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What does one mean by exothermic and endothermic reactions? Give examples.

Exothermic & Endothermic reactions refer to the flow of energy (usually in the form of heat) during a chemical reaction.

Exothermic Reactions:

An exothermic reaction releases energy, usually in the form of heat, to its surroundings. In these reactions, the energy of the products is lower than that of the reactants. For example:

  • Combustion (burning) of fuel like methane:--      CH₄ + 2O₂ → CO₂ + 2H₂O + energy (heat)
  • In this reaction, energy is released as heat when methane burns.

Endothermic Reactions:

An endothermic reaction absorbs energy from the surroundings. In these reactions, the energy of the products is higher than that of the reactants. For example:

  • Photosynthesis in plants:--        6CO₂ + 6H₂O + energy (sunlight) → C₆H₁₂O₆ + 6O₂
  • Plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

In summary:

  1. Exothermic = Energy released (e.g., burning fuels).
  2. Endothermic = Energy absorbed (e.g., photosynthesis).
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More Questions on Chemical Reactions and Equations

  1. A solution of substance ‘X’ is used for white washing. What is the substance ‘X’? State the chemical reaction of ‘X’ with water.
  2. Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water which is a compound made up of hydrogen and oxygen is used to extinguish fire. Why?
  3. What is meant by a chemical reaction?
  4. 1 g of copper powder was taken in a China dish and heated. What change takes place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equations of reactions, the name and the colour of the products formed in each case.
  5. Why should a magnesium ribbon be cleaned before burning in air?
  6. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
  7. What do you mean by a precipitation reaction? Explain by giving examples.
  8. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
  9. Oil and fat containing food items are flushed with nitrogen. Why?
  10. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
  11. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
  12. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions
  13. Why is respiration considered an exothermic reaction? Explain
  14. What does one mean by exothermic and endothermic reactions? Give examples.
  15. What is a balanced chemical equation? Why should chemical equations be balanced?
  16. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?