Introduction

INTRODUCTION

Chemistry is the science of substances, their properties, structures and their transformation. As all objects in the universe are made of matter. Chemistry is the branch of the science which deals with the study of material object. Study of chemistry is very interesting which covers various aspects of our culture and environment. All development in any science are based on scientific approach as in chemistry too. In order to achieve correct results, one has to rely upon the various skills connected with the measurements of quantities during a physical or chemical change. The degree of accuracy is closely linked with precision of the measuring instrument as well as on the skill of the person engaged in measurement. So we should be first familiar with some terminology used in chemistry.

Physical Property:

The property which can be measured without changing the chemical composition of the substance is known as physical property like mass, volume, density, refractive index etc.

Chemical Property:

The property which can be evaluated at the cost of matter itself is known as chemical property. For example combustible nature of hydrogen gas can be verified by burning of hydrogen. The sweet taste of sugar by consuming it.

Units for Measurement

All physical quantities have to be measured. The value of a physical quantity is expressed as the product of the numerical value and the unit in which it is expressed.

Fundamental Units:

Fundamental units are those units which can neither be derived from one another nor they can be further resolved into any other units.

The seven fundamental units of measurement in S.I. system.

Derived unit:

Some quantities are expressed as a function of more than one fundamental units known as derived units. For example velocity, acceleration, work, energy etc.

Units and Dimensional Analysis: Conversion of Units

The simplest way to carry out calculations that involve different units is to use dimensional analysis. In this method a quantity expressed in one unit is converted into an equivalent quantity with a different unit by using conversion factor which express the relationship between units:

Original quantity ´ conversion factor = equivalent quantity

(in former unit) (in other unit)

This is based on the fact that ratio of each fundamental quantity in one unit with their equivalent quantity in other unit is equal to one

For example in case of mass

$\frac{1kilogram}{2.205\mathrm{pound}}=1=\frac{1\mathrm{kilogram}}{1000\mathrm{gm}}$

So 1 kg = 2.205 pound = 1000 gm

In this way any derived unit first expressed in dimension and each fundamental quantities like mass length time are converted in other system of desired unit to work out the conversion factor.

For example: How unit of work / energy i.e. joule, in S.I. system is related with unit erg in C.G.S system

Dimension of work = force × displacement = MLT^-2 × L = ML²T^-2

1 joule = 1 kg (1 metre)² × (1see)^-2

$\Rightarrow$ $1\times Kg\times \frac{1000gm}{1Kg}\times {\left[1metre\times \frac{100cm}{1metre}\right]}^2\times {\left[1\sec \right]}^{-2}$

$\Rightarrow$ 100 gm × (100)²× 1 em²× (1 sec)^-2

$\Rightarrow$ 1000×10000 × 1 gm×1 cm² × 1 sec^-1

$\Rightarrow$ 1 joule = 10⁷ erg $

Similarly we can deduce other conversion factor for other quantity in different unit by the dimensional analysis method

Another interesting example is the conversion of litre – atmosphere to joule (the SI unit of energy) by multiplying with two successive unit factors. Thus,

$1Latm\times \left(\frac{{10}^{-3}{m}^3}{1L}\right)\times \frac{101.325Pa}{1atm}=101.325Pa{m}^3$

Knowing that

$Pa=\frac{N}{m^2}$

, we can write

$101.325Pa{m}^3=101.325\left(\frac{N}{m^2}\right)m^3=101.325nm=101.325J$

Hence 1 L atm = 101.325 J

Illustration: What is the mass of 1 L of mercury in grams and in kilograms if the density of liquid mercury is$13.6gc{m}^{-3}$?

Solution: We know the relationship,$1L=1000c{m}^3$ and

Also, $density=\frac{mass}{volume}$

We can write, mass = (volume) (density)

Therefore, the mass of 1 L of mercury is equal to

$(1L)\left(\frac{1000c{m}^3}{1L}\right)(13.6gc{m}^{-3})=(1000c{m}^3)(13.6gc{m}^{-3})=1.36\times 10^{4}g$

The mass in kilograms can be calculated as

(Remember, $\frac{1000c{m}^3}{1L}$and $\frac{1kg}{1000g}$ are conversion factors with which we have to multiply for getting our answer in appropriate units).

Matter

Anything that exhibits inertia is known as matter. The quantity of matter is its mass. e.g. chalk table.

CLASSIFICATION OF MATTER

This classification of matter is based upon chemical composition of various substances. According to this matter can be further divided into two types, pure substance and mixture. Mixtures are also of two types, homogenous mixtures and heterogeneous mixtures.

Elements:

The primary stuff present in all the substance is known as element, whose smallest unit is known as atom. Total 112 elements are known till date of which 92 are naturally occurring elements rest are results of artificial transmutation. There are 88 metals, 18 non metals and 6 metaloids.

Compound:

A non elemental pure substance is called a compound in which more than one atom of elements are linked by chemical bonds formed due to chemical reaction. The resulting molecule is a electrically neutral particle of constant continuous composition.

