## Wednesday, July 30, 2008

### Great Effort despite a very big Handicap

Read about the person who got JEE rank 992, despite a big handicap

### Ch.1 Basic concepts of Chemistry - July-Dec 08 Revision

JEE Syllabus

General topics:

The concept of atoms and molecules;
Dalton's atomic theory;
Mole concept;
Chemical formulae;
Balanced chemical equations;
Calculations (based on mole concept) involving common oxidation-reduction, neutralisation, and displacement reactions;
Concentration in terms of mole fraction, molarity, molality and normality.
---------
Accuracy

Accuracy is a measure of the difference between the true value (the value to be measured) and the value measured by an instrument.

Precision is depends totally on the instrument and it is the difference between measurements of the same dimension made number of times. It is expressed as the difference between one measurement and the arithmetic mean of the number of measurements.

Significant figures: Significant figures in a number are include all the certain digits plus one doubtful digit.

If a number has 4 significant figures or digits, it means 3 of them are certain and fourth one is doubtful.

Rules for determining the number of significant figures

1. All non-zero digits are significant
Decimal place does not determine the number of significant figures.

2. A zero becomes significant in case it comes between two non-zero numbers.
3. The zeros at the beginning of a number are not significant.
For example 0.0004 has only one significant figure.
4. All zeros placed to right of a number are significant. They represent the precision of the measuring scale.
For example 267.000 has six significant figures.
(The precision does not come by writing the number. It comes because the instrument has the ability to read a number certainly up to that level of measurement.)

Rules of calculations involving significant figures

Rule 1. The final result of addition or subtraction should be reported up to the same number of decimal places as are present in the term having the least number of decimal places.
6.414
2.3
0.501
------
9.215 is the answer but the answer should be reported up to one decimal place only as 2.3 is the term having least number of decimal places.

Rule 2. In multiplication or division, the final result should be reported up to the same number of significant figures as are present in the term with the least number of significant figures.
Example: 4.2345*1.25 = 5.293125
The final result should be reported up to three significant figures only as 1.25 has three significant figures. Hence the correct answer to be reported is 5.29.

Rounding off figures of retention of significant figures.

i) If the digit coming after the desired number of significant figures happens to be more than 5, the preceding digit or figure is increased by 1.
ii) If that digit is less than 5, it is neglected and hence the preceding significant figure remains unchanged.
iii) If that digit happens to be 5, the preceding digit is increased by one in case it is odd number. If preceding digit is an even number, it remains the same.

If the problem has number of steps, the rounding off is to be done at the final answer level only.

Chemical classification of matter

1. Element
Further classification: Metals, non-metals, metalloids
2. Compound
Further classification: Inorganic and organic
3. Mixture
Further classification: Homogeneous mixtures - they are called solutions.
Heterogeneous mixtures: They have visible boundaries of separation between the different constituents and they can be easily seen with naked eye.

Laws of chemical combination

a. Law of conservation of mass
During any physical o chemical change, the total mass of the products is equal to the total mass of reactants.

b. Law of constant proportions

A pure chemical compound always contains same elements combined together in the same definite proportion by weight.
c. Law of multiple proportions

When two elements combine to form two or more than two compounds, the weights of one of the elements which combine with a fixed weight of the other, bear a simple whole number ratio.

d. Law of reciprocal proportions
When two different elements combine separately with the same weight of a third element, the ratio in which they do so will be the same or some simple multiple of the ratio in which they combine with each other.

e. Gay Lussac’s law of combing volumes
Under similar conditions of temperature and pressure, whenever gases react together, the volumes of the reacting gases as well as products (if gases) bear a simple whole number ratio.
---------------
Dalton’s atomic theory

To provide theoretical justification to the laws of chemical combination which are experimentally verified, John Dalton postulated a simple theory of matter. The basic postulates of Dalton’s atomic theory are:

a. Matter is made up of extremely small indivisible and indestructible ultimate particles called atoms.
b. Atoms the same element are identical in all respects ie., in shape, size, mass and chemical properties.
c. Atoms of different elements are different in all respects and have different masses and chemical properties.
d. Atom is the smallest unit that takes part in chemical combinations.
d. Atoms of two or more elements combine in a simpler whole number ratio to form compound atoms (molecules).
e. Atoms can neither be created nor destroyed during any physical or chemical change.
f. Chemical reactions involve only combinations, separation or rearrangement of atoms.

