Chapter 1: Some Basic Concepts of Chemistry (CBSE Class 11)

Introduction

• The branch of science that studies the preparations, properties, structures, and reactions of material substances is called chemistry.
• Chemistry deals with the composition, structure, properties, and interaction of matter and is useful to humans in daily life. These aspects can be best described and understood in terms of basic constituents of matter, i.e., atoms and molecules. That is why chemistry is also called the science of atoms and molecules.

Importance of Chemistry

Chemistry is a branch of science that studies the properties and structures of substances and the changes undergone by them.

It plays a central role in science and is often intertwined with other branches of science.

Principles of chemistry are applicable in diverse areas such as weather patterns, functioning of brain and operation of a computer, manufacturing of fertilisers, alkalies, salts, acids, dyes, polymers, drugs, soaps,detergents, metals alloys, etc.

Chemistry provides methods for the isolation of life-saving drugs from natural sources and makes the synthesis of such drugs possible. Some of these drugs are Cisplatin and Taxol, which are effective in cancer therapy. The drug AZT (Azidothymidine) is used to help AIDS patients.

Chemistry also provides a safer alternative to environmentally hazardous refrigerants like CFCs (chlorofluorocarbons), responsible for ozone depletion in the stratosphere.

Nature of Matter

Matter

Anything that occupies some space and has some mass is called matter. Matter is made up of small particles with space between them. The matter particles attract each other and are in a state of continuous motion.

Classification of Matter

Physically, matter is classified into solid, liquid and gas, and chemically, matter is classified into pure substances and mixtures. A diagrammatic representation of a classification of matter is given in the picture below:

Pure Substance

They have characteristics different from the mixtures. They have fixed composition, whereas mixtures may contain the components in any ratio, and their composition is variable.

Element

It is the simplest form of pure substance, which can neither be decomposed nor be built from simpler substances by ordinary physical and chemical methods. It contains only one kind of atom. The number of elements known to date is 118.

An element can be a metal, a non-metal, or a metalloid.

-> Hydrogen is the most abundant element in the universe.
-> Oxygen (46.6%), a non-metal, is the most abundant element in the earth crust.
-> Aluminium is the most abundant metal in the earth crust.

Compounds

It is also the form of matter that can be formed by combining two or more elements in a definite ratio by mass. It can be decomposed into its constituent elements by suitable chemical methods, e.g. water (H₂O) is made of hydrogen and oxygen in the ratio 1 : 8 by mass.

Compounds can be of two types :

1. Inorganic compounds: Previously, it was believed that these compounds are derived from non-living sources, like rocks and minerals. But these are in fact the compounds of all the elements except hydrides of carbon (hydrocarbons) and their derivatives.
2. Organic compounds: According to earlier scientists, these compounds are derived from living sources like plants and animals, or they remain buried under the earth (e.g. petroleum). According to modern concept, these are the hydrides of carbon and their derivatives.

Mixtures

These are made up of two or more pure substances. They can possess variable composition and can be separated into their components by some physical methods.

Mixtures may be homogeneous (when composition is uniform throughout) or heterogeneous (when composition is not uniform throughout).

Mixture Separation Methods

Common methods for the separation of mixtures are:

1. Filtration: Filtration is the process of separating solids that are suspended in liquids by pouring the mixture into a filter funnel. As the liquid passes through the filter, the solid particles are held on the filter.
2. Distillation: Distillation is the process of heating a liquid to form vapours and then cooling the vapours to get back the liquid. This is a method by which a mixture containing volatile substances can be separated into its components.
3. Sublimation: This is the process of conversion of a solid directly into vapours on heating. Substances showing this property are called sublimate, e.g. iodine, naphthalene, camphor. This method is used to separate a sublimate from non-sublimate substances.
4. Crystallisation: It is a process of separating solids having different solubilities in a particular solvent.
5. Magnetic separation: This process is based upon the fact that a magnet attracts magnetic components of a mixture of magnetic and non-magnetic substances. The non-magnetic substance remains unaffected. Thus, it can be used to separate magnetic components from non-magnetic components.
6. Atmolysis: This method is based upon rates of diffusion of gases and used for their separation from a gaseous mixture.

