Sunday, May 17, 2015

IIT JEE Main Chemistry Ch 1. THE CONCEPTS - Core Points for Review

Importance of  Core Revision Points: Core Revision Points are important because if you remember them strongly, many more points related to them will come out of your memory and help you to answer question and problems. Read them many times and make sure you remember them very strongly.



JEE Main Syllabus



Contents of Chapter 1 of Jauhar

1.1 Importance of Studying Chemistry
1.2 Physical quantities and their S.I. Units
1.3 Dimensional Analysis
1.4 Measurement and Significant Figures
1.5 Chemical Classification of Matter
1.6 Laws of Chemical Combination
1.7 Dalton Atomic Theory
1.8 Avogadro's Hypothesis
1.9 Atoms and Molecules
1.10 Atomic and Molecular Mass
1.11 Mole Concept
1.12 Mass-Mole Conversions
1.13 Percentage Composition and Molecular Formula
1.14 Stoichiometry of Chemical Equations
1.15 Stoichiometric Calculations
1.16 Limiting Reactant
1.17 Solution Stoichiometry
1.18 Stoichiometry of of Reactions in Solutions

1.1 Importance of Studying Chemistry


Chemistry is the study of the materials that make uup the universe and the changes which these materials undergo. In chemistry, we study the composition of materials to find out what they are made of.  Now we come to know that all the matter in the universe is made up of various elements, compounds and mixtures. Each element has tiny smallest particles called atoms and elements have their stable combination of atoms called molecules. Compounds are made up of molecules which in turn have various atoms of elements.

There are 112 elements.

Elements and compounds when mixed in appropriate conditions undergo changes and new compounds and materials get formed. There changes in the language of chemistry are called chemical reactions.

Chemistry as a subject has helped in providing creating more food, health care products and other necessities of life. It also provided various materials that provide us comforts, pleasure and luxuries. It provided us building materials and synthetic fibres. In recent, new materials such super-conducting ceramics, conducting polymers, optical fibres, microalloys, nanomaterials like carbon fibres were developed by chemistry scholars and scientists.  Chemists are presently study chemical processes related to environmental damage by man made substances and bio-chemical processes to cure old age related diseases and malfunctions of the human body.


1.2 Physical quantities and their S.I. Units

SI (Systeme Internationale) units were adopted by General Conference of Weights and Measures in 1960. The SI system has seven basic units from which all other units are derived.

1.3 Dimensional Analysis

To convert one set of units say metres into another set of units say centimetres, conversion factor method is used. It is also called dimensional analysis.

Unit conversion factors are used in factor conversion method.

We know 1 m = 100 cm

So  1 = 100 cm/ 1 m  and this is called unit conversion factor between cm and m.

We can convert 25 metres into centimetres by writing 25 m (100 cm /1m) which will give 2500 cm. The metres dimension cancels out and cm dimension has come. So apart from numerical terms, dimensions are also multiplied or divided to get the required dimensions.

4. Measurement and significant figures

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

Accuracy  = Mean of measurements - true value

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.

Actual measurement value - mean of measurements

It can be understood as the lowest measurement - mean of measurements or highest measurement - mean 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.
Example- addition of three numbers
6.414
2.3
0.501
------
9.215  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.

Hence correct answer is 9.2

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.

5. 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.

6. 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.

7. 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.

1.8 Avogadro's Hypothesis


Avogadro's hypothesis or suggestion is that matter consists of two kinds of ultimate particles. These are atoms and molecules.

Atoms are the smallest particle of an element which may or may not have independent existence, but it takes part in chemical reactions.

Molecule is the smallest particle of a substance (element or compound) capable of independent existence.

The actual hypothesis is that under similar conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules.


1.9 Atoms and Molecules


Atoms are the smallest particle of an element which may or may not have independent existence, but it takes part in chemical reactions.

Molecule is the smallest particle of a substance (element or compound) capable of independent existence.

There are 112 elements in nature as per the present knowledge. Obviously there are 112 different types of atoms.

Molecules may have two or more atoms. Molecules are divided into two types:

1. Homoatomic molecules

2. Heteroatomic molecules



1.10 Atomic and Molecular Mass


Atomic Mass: An atom is such a small particle that its mass cannot be determined with the help of any available balance.  By an indirect method, the absolute mass of hydrogen atom has been found to be 1.66 X 10 to the power of -24 g.

