•TEMPERATURE:

The measure of average kinetic energy of all the particles of a system is called temperature.

It represents the kinetic energy of the molecules of the system.

It is independent of the quantity of the substance.

It is measured without the help of heat.

It is a state function.

•HEAT:

The property which flows from the hotter part of the body to the cooler part is called heat.

It is the measure of total energy in a given amount of substance.

It depends upon the quantity of the substance.

It is measured with the help of temperature.

It is not a state function because it is not the property of system.

HAIR DYE

 HAIR DYE:

•RAW MATERIAL:

In general,hair dyes include:

1. Dyes e.g 4-amino-2-hydroxytoluene

2. Modifiers e.g.resorcinol

3. Antioxidants e.g. sodium sulfite

4. Alkalizers e.g. ammonium hydroxide

Beyond these chemicals, many different chemical are used to impart special qualities. They may be shampoos, fragrances, chemicals that make the formula creamy,foamy,or thick, or contribute to the overall action of the formula.

•DIFFERENT TYPES OF HAIR DYE:

There are three types of hair dyes.

•Permanent: Causes some lightening of hair, contains 3-6% hydrogen peroxide, contains oxidative dyes that form during application, does not wash out significantly but may fade.

•Semi-permanent: causes mild lightening of hair, contains 1-2% hydrogen peroxide, contains oxidative dyes that form during application, last around 6 weeks.

•Temporary  dye(direct dye): no lightening of hair, does not contain peroxide, contains dye that are already formed before application, washes out in a few washes.

DIFFERENCE BETWEEN VALENCE BOND THEORY AND MOLECULAR ORBITAL THEORY

 DIFFERENCE BETWEEN VALENCE BOND THEORY AND MOLECULAR ORBITAL THEORY:

•VALENCE BOND THEORY:

(1)Only Valence shell electrons are involved in bond formation.

(2) The two concerned atoms don't lose their individual identity.

(3) It doesn't explain the Para magnetic behaviour of molecules.

(4) It doesn't give idea about bond order.

(5) It doesn't explain the non existence of He2 molecule.

•MOLECULAR ORBITAL THEORY:

(1) All the electrons of interacting atoms are involved in bond formation.

(2) The two atoms lose their individual identity.

(3) It explain the Para magnetic behaviour.

(4) It give idea about bond order by which we can determine that whether the bond is single, double or triple.

(5) It explain the non existence of He2 molecule.

Adsorption chromatography*and Column chromatography

 *Adsorption chromatography*

The chromatographic separation of the components in a mixture by absorption from a mobile liquid stream onto the surface of a powdered solid as a stationary phase is called adorption chromatography

*Column chromatography*

It is a type of adsoprtion chromatography.

Basic principle:

The principle behind column chromatography is adsorption, in which a mixture of components dissolved in the mobile phase is introduced in to the column and the components move depending on their relative affinities.

Factors:

The experiment factor of column chromatography are (I) the nature of adsorbent ..( ii ) the nature of solvent  ( iii ) the operating parameters 

1.. nature of adsorbent 

An adsorbent is usually a finely divided solid possessing an extremely large surface area . The characteristics of a solid that must b considered when selecting an adsorbent to effect the chromatographic seprations are ..

1  surface area ___ preferably 50m^2/g 

 2.  Particle size 100 -200 mesh (150-75) provide a minimum resistance to flow

3.  Surface activity --- higher surface activity of adsorbent will lower the separation efficiency..

2.. Nature of solvent 

The solvent in which a sample is dissolved will complete with the components of the sample for surface sites upon which to b absorbed . A solvent which is strongly adsorbed in the surface will act as a strong eluent and will displace solutes easily and thus results in shorter retention time of solutes

3.. Nature of solute

The most important feature of an adsorbent chromatography separation is the extent of separation of the components of the solute sample for a given solvent adsorbent system 

4.. Operating parameters.