Modern atomic theory
As a result of new discoveries made after Dalton developed his postulates, some modifications were done to atomic theory. They are:

1. Atom is no longer considered to be indivisible: It is found that atom is made up of subatomic particles such as electrons, protons and neutrons. We now state how many electrons are there, protons are there in an atom.

2. Atoms of same element may not be similar in all respects. Atoms of same elements have different atomic masses. These different atoms are called isotopes.

3. Atoms of different elements may have similar one or more properties. Atomic mass of calcium and argon (40 a.m.u.) are same. So the property of atomic mass is same for atoms of different elements. Isobars or elements or atoms having the same atomic mass.

4. Atom is the smallest unit which takes part in chemical reactions. Though electrons and protons are there, it is atom which takes part in chemical reactions and electrons exchange takes place between atoms.

5. The ratio in which the different atoms combine may be fixed and integral but may not always be simple. For example in sugar molecule the ratio of C,H and O atoms is 12:22:11, which is not simple.

6. Atom of one elements may be changed into atoms of other element. Transmutation is the process by which atoms one element can be changed inot elements of other elements by subjecting it to alpha rays.

7. The mass of atom can changed into energy. Mass and energy are inconvertible. The equation give for such conversion is E mc². Hence we cannot say that mass is not destructible. But in chemical reactions, atom remains unchanged and its mass is not destroyed to liberate energy.

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Mole concept
A mole is measuring unit like for example dozen.
A mole is a collection of 6.022*1023 particles

Stochiometric coefficients or numbers: The numbers which appear before the chemical symbols in a chemical equation.

Chemical equation gives information about moles of various reactants and products. Hence molar masses involved in the reaction and molar masses of products.

Mass percentage of substance in a system

Mole fraction of a substance in a system

--------------
Concentration in terms of mole fraction, molarity, molality and normality.

Mole fraction of a substance in a system: It is the ratio of number of moles one component to the total number of moles (solute and solvent) present in the solution. It is denoted by x.

So mole fraction of solute = Moles of solute/(Moles of solute + Moles of solvent)

Mole fraction of solvent = Moles of solvent/(Moles of solute + Moles of solvent)

MOLARITY = Amount of a substance (in mol)/Volume of solution expressed in dm^3
It is applicable to solutions only.

Also equal to No. of moles of solute/Vol. of solution in litres or dm^3

The unit of molarity is mol dm^-3. It is commonly abbreviated by the symbol M and is spelled as molar.

MOLALITY = Amount of a a substance (in mol)/Mass of solvent expressed in kg
It is also applicable to solutions only

CONCEPT OF EQUIVALENT
__________________________

"One equivalent of a substance in a reaction is defined as the amount of substance which reacts or liberates 1 mol of electrons or 1 mol of H^+ or 1 mol of OH^- ions.

The equivalent weight of a reagent may vary according to the reaction, but
if considering just acid and base moles and equivalents, then:-

H2SO4 + 2 NaOH -> 2H2O + Na2SO4

The equivalent weight of an acid is that which contains 1.0078 grams of replaceable hydrogen which, in the case of sulfuric acid, would be half the mole weight, because 1 mol of H2SO4 is liberating 2 moles of H^+ ions

In the case of hydrochloric acid, equivalent weight would be the mole weight as 1 mol of HCl liberates 1 mol of H^+ ions.

