chemical reactions

• A chemical reaction is the process of transformation of a substance where the substance undergoes a change in chemical composition to form new substances with new properties
• Substances that take part in chemical reactions are called reactants
• New substances that are formed in a chemical reaction are called products
• A chemical reaction is represented by a chemical equation
• A chemical reaction always involves the breaking of existing bonds and the formation of new bonds
• Types of Chemical Reactions:

  • Chemical Combination, Direct Combination, or Synthesis Combination
  • Decomposition
  • Double Displacement
  • Thermal Dissociation
• Chemical Combination Reaction:

  • Two or more reactants/substances combine to form a product/new substance: A+B → AB
  • Examples:
  • Calcium Oxide (Quicklime) reacts with water to give Calcium Hydroxide (Slacked Lime): CaO + H2O → Ca(OH)2
  • Carbon Dioxide reacts with Lime Water to give a white precipitate of Calcium Carbonate: CO2 + Ca(OH)2 → CaCO3 + H2O
  • Carbon combining in the presence of oxygen to give Carbon Dioxide is also a combination reaction that releases heat
  • When Hydrogen combines with Chlorine in the presence of sunlight, it is a combination reaction
• Decomposition Reaction:

  • A reactant/single compound decomposes to give two or more substances as products: AB → A + B
  • Examples:
  • Water decomposes into Hydrogen and Oxygen with the help of an electric current: 2H2O → 2H2 + O2
  • Thermal decomposition: Limestone (Calcium Carbonate) breaks down into Quicklime (CaO) and Carbon Dioxide
  • Lead(IV) Oxide on heating decomposes to give Lead(II) Oxide and Oxygen
  • Mercuric Oxide on heating decomposes to give Mercury and Oxygen
  • Metal chlorate on heating decomposes to give metal chloride and oxygen gas
• • Nitrates of Mercury and Silver decompose to give the respective metals, oxygen, and nitrogen dioxide
• Displacement Reaction:

  • An element displaces another element or a group of atoms to form a compound: A + BC → AC + B
  • Displacement reactions are common among metals, where a more reactive metal displaces a less reactive metal from its solution
  • Displacement reactions are also known as substitution reactions
• Double Displacement Reaction:

  • Two reactants exchange ions to form two new compounds
  • Precipitate Reaction: Two soluble salts react in an aqueous solution to form one of the products as an insoluble salt
• Neutralisation Reaction:

  • An acid and a base react to form a salt and water: Acid + Base → Salt + Water
  • The H+ ion in acids reacts with the OH- in bases to form an ionic salt
• Thermal Reaction:

  • A reversible decomposition reaction taking place in the presence of heat
• Exothermic Reaction:

  • A reaction characterized by the release of energy
• Endothermic Reaction:

  • A reaction in which energy/heat is absorbed
• Properties of Oxides:

  • Non-metals form acidic oxides
  • Acidic oxides are held together by covalent bonds and have a low melting and boiling point
  • Acidic oxides react with a base to form salt and water
  • Metals tend to form basic oxides
  • Basic oxides are usually ionic in nature and react with acids to form salt and water
  • Acidic oxides combine with water to form an acid
  • Basic oxides combine with water to form a base
  • Amphoteric oxides react with both acids and bases
  • Neutral solution: a substance that has neither basic nor acidic properties