Atoms and Molecules

An atom is the smallest particle of an element which can take part in a chemical reaction. It may or may not be capable of independent existence.

A molecule is the simplest particle of matter that has independent existence. It may be homoatomic, e.g. H₂ , Cl₂ , N₂ (diatomic), O₃ (triatomic) or heteroatomic, e.g. HCl, NH₃, CH₄ etc.

Properties of Matter and Their Measurement

Uncertainty in Measurement

Dimensional Analysis

Laws of Chemical Combinations

These are the simple statements that have wide applications during chemical reactions (though some of these laws are not perfect in today’s context):

Law of Conservation of Mass (Lavoisier, 1774)

According to this law, during any chemical or physical change, the system’s total mass (reactants + products) remains constant, or mass can neither be created nor destroyed.

Law of Definite/Constant Proportion (Proust, 1799)

This law states that a given chemical compound always contains its component elements in a fixed ratio (by weight) and does not depend on its source or method of preparation.

Example: H₂O obtained from any source always contains H and O in the ratio 1:8 (by mass).

Law of Multiple Proportions (Dalton, 1808)

It states that if two elements combine to form more than one compound, then the different masses of one element which combine with a fixed mass of the other element, bear a simple (whole number) ratio to one another.

For example: Oxides of nitrogen

Fixed weight of N = 28

Ratio = 1 : 2 : 3 : 4 : 5

Law of Reciprocal Equivalent and Combining Proportion (Richter, 1794)

According to this law, when two or more elements (say X , Y , Z, etc) combine with a fixed mass of another element (say A), then the ratio of their combining masses is same (or a simple multiple) as the proportion in which they combine with each other (XY , YZ , XZ, etc).

For example: CH₄ ,CO₂ and H₂O

Daltons Atomic Theory

On the basis of the laws of chemical combinations, Dalton (1803) proposed the atomic theory.

The main postulates of this theory are :

1. Matter is made up of indivisible and indestructible particles, called atoms.
2. Atoms are neither created nor destroyed in the course of an ordinary chemical reaction.
3. Atoms combine with each other to form compounds in a simple whole-number ratio.
4. An atom is the smallest portion of matter that takes part in a chemical reaction.
5. All atoms of an element have identical mass and similar chemical properties.

Limitations of Dalton’s Atomic Theory

1. It failed to explain how atoms combine to form molecules.
2. It does not explain the difference in masses, sizes, and valencies of the atoms of different elements.

Atomic and Molecular Masses

Atomic Mass

It may be defined as the relative mass of an atom of an element with respect to 1/12th of the mass of an atom
of C-12.

It is the average relative atomic mass of an atom. It indicates how many times an atom of that element is heavier as compared with 1/12th of the mass of one atom of carbon-12.

The word average has been used in the above definition and is very significant because elements occur in nature as mixture of several isotopes. So, atomic mass can be computed as:

Here, RA is the relative abundance of different isotopes.

In other words, if an element exists in the isotopic forms having atomic masses m1 , m2 and m3 in the ratio x , y and z, respectively, the average atomic mass:

Mole Concept and Molar Masses

The term mole was suggested by Ostwald (Latin word mole = heap)

A mole is defined as the amount of substance that contains the same number of elementary particles (atoms, molecules or ions) as the number of atoms present in 12 g of carbon (C-12).

1 mol = 6.023 × 10²³ atoms = one gram-atom = gram atomic mass

1 mol = 6.023 × 10²³ molecules = gram molecular mass
In gaseous state at STP (T = 273K, p=1atm): Gram molecular mass = 1 mol = 22.4 L = 6.022 × 10²³ molecules.

Standard number 6.023 × 10²³ is called Avogadro’s number in honour of Avogadro (he did not give this number) and is denoted by N_A.

The volume occupied by one mole molecules of a gaseous substance is called molar volume or gram molecular volume.

Number of molecules = number of moles × N_A

Number of molecules in 1g compound = N_A/g-molar mass
The number of molecules in 1 cm³ (1 mL) of an ideal gas at STP is called the Loschmidt number (2.69 × 1019 ).

Percentage Composition

Stoichiometry and Stoichiometric Calculations