Chapter - 1 Chemical Reactions and Equations

• Change in state, change in colour, evolution of a gas and change in temperature are indicators of a chemical reaction.
• A complete chemical equation represents the reactants, products and their physical states symbolically. A chemical equation is balanced so that the numbers of atoms of each type involved in a chemical reaction are the same on the reactant and product sides of the equation. Equations must always be balanced.
• When a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide.
• The word-equation for the above reaction would be – Magnesium + Oxygen → Magnesium oxide
• The substances that undergo chemical change in the reaction, magnesium and oxygen, are the reactants. The new substance, magnesium oxide, formed during the reaction, is the product. A word-equation shows change of reactants to products through an arrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products, and shows the direction of the reaction. A chemical equation represents a chemical reaction.
• Mg + O₂ → MgO
• Since the number of atoms of each element is not the same on both the sides, then the equation is unbalanced because the mass is not the same on both sides of the equation. Such a chemical equation is a skeletal chemical equation for a reaction.
• The number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation.
• Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
• The above word-equation may be represented by the following chemical equation –

                 Zn + H ₂SO ₄→ ZnSO ₄ + H ₂

• Calcium oxide and water combine to form a single product, calcium hydroxide. Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.

                 2FeSO ₄ → Fe ₂O ₃+ SO₂+ SO₃

• In this reaction a single reactant breaks down to give simpler products. This is a decomposition
• Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction used in various industries. Calcium oxide is called lime or quick lime. It has many uses – one is in the manufacture of cement. When a decomposition reaction is carried out by heating, it is called thermal decomposition.

                  Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

• In this reaction, iron has displaced or removed another element, copper, from copper sulphate solution. This reaction is known as displacement reaction.
• Any reaction that produces a precipitate can be called a precipitation reaction.
• Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.
• Reactions in which heat is given out along with the products are called exothermic reactions. „ Reactions in which energy is absorbed are known as endothermic reactions.

OXIDATION AND REDUCTION

• If a substance gains oxygen during a reaction, it is said to be oxidised. If a substance loses oxygen during a reaction, it is said to be reduced.
• When one reactant gets oxidised while the other gets reduced during a reaction, such reactions are called oxidation-reduction reactions or redox reactions.
• When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. The black coating on silver and the green coating on copper are other examples of corrosion. Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron. Corrosion of iron is a serious problem.
• When fats and oils are oxidised, they become rancid and their smell and taste change. Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation.