Chemical Changes and Reactions

Chemical Changes and Reactions

• Chemical reactions involve breaking chemical bonds in reactants and forming new bonds in products.
• Reactants are the substances present before the reaction, and products are the substances formed after the reaction.
• Chemical reactions may involve changes in state, color, formation of precipitates, evolution of gas.
• Conditions for chemical change: Mixing (tangible contact), solution (in liquid state), heat, light, electricity, pressure.

Types of Chemical Changes

• Direct combination (synthesis): Two or more substances combine to form a single substance.
  • Example: 2Mg + O₂ → 2MgO (formation of magnesium oxide)
• Decomposition: A single substance breaks down into two or more substances.
  • Example: 2KClO₃ → 2KCl + 3O₂ (decomposition of potassium chlorate)
• Displacement: One element displaces another element from a compound.
  • Example: CuSO₄ + Zn → ZnSO₄ + Cu (zinc displaces copper from copper sulfate)
• Double decomposition (double displacement): Two compounds exchange parts to form two new compounds.
  • Example: NaCl + AgNO₃ → AgCl + NaNO₃
• Photochemical reactions: Require light energy for the reaction to proceed.
  • Example: Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (formation of glucose using light energy)
• Electrochemical reactions: Require electrical energy for the reaction to proceed.
  • Example: Electrolysis of water: 2H₂O → 2H₂ + O₂ (producing hydrogen and oxygen)

Energy Changes

• Exothermic reactions: Release heat energy to the surroundings as part of reaction.
  • Example: Combustion reactions like burning of natural gas.
• Endothermic reactions: Absorb heat energy from the surroundings as part of reaction.
  • Example: Photosynthesis, thermal decomposition of metal carbonates

Catalysts

• Catalysts speed up or slow down a reaction without altering the products or reactants.
• Positive catalysts speed up the reaction.
• Negative catalysts slow down the reaction.

Other Important Chemical Changes

• Thermal decomposition: Decomposition of a compound brought about by heating.
  • Ex: CuCO₃(s)--heat-->CuO(s) + CO₂(g).
• Hydrolysis: A reaction where a substance reacts with water to form another substance.
• Precipitates: Formation of an insoluble solid substance during a chemical reaction.
  • Example: When silver nitrate is added to sodium chloride solution, a white precipitate of silver chloride is formed.
• Effervescence: Formation of gas bubbles during a reaction.
  • Example: When dilute acid is added to sodium carbonate, a gaseous product is produced.

Metal Hydroxides

• Many metal hydroxides are stable to heat.
• Upon heating, metal hydroxides decompose to form metal oxides and water vapour.

Metal Carbonates

• Many metal carbonates are stable to heat.
• Upon heating, metal carbonates decompose to form metal oxides and carbon dioxide gas.
• A more vigourous decomposition is possible in certain situations.

Metal Bicarbonates

• Metal bicarbonates decompose to form metal carbonates, water, and carbon dioxide.

Explore the fascinating world of chemical changes and reactions through this quiz. Test your understanding of various types of chemical reactions, including direct combination, decomposition, and displacement. Learn about the conditions required for chemical changes and the outcomes of different reactions.