Chemistry

Chemistry is the science of molecules and their transformations. It is the science not so much of the one hundred elements but of the infinite variety of molecules that may be built from them ...

Chemistry deals with the composition, structure and properties of matter. These aspects can be best described and understood in terms of basic constituents of matter: atoms and molecules. That is why chemistry is called the science of atoms and molecules. Can we see, weigh and perceive these entities? Is it possible to count the number of atoms and molecules in a given mass of matter and have a quantitative relationship between the mass and number of these particles (atoms and molecules)? We will like to answer some of these questions in this Unit. We would further describe how the physical properties of matter can be quantitatively described using numerical values with suitable units.

Why Study Chemistry?

Because understanding chemistry helps you to understand the world around you. Cooking is chemistry. Everything you can touch or taste or smell is a chemical. When you study chemistry, you come to understand a bit about how things work. Chemistry isn't secret knowledge, useless to anyone but a scientist. It's the explanation for everyday things, like why laundry detergent works better in hot water or how baking soda works or why not all pain relievers work equally well on a headache. If you know some chemistry, you can make educated choices about everyday products that you use.

IMPORTANCE OF CHEMISTRY

Science can be viewed as a continuing human effort to systematize knowledge for describing and understanding nature. For the sake of convenience, science is sub-divided into various disciplines: chemistry, physics, biology, geology etc. Chemistry is the branch of science that studies the composition, properties and interaction of matter. Chemists are interested in knowing how chemical transformations occur. Chemistry plays a central role in science and is often intertwined with other branches of science like physics, biology, geology etc. Chemistry also plays an important role in daily life.

Chemical principles are important in diverse areas, such as weather patterns, functioning of the brain and operation of a computer. Chemical industries manufacturing fertilizers, alkalis, acids, salts, dyes, polymers, drugs, soaps, detergents, metals, alloys and other inorganic and organic chemicals, including new materials, contribute in a big way to the national economy.

Chemistry plays an important role in meeting human needs for food, health care products and other materials aimed at improving the quality of life. This is exemplified by the large scale production of a variety of fertilizers, improved varieties of pesticides and insecticides. Similarly, many life-saving drugs such as cisplatin and taxol are effective in cancer therapy and AZT (Azidothymidine) used for helping AIDS victims, have been isolated from plant and animal sources or prepared by synthetic methods.

With a better understanding of chemical principles, it has now become possible to design and synthesize new materials having specific magnetic, electric and optical properties. This has to lead to the production of superconducting ceramics, conducting polymers, optical fibres and large scale miniaturization of solid-state devices. In recent years chemistry has tackled with a fair degree of success some of the pressing aspects of environmental degradation. Safer alternatives to environmentally hazardous refrigerants like CFCs (chlorofluorocarbons), responsible for ozone depletion in the stratosphere, have been successfully synthesised. However, many big environmental problems continue to be matters of grave concern to chemists. One such problem is the management of the Green House gases like methane, carbon dioxide etc. Understanding biochemical processes, the use of enzymes for large-scale production of chemicals and the synthesis of new exotic materials are some of the intellectual challenges for the future generation of chemists. A developing country like India needs talented and creative chemists for accepting such

challenges.

NATURE OF MATTER

Everything around us, for example, book, pen, pencil, water, air, all living beings etc. are composed of matter. You know that they have mass and they occupy space.

You are also aware that matter can exist in three physical states viz. solid, liquid and gas. The constituent particles of matter in these three states can be represented as shown in Fig. 1.1. In solids, these particles are held very close to each other in an orderly fashion and there is not much freedom of movement. In liquids, the particles are close to each other but they can move around. However, in gases, the particles are far apart as compared to those present in solid or liquid states and their movement is easy and fast. Because of such arrangement of particles, different states of matter exhibit the following characteristics:

(i)    Solids have definite volume and definite shape.

(ii)   Liquids have definite volume but not a definite shape. They take the shape of the container in which they are placed.

(iii)  Gases have neither definite volume nor definite shape. They completely occupy the container in which they are placed.

These three states of matter are interconvertible by changing the conditions of temperature and pressure.

Solid ↽ he at  ⇀   liquid ↽ heat ⇀   Gas

You are already familiar with the term matter. Anything which has mass and occupies space is called matter. On heating, a solid usually changes to a liquid and the liquid on further heating

Mole Concept

• What is Mole Concept?

Amedeo Avogadro

According to Amedeo Avogadro, one gram atom of any element contains the same number of atoms and one gram molecule of any substance contains the

the same number of molecules which is equal to 6.0221367 ×10²³                       

The amount of the substance containing 6.0221367 ×10²³atoms or molecules is called one mole.

For example, 1 mole of sodium means 6.022 ×10 ²³ atoms of sodium. 1 mole of any substance can be defined as the amount of a substance that contains as many particles (atoms, molecules or ions) as there are atoms in 12 g of the 12 C isotope.

The molar mass of a substance is the mass of one mole of a subset

Atoms and molecules are extremely small in size and their numbers in even a small amount of any substance is really very large. To handle such large numbers, a unit of similar magnitude is required. Just as we denote one dozen for 12 items, a score for 20 items, gross for 144 items, we use the idea of the mole to count entities at the microscopic level (i.e. atoms/molecules/particles, electrons, ions, etc).

In the SI system, mole (symbol, mol) was introduced as the seventh base quantity for the amount of a substance.

•  What is a mole?

Mole is the unit of the amount of substance.

One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012 kg) of the ¹²Cisotope

It may be emphasized that the mole of a substance always contains the same number of entities, no matter what

the substance may be. In order to determine this number precisely, the mass of a carbon– 12 atoms was determined by a mass spectrometer and found to be equal to

1.992648 × 10^-23 g. Knowing that one mole of carbon weighs 12 g, the number of atoms in it is equal to  6.0221367 × 10²³atom/mol

•  What is Avogadro's Number?

Avogadro's number is nothing but the number of particles present in 1 mole of any substance.

Numerically it is equal to 6.022 × 10 ²³ . It is also called Avogadro Constant and is represented by the symbol N_A.

• What is the difference between molecular mass and molar mass?

The molecular mass and molar mass are two different terms and should not be confused as implying the same meaning. 

The molecular mass of a compound is the sum of the atomic masses of all the atoms present in a molecule of that compound. It is expressed in the atomic mass unit (AMU) or unified mass (u).

In order to calculate the molecular mass of a compound, the mass of all the atoms present in it is added.

For example,

Molecular mass of glucose (C₆H₁₂O₆) is = 6 (C) + 12 (H) + 6 (O) = 180 u.

While the molar mass of any compound is the mass of one mole i.e. 6.023 x 1023 molecules of the compound.

Although molar mass is reported in gram and molecular mass is reported in an atomic mass unit, both are numerically equal.

For example, the molar mass of glucose is 180 grams while its molecular mass is 180 u. But the former is mass of 6.023 ×10²³ molecules of glucose while later one is mass of one molecule of glucose