# Railways Chemistry General Concepts of Chemistry General Concepts of Chemistry

General Concepts of Chemistry

Category : Railways

Introduction

• Definition

Chemistry is a branch of science which deals with study of matter and various changes it undergoes. It deals with the preparation, properties, reactions and structures of chemical elements and compounds. For convenience the study of chemistry is sub-divided into various branches such as:

(i) Inorganic chemistry                 (ii) Organic chemistry                   (iii) Physical chemistry

(iv) Analytical chemistry              (v) Industrial chemistry

• Valency

During the formation of molecules of compounds, atoms combine in certain fixed proportions. This is because of the fact that different atoms have different combining capacities. The combining capacity of an atom or radical is known as its valency.

In addition to atoms and molecules, a third type of particles occurs in substances. These particles, called ions, or atoms or group of atoms that carry an electrical charge. An ion is formed when electrons are removed from or added to an atoms or group of atoms. When electrons is/are removed the resulting ion is called a cation or basic radical. A cation is positively charged ion. (e.g$N{{a}^{+}}$. When electron is/are added the resulting ion is called an anion or acidic radicals. An anion is negatively charged ion e.g.,$(C{{l}^{-}},O_{2}^{2-})$An ion or radical is classified as monovalent, divalent, trivalent or tetravalent when the number of charges over it is 1,2,3 or 4 respectively.

• Formula of Elements and Compounds

Formula of dements: The molecule of an element is denoted by writing the symbol of the element and, to the right and below it, a number expressing the number of atoms in the molecule.

Formula of compound: A molecule of a compound maybe made up of atom of different elements linked up together chemically and in definite proportion by weight.

• Chemical formula: It is of two types:

(i) Molecular formulae: Chemical formulae that indicate the actual number and type of atoms in a molecule is called molecular formulae.

(ii) Empirical formulae: Chemical formulae that indicate only the relative number of atoms of each type in a molecule is called empirical formulae.

• Equivalent weight

(i) Equivalent weight of element $=\frac{Atomic\,\,weight\,\,of\,\,element}{Valency\,\,of\,\,element}$

(ii) Eq. wt of an acid/base$=\frac{Molecular\,\,mass}{Basicity\,\,of\,\,acid/Acidity\,\,of\,\,base}$

(iii) Eq. wt of salts$=\frac{Formula\,\,mass}{(Valency\,\,of\,\,cation)(No.of\,\,cations)}$

• Expression of strength /concentration of Solution

(i) Mass percent = $\frac{Weight\,\,of\,\,solute(gm)}{Weight\,\,of\,\,solution(gm)}\times 100$

(ii) Normality $=\frac{numer\,\,of\,\,gram\,\,equivalents\,\,of\,solute}{Volume\,\,of\,solution\,(lit.)}$

(iii) Molarity =$\frac{Number\,\,of\,\,gram\,\,moles\,\,of\,\,solute}{Volume\,\,of\,\,solution(lit.)}$

(iv) Molality $=\frac{Gram\,\,moes\,\,of\,\,solute}{weight\,\,of\,\,solent\,(kg)}$

(v) Mole fraction:  Mole fraction of solute $={{X}_{A}}=\frac{{{n}_{A}}}{{{n}_{A}}+{{n}_{B}}}$

Mole fraction of solvent $={{X}_{B}}=\frac{{{n}_{B}}}{{{n}_{A}}+{{n}_{B}}}$

${{X}_{A}}+{{X}_{B}}=1$

• Chemical reaction: The process in which a substance undergoes change to produce new substances with new properties are known as chemical reaction. For example magnesium carbonate when heated produces magnesium oxide and carbon dioxide (i.e. new substances with new properties). A chemical change is generally accompanied by a change of state, change of colour, evolution of a gas or change of temperature etc.
• Chemical equation: The qualitative representation of a chemical reaction in a short hand or concise form in term of symbols and formulae, is called a chemical equation.
• Skeletal chemical equation or symbol equation: A chemical equation written in the form of symbols and formulae is called a skeletal chemical equation.
• Balanced chemical equation: A chemical equation in which number of atoms of each elements on L.H.S. (i.e. reactants) and R.H.S. (i.e. products) is equal is called a balanced chemical equation.
• Balancing of chemical equations: The process of making the number of different elements on both side of the equation equal is known as balancing of chemical equation.A

Types of chemical reactions

• Various types of chemical reactions are:-

(i) Combination reactions: Combination reactions are those in which one element reacts with another to form a compound. This type of reactions are also known as synthesis reaction. For example, hydrogen combines with oxygen to give water.

$2{{H}_{2}}(g)+{{O}_{2}}(g)\to 2{{H}_{2}}O(l)$

(ii) Decomposition reactions: Decomposition reactions are those reactions in which a compound breaks down into simpler compounds (or substances). This type of reaction is simply the reverse of combination reactions. These reactions require energy in the form of heat, light, electricity, etc.

(iii) Simple displacement reaction and simple substitutions: A displacement reaction is a reaction in which an atom, or group of atoms, present in a molecule is displaced by another atom. This type of reaction can be represented as follows: $\underset{(Compound)}{\mathop{AB+C}}\,\to \underset{(New\,\,compound)}{\mathop{AC+B}}\,$

(iv) Double displacement reactions or Double decomposition: The reactions in which mutual exchange of radicals takes place are known as double decomposition reactions. As a result of double decomposition reactions two new substances are formed.

Examples: $2NaCl+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+2HCl$

Double-displacement reactions can be further classified as precipitation, gas formation, and acid-base neutralization reactions.

• Precipitation reactions: A precipitation reaction occurs when two solutions are mixed together and a solid separates from the solution. The solid part that forms and separates form the solutions is called the precipitate the reaction given above is a precipitation reaction.

(v) Oxidation-Reduction Reactions

• Oxidation: Oxidation is defined as a process which involve addition of oxygen or removal of hydrogen.
• Reduction: The term reduction is defined as a process which involve the removal of oxygen or addition of hydrogen.

(vi) Redox reactions: Those reactions in which oxidation and reduction takes place simultaneously, are known as redox reactions.

(vii) Exothermic and endothermic reaction: Chemical reactions usually proceed with either liberation of heat or the absorption of heat. When a chemical reaction liberates heat to the surroundings, it is said to be 'exothermic reaction' and when it absorbs the heat from the surroundings, it is said to be endothermic reaction.

${{N}_{2}}(g)+3{{H}_{2}}(g)\to 2N{{H}_{3}}(g)$+energy (Exothermic)

$CaC{{O}_{3}}(s)\xrightarrow{\Delta }CaO(s)+C{{O}_{2}}(g)$(endothermic)

• Corrosion (erosion by chemical action): Corrosion is the degradation of metals and generally called rust. Corrosion causes damage to car bodies, iron railings, ships and to all objects made of metals, especially those of iron. Corrosion of iron is a serious problem.
• Rancidity: The most important cause of deterioration in fats and fatty foods is oxidation of fats. What we perceive is an unpleasant change in the flavour and odour of a food, called rancidity.

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