## Wednesday, July 30, 2008

### 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.
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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
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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.
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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

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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
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"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).