Mixture:

Mixtures are the aggregate of more than one type of pure substance whose chemical identity remains maintained even in mixtures. Their constituent ratio may vary unlike compound.

For example – sugar + water = sugar syrup, Gun-powder 75 % KNO₃ 10% sulphur + 15% carbon

There are two types of mixture (a) homogeneous (b) heterogenous

(a) Homogeneous mixtures are those whose composition for each part remains constant. For example aqueous and gaseous solution.

(b) Heterogeneous mixtures are those whose composition may vary for each and every part. For example soil, concrete mixtures.

LAWS OF CHEMICAL COMBINATION

In order to understand the composition of the compounds, it is necessary to have a theory which accounts for both qualitative and quantitative observations during chemical changes. Observations of chemical reactions were most significant in the development of a satisfactory theory of the nature of matter. These observations of chemical reactions are summarized in certain statements known as laws of chemical combination.

Law of conservation of mass:

For any chemical change total mass of active reactants are always equal to the mass of the product formed. It is a derivation of Dalton's atomic theory 'atoms neither created nor destroyed'.

Total masses of reactants = Total masses of products + Masses of unreacted reactants

Illustration :-. 5.2 g of CaCo${}_3$​hen heated produced 1.99 g of Carbon dioxide and the residue (CaO) left behind weighs 3.2g. Show that these results illustrate the law of conservation of mass.Solution: Weight of CaCo${}_3$​taken = 5.2 g

Total weight of the products (CaO +CO${}_2$)= 3.20+ 1.99 = 5.19 g

Difference between the wt. of the reactant and the total wt. of the products

= 5.20 – 5.19 =0.01 g.

This small difference may be due to experimental error.

Thus law of conservation of mass holds good within experimental errors.

Law of constant composition

A chemical compound always contains same elements in definite proportion by mass and it does not depend on the source of compound.

For example

$C+O_2\to C{O}_2$

$CaC{O}_3\stackrel{\Delta }{\to }CaO+C{O}_2$

$N{a}_{2}C{O}_3+2HCl\to 2NaCl+H_{2}O+C{O}_2$

$C{H}_4+2O_2\to C{O}_2+2H_{2}O$

The composition of CO₂ obtained by different means always having same C:O ratio =$\frac{12}{32}=0.375$.

Law of multiple proportion:

When two elements combine to form two or more than two different compounds then the different masses of one element B which combine with fixed mass of the other element bear a simple ratio to one another.

For example: Carbon forms two oxides in oxygen

Carbon monoxide $\underset{12}{C}+\frac{1}{2}\underset{16}{O_2}=CO$

Carbon oxide $\underset{12}{C}+\underset{32}{O_2}=C{O}_2$

The ratio of masses of oxygen in CO and CO₂ for fixed mass of carbon (12)

is 16 : 32 = 1: 2.

Illustration :- The law of multiple proportions is illustrated by the pair of compounds:

(A) sodium chloride and sodium bromide

(B) water and heavy water

(C) sulphur dioxide and sulphur trioxide

(D) magnesium hydroxide and magnesium oxide

Solution: In SO₂ 32 gram of suphur react with 32 gram of oxygen. Similarly for SO₃ fixed mass of sulphur (32 gram) react with 48 gram of oxygen. The ratio of oxygen's mass = 32:48 = 2:3

Which support law of multiple proportions.

Hence (C) is correct answer.

Law of reciprocal proportion:

If two elements B and C react with the same mass of a third element (A), the ratio in which they do so will be the same or simple multiple if B and C reacts with each other.

The above law is the basis of law of equivalent masses, which will be described latter in details.

For example:

ratio of masses of carbon and sulphur which combine with fixed mass (32 parts) of oxygen is

12:32 or 3:8 …(1)

In $\mathrm{C}{\mathrm{S}}_2$ratio of masses of carbon and sulphur is

12:64 or 3:16 …(2)

The two ratios (1) and (2) are related to each other by $\frac{3}{8}:\frac{3}{16}$ or 2:1

or $\frac{3}{8}:2\left(\frac{3}{16}\right)$

i.e. first ratio is integral multiple of second.

Illustration:- One part of an element A combines with two parts of B (another element). Six parts of element C combine with four parts of element B. If A and C combines together, the ratio of their masses will be governed by:

(A) law of definite proportions (B) law of multiple proportions

(C) law of reciprocal proportions (D) law of conservation of mass

Solution: $\frac{B}{A}=\frac{2}{1};\frac{B}{C}=\frac{4}{6}=\frac{2}{3}$from this when A combined with C the ratio is A/C = 1/3

Hence (C) is correct.

Gay Lussac's law of Combining Volumes:

At given temperature and pressure the volumes of all gaseous reactants and products bear a simple whole number ratio to each other.

