Tiêu đề Chapter 1 Chemical Reactions and Equations.pdf ✅

SCIENCE (Chemistry) Chapter 1: Chemical Reactions and Equations
(1) 01 CHEMICAL REACTIONS AND EQUATIONS Chemical Reactions and Equations Chemical Reactions: A chemical reaction occurs when one or more reactants (also known as reactants) are changed into one or more products (also known as products). The constituent atoms of the reactants are rearranged in a chemical reaction, resulting in the formation of various substances as products. A change that results in the formation of one or more new compounds. Chemical changes are also known as chemical reactions. • 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
(2) 01 CHEMICAL REACTIONS AND EQUATIONS 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 arrow pointing downwards(↓). Balancing the Chemical Equations • In a balanced chemical equation, the total number of atoms of each element of the reactants on the left hand side of the equation is equal to the number of atoms of the products formed on the right hand side of the equation. • The total mass of the reactants is equal to the total mass of the products or the number of atoms of each element before the reaction and after the reaction is equal. Steps Involved in Balancing a Chemical Equation Consider the chemical reaction between magnesium and oxygen to understand the steps involved in balancing a chemical equation. Step 1 Let us first write the word equation for thisreaction. Magnesium + Oxygen → Magnesium oxide Step 2
(3) 01 CHEMICAL REACTIONS AND EQUATIONS Write the chemical equation for the reaction between magnesium and oxygen. Mg + O2 → MgO Step 3 Count the number atoms of an element occurring on both L.H.S. and R.H.S. in this equation. Mg + O2 → MgO Component Reactant Product Magnesium 1 1 Oxygen 2 1 This is an unbalanced equation. Step 4 • To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation. • Choose a reactant or a product which has the maximum number of atoms in it. In that compound, select the element which has the maximum number of atoms. In this equation we shall select MgO i.e. magnesium oxide and the element oxygen in it. • To balance the oxygen atoms, let us multiply magnesium oxide molecule by 2 on the right hand side. The equation can now be expressed as, Component Reactant Product Magnesium 1 1 × 2 = 2 Oxygen 2 1 × 2 = 2 Step 5 There are two oxygen atoms on either side of the equation but one magnesium atom on the reactant's side and two on the product's side. Therefore, multiply the magnesium atom by 2 on the left hand side. Component Reactant Product Magnesium 1 × 2 = 2 2 Oxygen 2 2 Balanced equation is, ∴ The number of atoms of each element of reactants = The number of atoms of each element of products Step 6