Introduction
A dynamic environment of matter and material surrounds man. Since his birth he has been exposed to these changing surroundings. His curiosity to understand his surroundings and the events occurring around him, has led him to systematically enquire and collect information through experiments and observations. This knowledge is referred to as science. Science may be defined as systematized knowledge gained by mankind through observations and experimentation.
Importance of Chemistry
The branch of science, which deals with the study of matter, its composition, its properties and the changes that it undergoes in composition as well as in energy during various processes, is defined as chemistry. It has been further divided into different branches depending upon specialized fields of study.
Physical Quantities and their Measurement
The development of chemistry is based upon the systematic approach of the scientists in carrying out different experiments. Scientific theories, inferences and generalizations are made on the observations recorded. The quantitative information involves the measurement of one or more quantities. These measurements are often used in calculations to obtain other related quantities. The ability of a chemist to measure properties as accurately as possible depends upon his skill and the degree of accuracy of the instrument.
S.I. Units
The value of any physical quantity is always expressed as its numerical value as well as a unit. One often hears the weight expressed as grams or kilograms; the distance as foot, yard or mile; the volume as litres or gallons. The use of a number of units cause confusion and complications. Hence, the need for a uniform system of measurement was felt as a necessity. In 1971, the French Academy of Science devised the metric system of measurement based on the decimal system. In October 1960, The XIth General Conference of Weights and Measures (Conference Generale des Poids et Mesures) adopted an International System of Units. This system is known as SI system of units after the French word, Systeme' International d'Unites.
Derived Units
The units of all physical quantities can be derived from the seven basic units. These units are called derived units because they can be derived from the basic units algebraically by multiplication and division. It is frequently necessary to convert one set of units to another.
Uncertainty in Measurement
When dealing with objects that can be counted, one can always get an exact answer. But while measuring the weight of a person, the volume of milk in a container, the length of a cloth etc. it is not possible to measure them exactly.
Significant Figures
The reliability of a measurement is indicated by the number of digits used to represent it. To express it more accurately we express it with digits that are known with certainty. These are called as Significant figures. They contain all the certain digits plus one doubtful digit in a number.
Problems - Significant Figures
1. State the number of significant figures in each of the following:
(i) 208.91 (ii) 0.00456 (iii) 453 (iv) 2.945 x 104 (v) 0.346
Classification of Matter
All around us are substances that are made of matter. Matter has mass and occupies some space. Different kinds of substances are made up of different matter. Matter can be thus classified in many ways. Two basic classifications of matter are based on its physical and chemical properties. The physical classification of matter shows its physical appearance that describe the shape, hardness, softness, melting and boiling points. The chemical classification of matter shows the ability of matter to change its composition to form new substances.
Mixtures
A mixture is a combination of two or more elements or compounds in any proportion so that the components do not lose their identity. Air is an example of a mixture (several gases mix to form air).
Laws of Chemical Combination
The study of reactions is one of the important areas of Chemistry. There are many aspects of a reaction like the rate of reaction, energy absorbed or evolved, mechanism of a reaction that can be evaluated only with weight and volume relationships. A study of reactions is possible by understanding the concept of atoms and molecules and the definite laws and patterns regulating them.
Dalton's Atomic Theory
John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805.
Modern Atomic Theory
In view of new discoveries about atoms, Dalton's atomic theory was modified as given below:
* Atom is no longer indivisible as it has a complex structure of sub atomic particles (electrons, protons and neutrons).
* Atoms of the same element may posses different relative masses (isotopes).
* Certain atoms of different elements have same relative masses but their chemical properties are entirely different (isobars).
Avogadro's Hypothesis
In trying to correlate Dalton's atomic theory with Gay Lussac's law of gaseous volume, Berzelius put forward the hypothesis that equal volumes of all gases contain equal number of atoms under similar conditions of temperature. However, it was found that even a fraction of an atom could be involved in certain chemical reactions. This went against Dalton's atomic theory. Therefore, Avogadro postulated the existence of molecules along with atoms as two kinds of ultimate particles.
Problems
7. 1.375 g of cupric oxide on reduction in hydrogen gas gives 1.098 g of copper. In another experiment, 1.179 g of metallic copper produced 1.476 g of copper oxide. Show that these results illustrate the law of constant proportions.
Atoms and Molecules
The smallest particle of an element, which may or may not have independent existence is called an atom, while the smallest particle of a substance which is capable of independent existence is called a molecule. Molecules are classified as homoatomic and heteroatomic. Homoatomic molecules are made up of the atoms of the same element and have different atomicity (number of atoms in a molecule of an element) like monoatomic, diatomic, triatomic and polyatomic.
Atomic and Molecular Masses
It is known that the atomic mass of an element/compound is not the mass of one atom or molecule of the said element/compound because they occur as a mixture of isotopes. As the atomic and molecular masses are expressed on a relative scale based on a mass of 12C atom, all atomic and molecular masses are in fact the weighted average of the mass of these isotopes. Thus, atomic mass of an element is defined as the average relative mass of an atom of an element as compared to the mass of an atom of carbon (12C ) taken as 12.
Mole Concept
In chemistry, the term mole represents a pile or mass of atoms, molecules, ions or electrons. Just as common man measures quantity in terms of kilograms or dozens, a chemical scientist deals with a 'mole' of atoms, molecules, ions or electrons.
Chemical Formulae
Every chemical substance is known by a specific name. But many a times these names are cumbersome, confusing, and do not provide information about its chemical composition. To overcome this, each chemical compound is represented by a chemical formula that gives its composition (constituent elements present) and the number of elements of each type present.
Determination of the Molecular Formula of a Compound
* The empirical formula of the compound is written.
* The empirical formula mass is calculated by adding the atomic masses of all atoms present in the empirical formula.
Stoichiometry of Chemical Reactions
A chemical reaction can be represented by a chemical equation, which may be defined as a chemical change in terms of symbols and formulae of the substances involved in the reaction. The substances that react among themselves to bring about the chemical changes are known as reactants, where as the substances that are produced as a result of the chemical change, are known as products.
Balancing of Chemical Equation
A chemical equation should be balanced so as to satisfy the requirements of the law of conservation of mass, as no matter is destroyed or created during a chemical reaction.
Stoichiometric Calculations Involving Reactions/Solutions
Most chemical reactions are carried out in solutions. It is therefore important to know how to express the concentration of the substance in its solution.
Normality
Normality is defined as the number of gram equivalents of solute present per litre (dm3) of the solution at any given temperature.
Normality Equation
The normality equation is commonly used to calculate the normality of solutions after dilution. The above equation is also called as dilution formula because it helps in calculating the volume of the solvent required in diluting a concentrated solution. The same equation holds good for calculations involving molarity (M).
Relationship between Normality and Molarity of Solutions
Normality and Molarity are related as:
Other Methods of Expressing Concentration of Solution
Molality is defined as the number of moles of solute dissolved per 1000 g (1 kg) of solvent. Molality is expressed as 'm'.