Chemists have defined the atomic mass of hydrogen as one and expressed the atomic mass of other elements as multiples of hydrogen's atomic mass. The relative atomic masses expressed in terms of hydrogen as termed atomic weights.

Atomic weights of some common elements

Metals

Magnesium  24.3
Aluminium  27.0
Iron              55.8
Zinc             65.4
Tin             118.7

Gram Atomic Mass: Gram atomic mass is the quantity of an element whose mass in grams is numerically equal to its atomic mass.

Molecular Mass: Measuring the mass of a single molecule is not possible. Hence as in the case of atoms,  molecular mass is defined as the average relative mass of its molecule as compared to the mass of an atom of carbon (C12) having mass number 12.


11. Mole concept

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



1.12 Mass-Mole Conversions


The mass of 6.022 X 10 to the power 23 molecules of a substance is equal to its gram moleculuar mass or gram molecule.

Ionic compound have the formula of composition of ions in the compound.  In their case the mass of one mole of formula units in grams is equal to  formula mass expressed in grams or gram formula mass of the compound. Thus, mass of 6.022 X 10 to the power 23 formula units (or one mole formula units) of any ionic substance in gram is equal to its gram formula mass.

Molar Mass: The mass of 1 mol of a substance is called its molar mass (M). The units of molar mass are g mol to the power -1 or kg mol to the power -1.

Mole in Terms of Volume

It has been observed that one mole (6.022 X 10 to the power 23 molecules) of an ideal gas occupies 22.4 litres at N.T.P. (0 degrees C and 1 atm pressure)


1.13 Percentage Composition and Molecular Formula


Molecular formula is also the chemical formula of a compound. It gives the representation of a molecule of a substance in terms of symbols of various elements present in it. The determiination of chemical formula requires chemical analysis to determine:

1. the elements present in it.
2. the relative of each element in the given mass of the compound.

Percentage Composition
The composition is generally expressed as the mass percentage composition. It gives the mass of each element expressed as the percentage of the total mass of the compound. It can also be expressed as the number of grams of the element present in 100 g of the compound.

Empirical Formula and Molecular Formula

Empirical formula gives the elements present in a compound.
Molecular formula gives the number of atoms of each element present in the compound. This is ascertained from the percentage composition calculations.

Stoichiometry


1.14 Stoichiometry of Chemical Equations
1.15 Stoichiometric Calculations
1.16 Limiting Reactant
1.17 Solution Stoichiometry
1.18 Stoichiometry of of Reactions in Solutions

1.14 Stoichiometry of Chemical Equations


Stoichiometry is derived from the Greek words stoicheion meaning elementand metron meaning measure.

It means measuring elements in chemical compounds. In chemical reactions, it is measuring reactants and products.

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.

Balancing of Chemical Equations

1. Trial and error method
2. Partial equation method
3. Oxidation Number method
4. Ion-electron method


1.15 Stoichiometric Calculations


In stoichiometric calculations we find to mole to mole relationships in chemical equations. Mass to mass relationships are also calculated. Mass, volume relationships are calculated. Volume - volume relationships are calculated. You can imagine each of them based on the discussion we made above regarding mole concept, molecular mass, and mole volume.

1.16 Limiting Reactant


If reactants of different masses are mixed in a reaction vessel, the reactant that is completely consumed when a reaction goes to completion is called the limiting reactant of that mixture of reactants.  The other reactants present are called excess reagents. We need to minimize the excess reagents as it is a waste and we may not be able use them in other reactions due to their contamination in various ways because of using them in this reaction.


1.17 Solution stoichiometry


In solution generally one component is present in lesser amount and it is called solute.

The other present in excess is called the solvent.

The amount of solute present in a given quantity of solvent or solution is expressed in terms of concentration.




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

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

Mass percentage of substance in a system

Mole fraction of a substance in a system

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 H^+ or OH^- ions).





1.18 Stochiometry of reactions in solutions


Many reactions are carried out in aqueous solutions. In this case their concentration is important measure. The amounts of the products of a reaction can be calculated from the volumes of the solutions of the reactants and their concentrations. In the book, the calculations are illustrated through examples.


Updated  17 May, 16 May 2015

Last updated 19 Jan 2008, 30 Nov 2014

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