Latent Heat of Vaporization

Introduction :

Latent is a Latin word which means hidden. Latent heat is hidden heat, which a body absorbs or releases when it changes form one state to another.

Latent heat of vaporization of a substance is the amount of heat required to change one unit mass of that substance from liquid to gaseous state. During this process, the temperature of the substance remains constant. The heat absorbed in the process used to change the state of that substance from liquid to gaseous state. The heat energy increases the internal energy of the substance in form of change of state with no rise in temperature. A substance starts changing its state from liquid to gaseous state upon reaching its boiling point. When change of state starts taking place, the temperature of the liquid becomes constant at the boiling point. The heat absorbed there after used up in changing its state from liquid to gaseous state.

Latent Heat of Vaporization :

Similar to the latent heat of vaporization, there is latent heat of fusion. Just like the latent heat of vaporization, it is the heat required by a substance to change from solid to liquid state. In this case, also there is no rise in temperature when change of state takes place.

Latent Heat of Vaporization :

For a particular substance, Latent heat of vaporization per unit mass is more than its specific heat. For example, Latent heat of vaporization of water is 2260 J/g whereas its specific heat is just 4.2 J/g per degree centigrade rise in temperature. That is why burn caused by steam at 100 degree centigrade is more severe than burn caused by water at 100 degree centigrade.  Similarly, ice at zero degrees centigrade is cooler than water at zero degrees centigrade.

We can conclude one should be more cautions in burns caused be steam because latent heat of vaporization comes into play causing more severe burns.

J.J. Thomson's atomic model

Joseph John Thomson was Danish Physicist, who discovered the 'electrons' in 1897 and put forward his famous ‘plum pudding model’ of atom in 1904.The structure of atom was put forth by many scientists. John Dalton, J.J. Thomson, Rutherford and Bohr are the important scientists who worked in this field.

The following are the work of various scientists in framing the structure of atom:

• John Dalton discovered that matter is made of very small particles called atoms which can not be divided.
• J.J. Thomson discovered electrons and found that the atom can be divided and it is made of positive core and negatively charged particles embedded in it.
• James Chadwick discovered the protons.
• Goldstein discovered neutrons.
• Rutherford discovered that atom has a positive central core called nucleus and the electrons move around the nucleus in great speed.
• Bohr stated that the electrons move around the nucleus in a definite energy levels called shells and gave the explanation for the stability of atom.

Discovery of Atom

It was John Dalton, who stated that the matter is made of indivisible small particles called atoms. Atom means 'that which can not be divided'. This model was discarded by J.J. Thomson when he discovered the electrons.

J.j. Thosmson Atomic Model

 Michael Faraday worked on the passage of electricity through liquids while Thomson worked on the passage of electricity through gases. During this experiment, Thomson discovered the electrons.
According to Thomson, when a glass tube fitted with two electrodes filled with a gas at low pressure and when a high voltage of electricity is applied, some rays are generated from the cathode. He named these rays as cathode rays. When two charged plates are placed on either side of this discharge tube, these rays (cathode rays) are attracted to the positive plate. Hence he said that the cathode rays are made of negatively charged particles. He called these particles as ‘corpuscles’. Corpuscles mean particles. G. J. Stoney named them as ‘electrons’

In J.J. Thomson atomic model, the atom consists of a sphere of uniform distribution of positive charge with electrons embedded in it. The number of positive particles is equal to the number of electrons and the 'atom as a whole is eclectically neutral'. This model accounted for the neutrality of atom. This model is popularly known as"plum pudding model", like negatively-charged "plums" surrounded by positively-charged "pudding".

But later Rutherford proved that the electrons are not embedded in the positive sphere, but they revolve around the positive central core and he named it as nucleus

Molecular Formula for Alkynes

Introduction :

Alkynes are hydrocarbons which contains carbon-carbon triple bond. They show neither geometric nor optical isomerism. The simplest alkyne consist of one triple bond i.e. ethyne (HCCH) which is commonly termed as acetylene. The structural formula of acetylene is



Molecular Formula and Structural Formulas:

The alkynes consist of compounds which are based on carbon and hydrogen series containing leastways one triple bond. The alkyne is a homologous series having the formula of CnH2n-2, where n is any integer which is greater than one.

In the chain of long alkynes, the carbon atoms added are closed to each other via single covalent bonds. Each carbon atom in the alkynes is associated with sufficient hydrogen atom to complete the valency of carbon as four. In alkynes, if there are four or more than four carbon atoms, the triple bond can be situated in different positions next to the chain which results in the formation of isomers.

For example, the alkyne of molecular formulas C4H6 has two isomers,



As alkynes have restricted rotation because of the presence of triple bond, they do not hold stereo isomers as alkenes because there is sp hybridized bonding present in a carbon-carbon triple bond. The maximum separation is 180o between the hybridized orbitals in the sp hybridization so the molecule is in linear form. Thus the alkynes are located in straight line because of this stereo isomers are impossible.

Physical Properties of Molecular Formula for Alkynes:

Most of the alkynes are less dense than water but some are exceptions.

Chemical Properties of Molecular Formula for Alkynes;

Alkynes are more polar than alkanes and alkenes. Liquid alkynes are non-polar solvents so immiscible in water. Alkynes having low ratio of hydrogen atoms to carbon atoms are highly combustible.

These alkynes are highly reactive so can be easily broken into alkanes and alkenes. They store large amount of chemical energy that’s why highly exothermic when broken. The released heat can cause rapid expansion so care must be taken when managing with alkynes.

Conclusion on Molecular Formula for Alkynes:

The triple carbon bonds are formed in alkenes due to absence of hydrogen’s. Thus they allow carbon bonds to be stronger, because of the nucleus central force which pulls in nearby atoms.

