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CHEMICAL REACTIONS AND EQUATIONS, Important concepts related to the unit:, Molecule: A group of two or more atoms held together strongly by chemical bonds. A, molecule is the smallest particle of an element or a compound which can exist freely under, ordinary conditions. It shows all the properties of that substance., Molecule of an element: When one, two or more atoms of the same element existing as, one species in the free state, is called molecule of an element. Molecules of elements may, be monoatomic, diatomic, triatomic, tetraatomic or polyatomic., , , , , , , , , , , , , , , , S.No. | Type of molecule Examples, , 1 Monoatomic He, Ne, Ar, , 2 Diatomic H2, Oz, Cl2, Fz, 3 Triatomic Os, , 4 Tetratomic Ps, , 5 Polyatomic Ss, Ceo, , , , , , , , , , Molecules of a compound: When two or more atoms of different elements combined, together in a definite proportion by mass to form a species that can exist freely, is called, molecule of a compound. Molecules of a compound can be diatomic, triatomic or, tetratomic., , Diatomic molecule of a compound HCI, CO, Triatomic molecule of a compound H20, COz, Tetratomic molecule of a compound NHs3, H202, , Valency: The number of electrons lost, gained or shared by an atom of an element in order, to get the electron configuration of the nearest noble gases, is called the valency of that, element., , Valency of some important elements is given below:, , Hydrogen 1 Oxygen 2, Flourine 1 Sulphur 2,4,6, Chlorine 1 Nitrogen 3, , Bromine 1 Phosphorus 3,5, , lodine 1 Carbon 4, , Valency of an lon is the units of positive or negative charge present on the ion. Depending, on the valency of anions, they can be monovalent, divalent or trivalent ions., , , , , , , , , , , , , , , , , , , , , , , , , , , , Monovalent ions Divalent ions Trivalent ions, , Hydroden H* Magnesium Mg?* Aluminium AP, , Sodium Na* Calcium Ca?* lron(111) Fe, , Copper(1) Cu* Copper(11) Cu?* Phosphate POs, , Ammonium NHa* lron(11) Fe?* Nitride N, , Hydroxide OH Zine Zn2* Phosphide P, , Naveed Gull Contact No. 9596477507 Page 1
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Chloride Cl Carbonate COs, Nitrite NOz Sulphate SO, , , , , , , , , , , , Nitrate NOs Oxide O, , , , Laws of chemical combination:, , There are two laws of chemical combination which are discussed below:, , 1. Law of conservation of mass: In any chemical reaction, the total mass of the reactants is, equal to the total mass of the products., , 2. Law of definite proportions: A chemical compound is always made up of the same, elements combined together in the same fixed proportion by mass., , Physical and chemical change:, , Physical change: In a process when non new chemical substances are formed is called a, physical change., , Chemical change: In a process when the original substances lose their nature and identity, and form new chemical substances with different properties, is called a chemical change., , , , Chemical reaction: The process involving a chemical change is called a chemical reaction., Thus a chemical reaction is the process in which a substance or substances undergo a, change to produce new substances with different properties. For example, when calcium, carbonate is heated, calcium oxide and carbon dioxide are formed. It is an example of a, chemical reaction because calcium carbonate changes into calcium oxide and carbon, dioxide which are having entirely different properties., , Calcium carbonate——______,» Calcium oxide + Carbon dioxide, , Reaction between sodium and water to form sodium hydroxide and hydrogen is an, another example of chemical reaction., , Sodium + Water —_______» Sodium hydroxide + Hydrogen, , A chemical reaction simply involves rearrangement of atoms. In a chemical reaction, breaking of chemical bonds in the reactants is followed by the making of new chemical, bonds to form products., , Reactants and products;, , Reactants: The substance or substances which take part in a chemical reaction are called, reactants, For example, in the breaking up of calcium carbonate into calcium oxide and, carbon dioxide, calcium carbonate is the reactant. Similarly, when sodium reacts with water, to form sodium hydroxide and hydrogen, sodium and water are the reactants,, , Products: The substance or substances formed in a chemical reaction are called products., For example, when calcium carbonate breaks to form calcium oxide and carbon dioxide,, calcium oxide and carbon dioxide are the products. Similarly, when sodium reacts with, , , , Naveed Gull Contact No, 9596477507 Page 2