The equivalent weight of a base is that which contains one replaceable hydroxyl group ( i.e., 17.008g of ionisable hydroxyl ). Thus the equivalent weight of sodium hydroxide ( NaOH ) and potassium hydroxide ( KOH ) would be the mole weight, but for calcium hydroxide ( Ca(OH)2 ) it would be half the mole weight.

Normality

A Normal solution contains one gram equivalent weight ( aka equivalent )
of the reagent in one litre of solution, and is represented by " N ".

Normality = (equivalents of X (solute))/volume of solution in Liters or dm³

Normality = molarity x n (where n = the number of protons exchanged in a reaction).

## Sunday, July 27, 2008

### July - December 2008 Revision

I plan to go through each chapter in Chemistry, Physics and Mathematics in revision mode during July-December 2008 apart from reading or studying chapter which I have not read so far.

My thinking is that from 1st January 2009 onwards, the aspirants should focus on memorizing things and a revision during July-Dec at leisurely pace, one chapter per day would help in that. From 1 January 2009, the memorization process should take up three chapters per day.

### Carboxylic Acid - July-December Revision

JEE syllabus

Carboxylic acids:
Preparation, properties
Characteristic reactions
formation of esters,
acid chlorides and amides,
ester hydrolysis;
---------
I. Carboxylic acids are the compound containing carboxyl group in their molecules.

-C with a double bond with oxygen and single bond with OH

O

C
|
OH

II. These acids can be aliphatic or aromatic.

aliphatic acids:

Formic acid HCOOH
Acetic acid CH-3COOH
Isobutyric acid (Branched)

Aromatic acids

Bezoic acid : H in benzene substituted by COOH. It is the simplest aromatic carboxylic acid.

m-Nitrobenzoic acid: One more H substituted by NO-2

o-Toluic acid (o refers to ortho) Benzoic acid with one more H substituted by CH-3

III. Methods of Preparation of Monocarboxylic Acids:

1. From oxidation of primary alcohols

2. By oxidation of aldehydes and ketones.

3. From hydrolysis of nitriles and cyanides
the nitriles are hydrolysed in dilute acqueous acidic or alkaline medium.

4. From Grignard reagents
The reaction is carried out by bubbling CO2 through the etheral solution of suitable Grignard reagent.

5. By hydrolysis of esters
Hydrolysis of esters with mineral acids or alkalines gives carboxylic acids

6. Carboxylation of alkenes
Heating alkenes with CO and steam under pressure with phospoiric acid at 673 K. This reaction is called Koch reaction.

7. From trihalogen derivatives of hydrocarbons
Hydroysis of 1,1,1,-trihalogen derivatives of alkanes with acqueous KOH.

8. Preparation of aromatic acids from alkyl benzenes
the alkyl side chain of benzene ring can be easily oxidized to carboxylic group with alkalines KMnO4, chromic anhydride or conc. HNO3.

## Saturday, July 26, 2008

### More Revision of Carboxylic Acid

One of the carboxylic acid of relevance in soap making

common name: Strearic acid

CH3CH2)16COOH

Sodium stearate is soap.

Triglyceride is a triester of glycerol with fatty acids.

Fatty acids are carboxylic acids.

Formation of esters

When carboxylic acids are heated with alcohols in the presence of concentrated H2SO4, esters are formed. The reaction is reversible and is called esterification.

From the materials on esters

Hydrolysis of esters

Esters are slowly hydrolysed by water at reflux temperatures. The reaction is accelerated by dilute mineral acids (HCl, H2SO4) or alkalies.

Hydrolysis of esters by alkalies is also known as saponification and leads to formation of soaps.

### Glycerol - Revision

I came across glycerol as I am browsing through a CET guide on chemistry in the section on oils and fats. I went back to organic chemistry text to locate glycerol. There is a need to think of various concepts repeatedly to recollect them when needed.

Glycerol is a triol.

The alcohols with three –OH groups are named as triols.

It is also termed as trihydroxy alcohol.

