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ICSE Class 8 Chemistry Question 9 of 15

Chemical Reactions — Question 13

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Question 11

Define exothermic and endothermic reactions. Give two examples of each.

Answer

Exothermic Reaction

A chemical reaction in which heat is given out is called an exothermic reaction.

Examples:

  1. When carbon burns in oxygen, carbon dioxide is formed and a lot of heat is produced.
    C + O2 ⟶ CO2 + Heat
  2. When water is added to quicklime, slaked lime is formed and along with it a large amount of heat energy is produced.
    CaO + H2O ⟶ Ca(OH)2 + Heat

Endothermic Reaction

A chemical reaction in which heat is absorbed is called an endothermic reaction.

Examples:

  1. When nitrogen and oxygen gas are heated together to a temperature of about 3000°C, nitric oxide gas is formed.
    N2 + O2 →3000°Cheat\xrightarrow[3000\degree\text{C}]{\text{heat}} 2NO
  2. When calcium carbonate is heated at 1000°C, it decomposes to give calcium oxide and carbon dioxide.
    CaCO3 →1000°Cheat\xrightarrow[1000\degree\text{C}]{\text{heat}} CaO + CO2
Chemical Reactions - Study Guide | Bright Tutorials
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Chemistry | Chapter 5: Chemical ReactionsWeb Content / Study Guide

Chemical Reactions — Interactive Study Guide

Quick Concept Map

CHANGES: Physical (reversible, no new substance) vs Chemical (irreversible, new substance)
REACTION TYPES: Combination | Decomposition | Displacement | Double Displacement
ENERGY: Exothermic (releases heat) vs Endothermic (absorbs heat)
REDOX: Oxidation (gain O / lose H) + Reduction (lose O / gain H) happening together

Four Types of Reactions — Quick Reference

TypePatternEasy Memory AidExample
CombinationA + B → ABJoining togetherMg + O2 → MgO
DecompositionAB → A + BBreaking apartCaCO3 → CaO + CO2
DisplacementA + BC → AC + BBully kicks out the weakFe + CuSO4 → FeSO4 + Cu
Double DisplacementAB + CD → AD + CBExchange partnersNaCl + AgNO3 → AgCl + NaNO3

Signs of a Chemical Reaction

  1. Change in colour
  2. Evolution of gas (bubbles)
  3. Change in temperature (hot or cold)
  4. Formation of precipitate (insoluble solid)
  5. Change in smell
  6. Change in state
  7. Sound or light produced

Oxidation vs Reduction

OxidationReduction
Gain of oxygenLoss of oxygen
Loss of hydrogenGain of hydrogen
Example: C + O2 → CO2 (C is oxidised)Example: CuO + H2 → Cu + H2O (CuO is reduced)

Memory aid: OIL RIG — Oxidation Is Loss (of hydrogen), Reduction Is Gain (of hydrogen)

Self-Check Questions

  1. Is dissolving salt in water a physical or chemical change? Why?
  2. Classify: Iron + Copper sulphate → Iron sulphate + Copper. What type of reaction is this?
  3. What is the difference between exothermic and endothermic reactions? Give one example of each.
  4. In the reaction CuO + H2 → Cu + H2O, which substance is oxidised and which is reduced?
  5. Write word equations for one example of each: combination, decomposition, displacement, double displacement.

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