Benzene Molecular Formula

Introduction:

Carbon has valency of 4.Hydrogen has valency of 1.Untill the discovery of benzene,no compound with empirical formula C_nH_n was discovered.As such benzene with same empirical formula posed a great challenge.It was predicted that the molecule would invariably contain unsaturation.

Progressive Attempts on Benzene Molecular Formula:

Since it was first isolated and identified in 1825,the knowledge of structure of eluded the chemists for many decades

The existence of double bond in ethene was discovered by Scottish chemist Alexander Brown in 1864.However the discovery of the probable structure of C6H6 continued to dodge the chemists.It was so because never before was any structure derived for a compound with the empirical  formula C_nH_n.

The chemists were confused as to how all the valencies of carbon would stand satisfied in such a compound.

Kekule's Contribution in Benzene Molecular Formulas

Simultaneously attempts were also made by Adolph Carl Ludwig Clause ,Henry Armstrong etc. to propose certain structures.However they failed.

It was only in 1865 that Kekule could make a breakthrough in devising this structure.He devised the structure with hexagonal ring. He said that he had discovered the ring shape of the benzene molecule after having  day-dream of a snake seizing its own tail .

This structure  met with lot of criticism in the beginning and was further refined.In 1872 he put forth that the atoms are oscillating and were in reverse and forward collision with the neighbouring carbon atoms.

pi bonds in benzene

Kekule put forth the correct structure in 1865.According to it ,benzene has aromatic structure. He revealed that he had discovered the ring shape of the benzene molecule after having  day-dream of a snake seizing its own tail .This structure  met with lot of criticism in the beginning and was further refined.
The only way in which this could be explained is pi bonds in the aromatic ring.

Unconventional pi bonds

It means it consists of a conjugated planar ring system with delocalized pi electron clouds.These electtrons which form the double pi bond keep on 'hopping' between subsequent bonds.This also can be expressed in following sentence.

The electrons for C–C bonding are distributed equally between each of the six carbon atoms.
Average length between the C-C and C=C is at 0.139 nm. This is called as 'resonance'. Resonance adds to the energy of the structure. As a result benzene is more stable by 150 kJ mol^-1than predicted by Kekule because of resonance.

Each carbon atom is attached to one hydrogen atom in addition to two carbon atoms.The electrons of the pi bonds keep on oscillating forth and back.In this way an the valencies of the atoms stand satisfied,albeit in a unconventional way.Thus in order to facilitate this,the structure would be planar and not three dimensional.This enhanced stability is the fundamental property of aromatic molecules that differentiates them from non-aromatic molecules.

Because of these pi bonds, benzene undergoes nucleophilic as well as electrophilic addition reactions at one end of any double bond.

Structure with Respect to Moleculr Formula

The structure of benzene has aromatic nature.
It means it consists of a conjugated planar ring system with delocalized pi electron clouds.The electrons for C–C bonding are distributed equally between each of the six carbon atoms. Each carbon atom is attached to one hydrogen atom.The electrons of the pi bonds keep on oscillating forth and back.In this way an the valencies of the atoms stand satisfied,albeit in a unconventional way.Thus in order to facilitate this,the structure would be planar and not three dimensional.This enhanced stability is the fundamental property of aromatic molecules that differentiates them from non-aromatic molecules .

The discovery of ring structure of benzene has led to vast field of aromatic compounds in organic chemistry. Many important chemicals are derived from benzene by replacing one or more of its hydrogen atoms with another functional groups.e.g toluene,phenol etc.The ring structure is also the basic unit of many biochemicals like hormones.

Brownian motion

The zig-zag, random, irregular motion exhibit by small particles of matter suspended in a fluid is called as Brownian motion or Brownian movement. This type of movement can observed in almost all type of colloidal suspensions such as liquid in liquid, gas-in-liquid, solid in liquid, liquid-in-gas and solid-in-gas.

Let’s define Brownian motion. Brownian motion definition can write as the random motion of particles in all possible directions.

Brownian motion was first observed by botanist Robert Brown in 1827 in pollen grains floating in water. The movement of particles is not depends on any external force. The velocity of particles is proportional to the square root of the temperature. Generally particles of about 0.001 mm in diameter can show Brownian movement as these particles are small enough to share in the thermal motion, yet large enough to be seen with an ultra-microscope.

In 1905 Albert Einstein explained, ‘What is Brownian Motion’ and purposed the theoretical treatment of Brownian motion. Further Jean Perrin made a quantitative experimental study and proved the dependence of Brownian motion on temperature and particle size and also provided verification for Einstein's mathematical formulation.

Brownian movement can easily observe in colloidal solution. The colloidal particles and molecules of dispersion medium are in continuous collisions due to their constant motion. Due to their motion, particles pass their kinetic energy and strike in all sides results into zigzag movement. That is the reason when colloidal solutions are viewed under an ultra microscope; particles are seen continuously moving in a zigzag path. The main cause of Brownian movement is the unequal bombardments of the moving molecules with other particles. Like in colloidal solution, the molecules of dispersion medium continuously attack on colloidal particles from all sides and impart momentum to them.

Due to collisions between molecules, colloidal particles change its direction and again move in another direction to colloid with another molecule. The collision between molecules remains continuous and results in a random zigzag movement of particle. As the size of colloidal particles increases, the Brownian movement decreases and that is the reason suspensions cannot exhibit this type of movement.

The stability of colloidal sol is mainly depends upon the Brownian movements of particles. As Brownian movement opposes the gravitational forces acting on colloidal particles, therefore particles would not settle down. These movements are also helpful to explain the force of gravity on colloidal particles.

So we can briefly describe Brownian moment of a particle as the zig zag motion of the particles in simple words. Brownian moment hence plays a significant role when we describe the characteristics of the particles.