Glycerol is formed when 3 Hs of Propane (C3H8) are replaced by 3 OH groups.

Glycerol is C3H5(OH)3 = OHCH2-OHCH-OHCH2

CH2-OH
|
CH-OH
|
CH2-OH

Triglycerides (glyceride or triacyl glycerol or oils or fats)

When glycerol (glycerine) is heated with fatty acid, triglycerides are produced. Tyiglycerides have wide application in preparation of soap, paints, varnishes, ink, ointments and cream.

Fatty acid is represented is a monocarboxylic acid and is represented as HOOCR

CH2-OH
|
CH-OH + 3HOOCR
|
CH2-OH

will give

CH2-OOCR
|
CH-OOCR
|
CH2-OOCR

and 3H2O

Formation of soap

Soap is a sodium or potassium salt of fatty acid. Alkaline hydrolysis of triglycerides to form soap and glycerol is known as saponification.

Fatty acid + NaOH (Acqueous) → soap + glycerol

On completion of saponification precipitate of soap is completed by common ion effect. For this saturated solution of NaCl (brine) is used.

Sodium soaps are hard and hence used as washing soap. (ex. Sodium stearate)
Potassium soaps are soft and hence used as bathing soap.(Ex. Potassium oleate (remember Oil of Olay), Potassium stearate)

## Wednesday, July 23, 2008

### Nomenclature - Carboxylic Acids

Nomenclature

The common names of carboxylic acids are based on their source of origin. Formic acid was first obtained from red ants (Latin formica means red ants) and it was named from that. Acetic acid was obtained from vinegar (Latin aceum means vinegar) and so got that name.

In the common system the position of substituents is indicated by the Greek letters α, β, γ, ō.

α, β, γ, ō Carbon atoms

The carbon atom next to the carboxyl carbon is assigned the letter α. The carbon next to α-carbon is the β-carbon. The carbon next to β-carbon is the γ-carbon. The carbon next to γ-carbon is the ō Carbon.

ō-γ-β-α carbons
C-C-C-C-COOH

IUPAC System

According to IUPAC system, the name of the monocarboxylic acid is derived by changing the final 'e' from the name of the corresponding hydrocarbon with 'oic' and adding the word acid.

Formic acid - Methanoic acid
Acetic acid - Ethanoic acid
n-Butyric acid - Butanoic acid
Isobutyric acid - 2-Mehtylpropanoic acid

Dicarboxylic acids

Oxalic acid - Ethanedioic acid
Malonic acid - Propanedioic acid

The position of substituents is indicated by the following rules.

1. The longest chain of carbon atoms containing the carboxylic group(-COOH) is selected.
2. The numbering of carbons starts from the carboxylic acid group and the carbon of carboxyl group is given number 1.
3. The position of the substituents is indicated by the number of carbon atom to which they are attached.

Aromatic carboxylic acids

The simplest aromatic carboxylic acid is benzoic acid.

The IUPAC names of substituted aromatic carboxylic acids are derived by prefixing the name of the substituent to the name of the parent acid i.e., benzoic acid and the position is indicated by an Arabic numeral with the carbon atom carrying the –COOH group being numbered as 1.

## Thursday, July 10, 2008

### Ionisation

When an elctrolyte is dissolved in water, it splits up into ions.

For example, sodium chloride is the solid state exists as a collection of positively charged sodium ions and negatively charged chloride ions. These are held together by strong electrostatic forces of attraction in the solid state. Due to these strong attractive forces, the ions do not move freely and therefore do not conduct electricity in the solid state.

Ionisation process: When such a compound (NaCl) is dissolved in water, a high dielectric constant of water cuts down the forces of attraction between the ions. As a result, the ions get separated and dissolve in water.

The forces of attraction between oppositely charged ions are inversely proportional to the dielectric constant of the medium. The grreaer the dielectric constant, teh greater is the dissolving power of the solvent. The dielectric constant of water is 80 and therefore, these forces are reduced by a factor of 80 in aqueous solutions and ions become free to move.

