Tiêu đề Chapter - 1 Chemical_Reactions_and_Equations.pdf ✅

ChemContent Chemical Reactions and Equations ● Most of the substance around us undergoes various changes. Some of these changes are temporary with no new substance being formed. They are called physical changes. Example: Water changes to steam on boiling but no new substance is formed(Even though steam and water look different when they are made to react with a piece of Na, they react the same way and give the exact same products). This involves only a change in state (liquid to vapour). ● A substance is said to undergo a chemical change when the chemical properties of a substance alter. As a result, there is either formation or breaking of atomic bonds at the molecular level. Some characteristics of a chemical change are: o New substances are produced during a chemical reaction. o Changes in energy are involved. o During the reaction, there occurs a change in mass. o There is a permanent alteration. Examples: ● Cooking of food ● Rusting of iron ● Heating of Lead nitrate ● Souring of milk ● Ripening of fruit. Chemical Equation Writing Chemical Equations ● In a chemical reaction, the reactants are written on the left hand side and the products on the right hand side of the equation. ● An arrow (→) pointing towards the products is inserted between the reactants and the products. It also represents the direction of the reaction. ● A single arrow (→) indicates the direction in which the reaction proceeds. ● A double arrow ( ) indicates a reversible reaction, i.e. the products recombine to form the reactants. ● A plus sign (+) is inserted between two or more reactants or products formed. ● If reactions are carried out under specific conditions of temperature, pressure, catalyst etc., then these conditions are mentioned on the arrow. ● The chemical equation can be made more informative by mentioning the physical states of the reactants and products. ● If gas is liberated as a product, then it is represented by an arrow pointing upwards (↑). If the product formed is in the form of a precipitate, it is represented by an


ChemContent ● Exothermic Reaction: The reactions accompanied by the evolution of heat are called endothermic reactions. 2. Decomposition Reaction A chemical reaction in which a single compound splits into two or more simple substances is called a decomposition reaction. Correct way of heating the boiling tube containing crystals of ferrous sulphate and of smelling the odour For example: When mercuric oxide is heated in a crucible, the orange-red powder begins to darken and a silver mirror begins to deposit on the cooler parts of the crucible. ● Thermal Decomposition Reaction: The decomposition reactions carried out by heating are known as thermal decomposition reactions. ● Photochemical reaction: The chemical reactions which proceed with the absorption of light energy are called photochemical reactions. 3. Displacement Reaction Reactions in which the more reactive element displaces the less reactive element from its compound are called displacement reactions. Iron nails dipped in copper sulphate solution For example: Zinc displaces copper in copper sulphate to form zinc sulphate.