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Chapter 1:Chemical Reactions and Equations

Introduction

  • Any process that involves the rearrangement of the structure of the substance or conversion of reactants into products is defined as a Chemical Reaction
  • In a chemical reaction the nature and the identity of any substance change.
  • Some common examples of such change from everyday life are:
  • Formation of curd from milk. (Lactose present in milk is converted into lactic acid by the action of bacteria.)
  • Rusting of iron nails or iron rods or pans when exposed to the humid atmosphere. (Iron gets converted into oxides of iron i.e., rust when exposed to water and oxygen).
  • Fermentation of grapes (glucose present in grapes gets converted into alcohol and carbon dioxide gas by action of yeast).
  • Digestion of food in our body. (Complex food i.e., sucrose is broken down into simpler particles like glucose)
  • Our respiration process generates energy.
    • Glucose in the presence of oxygen broken down to release energy along with carbon dioxide and water.
  • Photosynthesis is done by plants.
    • Carbon dioxide and water combine in the presence of sunlight and chlorophyll to form glucose.

Chapter Summary

  • 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.
  • In a combination reaction, two or more substances combine to form a new single substance.
  • Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances.
  • 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.
  • When an element displaces another element from its compound, a displacement reaction occurs.
  • Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
  • Precipitation reactions produce insoluble salts.
  • Reactions also involve the gain or loss of oxygen or hydrogen by substances.
  • Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or the gain of hydrogen.
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