For example

$\underset{1voume}{H_{2(g)}}+\underset{1volume}{C{l}_{2(g)}}\to \underset{2volumes}{2HC{l}_{(g)}}\left[1:1:2\right]$

i.e. one volume of hydrogen reacts with one volume of chlorine to form two volumes of HCl gas. i.e. the ratio by volume which gases bears is 1:1:2 which is a simple whole number ratio.

Similarly other examples are:

$\underset{2volume}{2S{O}_2}+\underset{1volume}{O_2}=\underset{2volume}{2S{O}_3}\left[2:1:2\right]$

$\underset{2volume}{2N{H}_3}\rightleftharpoons \underset{1volume}{N_2}+\underset{3volume}{3H_2}[2:1:3]$

$\underset{1volume}{N{H}_3}+\underset{1volume}{\mathrm{HCl}}\rightleftharpoons \underset{1volume}{N{H}_{4}Cl}\left[Vapour\right][1:1:1]$

Illustration :- In the reaction, the ratio of volumes of nitrogen, hydrogen and ammonia is 1 : 3 : 2. These figures illustrate the law of:

(A) constant proportions (B) Gay-Lussac

(C) multiple proportions (D) reciprocal proportions

Solution: The above ratio of 1 : 3 : 2 illustrates the Gay-Lussac law of combining volume.

Hence (B) is correct.

PERCENTAGE YIELD

Reactants, often yield quantities of products that are less than those calculated from the balanced chemical equation.

Reason behind such discrepancy may be:

(i) Some of the reactants fail to undergo reaction due to lower amount at the end.

(ii) Some of the reactant follows other route of reaction resulting unexpected, undesired side product.

(iii) Some of the expected product recombines to form other undesired product or reverting toward reactant also known as backward reaction.

(iv) The total recovery, isolation of products is not possible due to some unavoidable reason.

So percentage yield is the ratio of actual yield (recovered) to theoretical yield multiplied by 100.

$\%yield=\frac{actualyield}{theoreticalyield}\times 100$

DALTON'S ATOMIC THEORY

By observing the laws of chemical combination discussed above, John Dalton (1808) proposed atomic theory of matter. The main points of Dalton's atomic theory are as follows:

$\bullet$Matter is made up of extremely small, indivisible particles called atoms.

Atom of same substance are identical in all respect i.e. they posses same size, shape, mass, chemical properties etc.

$\bullet$ Atom of different substances are different in all respect i.e. they posses different shape, size, mass and chemical properties etc.

$\bullet$ Atom is the smallest particle that takes part in chemical reactions.

$\bullet$ Atom of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.

$\bullet$ Atom can neither be created nor destroyed i.e. atoms are indestructible.

Limitations

The main failures of Dalton's atomic theory are:

Atom was no more indivisible. It is made up of various sub-atomic particles like electrons, proton and neutron etc.

$\bullet$ It failed to explain how atoms of different elements differ from each other.

$\bullet$ It failed to explain how and why atoms of elements combine with each other to form compound atoms or molecules.

$\bullet$ It failed to explain the nature of forces that bind together different atoms in molecules.

$\bullet$ It failed to explain Gay Lussac's law of combining volumes.

$\bullet$ It did not make any distinction between ultimate particle of an element that takes part in reaction (atoms) and the ultimate particle that has independent existence (molecules).

Modern Atomic theory or Modified Atomic Theory

$\bullet$ Atom is no longer supposed to be indivisible. Atom has a complex structure and is composed of sub-atomic particles such as electrons protons and neutrons.

$\bullet$ Atom of the same element may not be similar in all respects e.g. isotopes.

$\bullet$ Atom of different elements may be similar in one or more respects e.g. isobars.

$\bullet$ Atom is the smallest unit which takes part in chemical reactions.

$\bullet$ The ratio in which atoms unite may be fixed and integral but may not be simple. e.g. In sugar molecules${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}$, the ratio of C, H and O atoms is 12:22:11 which is not simple.

$\bullet$ Atom of one element can be changed into atoms of other element for e.g. transmutation.

$\bullet$ Mass of atom can be changed in energy.

$\mathrm{E}=\mathrm{M}{\mathrm{C}}^2$according to Eienstein mass energy relationship, mass and energy are inter-convertible. Thus atom is no longer indestructible.

Illustration 14. An important postulate of Dalton's atomic theory is:

(A) an atom contains electrons, protons and neutrons

(B) atom can neither be created nor destroyed nor divisible

(C) all the atoms of an element are not identical

(D) all the elements are available in nature in the form of atoms

Solution: The statement written in (B) is about law of mass conservation which is true for all chemical reaction.

Hence (B) is correct.

Atom: Atom is the ultimate electrically neutral, made up of fundamental particle (Electron, neutron, Proton) which shows the characteristic properties of the element and exist freely in a chemical reaction.

Molecules: A molecule is the smallest particle made up of one or more than one atom in a definite ratio having stable and independent existence e.g.${\mathrm{H}}_2,{\mathrm{O}}_2,{\mathrm{N}}_2,He,HCl,CaC{O}_3$ etc.