(Topic: Ionic equilibrium)

## Wednesday, July 9, 2008

### Lewis Concept of Acids and Bases (1923)

An acid is a substance (molecule or ion) which can accept a pair of electrons.

A base is a substance (molecule or ion) which can donate a pair of electrons.

Species that can be Lewis bases

(i) Neutral species (substances) having at least one lone pair of electrons

Ammonia, amines, alcohols

(ii)Negatively charged species (anions)

chloride ion, cyanide ion, hydroxide ion etc.

Species that can be Lewis acids

(i)Molecules in which the central atom has incomplete octet

BF3, AlCl3, FeCl3

(ii) Simple cations.
Some cations like Na+, K+, Ca²+ have very little tendency ot accept electrons.
But cations like H+, Ag+ etc. hve a greater electron accepting tendency and therefore act as Lewis acids.

(iii) Molecules in which the central atom has empty d-orbitals.
Molecules such as SiF4, SnCl4, and PF5 have their central atoms with empy d- orbitals. They can expand their outer octet by accepting electrons from other substances.

(iv) Molecules in which atoms dissimilar electronegativities are joined by multiple bonds.

In these molecules, electrons one atoms are pulled towards it by the other atom. Due to this, under the influence of the attacking Lewis base, the pair of electrons in the π-bond or one of the π-bonds(if more than one multiple bond is there like CO2) will be shifted toward the oxygen atom and the pair donated by the Lewis base is accepted.

All Bronsted bases are also Lewis bases.
But all Bronsted acids are not Lewis acids. For instance, HCL, H2SO4 can give a proton but cannot accept a pair of electrons.

### Bronsted - Lowry Concept of Acids and Bases (1923)

According to Bronsted - Lowry Concept:

An acid is a substance which can donate proton (H+):

A base is a substance which can accept a proton (H+).

All Arrhenius acids are also Bronsted acids.
But all Arrehenius bases are not Bronsted bases.

For example, NaOH is a base according to Arrhenius theory because it gives OH- ions in aqueous solution. But NaOH does not accept a proton. Thus it may not be classified as a base according to Bronsted theory.

Conjugate acids, bases

The pairs of acids and bases whihc are formed from each other by the gain of loss of a proton are called conjugate acid-base pairs.

(Topic: Ionic equilibrium, Redox reactions)

### Arrhenius Concept of Acids and Bases

An acid is a substance which dissociates in acqueous solution to give hydrogen ions.

A base is a substance which dissociates in acqueous solution to give hydroxyl ions

Acids give H+ ions in water, bases give OH- ions in water

## Monday, July 7, 2008

Under similar conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules.

(Basic concepts of chemistry)

## Sunday, July 6, 2008

### Laws of Chemical Combination

1. Law of conservation of mass

During any physical or chemical change, the total mass of the products is equal to the total mass of reactants.

2. Law of constant proportions

A pure chemical compound always contains same elements combined together in the same definite proportion by weight.

3. Law of multiple proportions

When two elements combine to form two or more than two compounds, the weights of one of the elements which combine with a fixed weight of the other, bear a simple whole number ratio.

4. Law of reciprocal proportions

When two different elements combine separately with the same weight of a third element, the ratio in which they do so will be the same or some simple multiple of the ratio in which they combine with each other.

5. Gay Lussac’s law of combing volumes

Under similar conditions of temperature and pressure, whenever gases react together, the volumes of the reacting gases as well as products (if gases) bear a simple whole number ratio.

### If you can dream it, you can do it

The starting point of all achievement is desire.

Keep this constantly in mind. Weak desires bring weak results, just as a small amount of fire makes a small amount of heat.

NAPOLEAN HILL, PIONEER OF PERSONAL-SUCCESS LITERATURE

Read the article by Ankur Gupta in Hindustan Times, 6th July 2008 page No. 9

I shall give the summary and important points at a later date.