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water to form sodium hydroxide and hydrogen, sodium hydroxide and hydrogen are the, products., , Calcium carbonate——————»_ Calcium oxide + Carbon dioxide, ( Reactant ) ( Products ), , Sodium + Water ——__—_———» Sodium hydroxide + Hydrogen, ( Reactants ) ( Products ), , Chemical equations: The short-hand representation of a chemical reaction in terms of, symbols and formulae of substances is called a chemical equation., , The symbols and formulae of the reactants are written on the left hand side and the, symbols and formulae of products are written on the right hand side. An arrow sign pointing, from reactants to products is put between. For example, when potassium nitrate is heated,, it gives potassium nitrite and oxygen. It can be represented in the form of a chemical, equation as follows., , KNOs ————_> KNOz + Oz, Potassium nitrate Potassium nitrite Oxygen, , Characteristics of chemical reactions: During a chemical reaction some easily observable, changes take place. These are called the characteristics of a chemical reaction., , 1. Evolution of gas: Some of the chemical reactions occur with the evolution of a gas or a, mixture of gases. For example,, , Metals like zinc, magnesium, iron etc., react with dilute hydrochloric acid with the evolution, of hydrogen gas., , Zn+ 2HC| —_» ZnClz + H2, Zinc Hydrochloric acid zinc chloride hydrogen gas, , 2. Formation of precipitate: Sometimes when two solutions are mixed together, a solid gets, separated from the solutions. The solid thus separated is called the precipitate. For, example, when an aqueous solution of sodium sulphate is mixed with a solution of barium, chloride, a white precipitate of barium sulphate is produced., NazSOq + BaClz -_—-» BaSOa + 2NaCl, Sodium Sulphate Barium chloride Barium sulphate, sodium chloride, , 3. Change in temperature: Some chemical reactions are accompanied by rise or fall in, temperature. Reactions in which temperature increases, heat is evolved and such reactions, , , , , , Naveed Gull Contact No. 9596477507 Page 3
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are called exothermic reactions. For example, when sodium reacts with water to form, sodium hydroxide and hydrogen gas, heat is evolved., , 2Na + 2H2.0 —————® 2Na0H + H2, Sodium Water Sodium hydroxide, Hydrogen, , Whereas reactions in which temperature decreases, heat is absorbed and such reactions, are called endothermic reactions. For example, when ammonium chloride and barium, hydroxide are mixed together in a test tube, the bottom of the test tube becomes cold, It is, because heat is absorbed., 2NHACI + Ba(OH)z ———» 2NHsOH + BaClz, Ammonium chloride barium hydroxide ammonium hydroxide — barium, chloride, 4, Change in colour: In some chemical reactions a change in colour is observed. For, example, when chlorine water (yellowish) is added to a solution of potassium iodide, (colourless), a brown solution is obtained., , Cl + 2Kl ———> _—2KCI + la, Chlorine water potassium iodide potassium chloride, iodine, (Brown), , 5. Change in state: Some chemical reactions are accompanied by change in state. For, example, solid wax burns to form water vapour and carbon dioxide which are gases., , Balanced and unbalanced chemical equations:, , Balanced chemical equation is a chemical equation in which the number of atoms of each, element is equal on both the sides of a chemical equation., , For example the reaction between zinc and sulphuric acid can be shown as:, , Zn + H2SOa > ZnSOa+ H2, The above equation contains an equal number of atoms of each element on both sides of, the equation,, Unbalanced chemical equation is a chemical equation in which the number of elements of, the atoms on the two sides of the equation is not the same., Forexample, the reaction between hydrogen and oxygen forming water is represented by a, chemical equation as, , H2 +02 H20, In the above equation the number of atoms of elements on bothe sides of an equation is, not equal. We need to balance such equations with tyhe help of appropriate numbers., , Types of Chemical reactions, , , , Naveed Gull Contact No, 9596477507 Page 4
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Combination reactions, Decomposition reactions, Displacement reactions, Double-displacement reactions, Oxidation & Reduction, , Pe Pp, , Combination reactions, Those reactions in which two or more elements or compounds combine together to, form a single compound are called combination reactions., Foe example, magnesium burns and react with air to form magnesium oxide is an, example a combination reaction., , , , Me + Oz > MgO, Subtypes of combination reactions:, > combination between two elements, > combination between two compounds, > combination between an element anda compound, , Combination between two elements, Cc +02 > C02, 2Na +Clz > 2NaCl, Combination between two compounds, Combination between ammonia and hydrogen chloride to form ammonium chloride., NHs + HCl > NHaCl, Combination between an element and a compound, Reaction between carbon dioxide and carbon to form carbon monoxide, , cOz +C > 2CcO, , , , Naveed Gull Contact No. 9596477507 Page 5