The effect of general experimental conditions such as method of packing , flow rate and temperature are important for the effiency of a column adsorption 

(a) packing

Technique if column packing is important . One common method is to pour dry powered solid slowly onto the top of the glass column . The column is gently tapped or vibrated till the solid settles

(b) Flow rate

The rate of flow rate of solvent through the column is very important in achieving successful chromatographic seprations . If the flow rate is too rapid , there will b issuffient time for the adsorption..

(c) Temperature

Increasing the temperature of the column will cause a slight decrease in the adsorption of all the components into sample  . This decrease is on the order of 1-2% per degree centigrade

Differences between Fission and Fusion reaction

*Nuclear fission* 

1- Heavy nuclei split into lighter nuclei of comparable atomic masses

2- nuclei of heavy elements undergo fission

3- The reaction initiates at normal temperature

4- The fission process liberates about 200 MeV energy

5- Fast neutrons are also released

6- Fuel is either solid or liquid 

*Nuclear fusion..*

1- lighter nuclei fuse to form heavy nuclei

2- nuclei of light elements are in involved in the process of fusion

3- Fusion initiates at 10^8 K

4- the energy released in 24 MeV

5- nature of the ejected particles depends uopn the type of thermonuclear reaction

6- Fuel is in plasma state

DEFFECTS OF BOHR'S ATOMIC MODEL

 DEFFECTS OF BOHR'S ATOMIC MODEL:

◉MONO-ELECTRONIC SYSTEM:

Bohr's theory can not explain the origin of the spectrum of multi electrons or poly electronic system like He, Li,Be. 

◉HIGH RESOLVING SPECTROMETER:

When a spectrum of hydrogen gas is seen through a powerful spectrometer, the original spectral lines are replaced by several very fine lines i.e, original lines are divided into other fine lines. These lines cannot be explained by Bohr's theory. 

◉THREE DIMENSIONAL MOTION OF ELECTRONS: 

Bohr suggested circular orbits of electrons around the nucleus of H-atom. But it is proved that the motion of electrons is not in a single plane but takes place in three dimensional space. 

◉ZEEMAN EFFECT:

When the excited atoms of hydrogen giving atomic emission spectrum is placed in a magnetic field, it's spectral lines further split up into closely spaced lines. This type of splitting up of spectral lines is called "Zeeman effect".Bohr's theory doesn't explain zeeman effect. 

◉STARK EFFECT:

When the excited hydrogen atom giving atomic emission spectrum is placed in a strong electric field, its spectral lines further split up into closely spaced lines. This type of splitting up of spectral lines is called " stark effect". Bohr's theory doesn't explain stark effect

Le Chatelier's principle

                Le-chatlier Principle

Defination: If you imppose a change on a system that is at equilibrium system behave in a way as to oppose that change.

Change in concentration :If concentration of reactant is increased reaction will move in farward direction towards product to nuteralize the effect of that change. 

If concentration of product is increased reaction will move in backward direction. For example consider a reversable reaction 

H2 + I2  <=====> 2HI 

In this reaction if concentration of Hydrogen and Iodine is increased reaction will move in farward direction.

Change in pressure :

If pressure is increased reaction will move towards less volume side (less no of mole side).If pressure is decreased reaction will move towards greater volume side(more no of mole side).

N204    <=====>  2NO2 H= +57.2

In this reaction prooduct have 2 moles and reactant have 1 mole since volume is directly proprtional to no of moles and inversly proportional to pressure .hence increase in pressure will move reaction towards backward direction.

If no of moles of both ractant and product are same pressure change will not effect eqilibrium .

H2 + I2  <=====> 2HI

Change in Temperature:

Reaction is of two types exothermic (Heat is released -ve sign)  or endothermic (heat is absorbed+ve sign).

If reaction is exothermic increase in temperature will move reaction in backward direction and vice versa

If reaction is endothermic Increase in temperature will favour farward reaction . 

For example consider the following reaction 

N204 <=====> 2NO2 H= +57.2

+ve sign show it is an endothermic reaction.If we increaese temperature reaction will move in farward direction.

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