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Chemical Reactions — Question 3

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Question 3

How do the following help in bringing about a chemical change? Explain each with a suitable example.

(a) Pressure

(b) Light

(c) Catalyst

(d) Heat

Answer

(a) Some chemical reactions take place when the reactants are subjected to high pressure.

When nitrogen and hydrogen gas are subjected to high pressure, ammonia gas is produced.

N2+3H2⇌200 atmos.pressure450 °C 2NH3\text{N}_2 + 3\text{H}_2 \xrightleftharpoons[\underset{\text{pressure}}{\text{200 atmos.}}]{\text{450 \degree C }} \text{2NH}_3

(b) Some chemical reactions can take place only in the presence of light and are called photochemical reactions.

Photosynthesis is a chemical reaction in which glucose is prepared by the green leaves of a plant but light is necessary for the reaction to take place.

6CO2[carbon dioxide]+6H2O→SunlightChlorophyllC6H12O6[glucose]+6O2[oxygen]\underset{\text{[carbon dioxide]}}{6\text{CO}_2} + 6\text{H}_2\text{O} \xrightarrow[\text{Sunlight}]{\text{Chlorophyll}} \underset{\text{[glucose]}}{\text{C}_6\text{H}_{12}\text{O}_6} + \underset{\text{[oxygen]}}{6\text{O}_2}

(c) Some chemical reactions need a catalyst to change the rate of reaction, if the reaction is too slow or too fast. Positive catalyst increases the rate of chemical reaction whereas negative catalyst decreases the rate of reaction.

Finely divided iron is used as a positive catalyst in the manufacture of ammonia from hydrogen and oxygen.

     N2+3H2⇌200-900 atmos.Fe catalyst/450°C 2NH3\text{N}_2 + 3\text{H}_2 \xrightleftharpoons[\text{200-900 atmos.}]{\text{Fe catalyst/450\degree C }} \text{2NH}_3

(d) Some chemical reactions take place only in the presence of heat.

If iron powder and sulphur powder are mixed, they do not react. But they are heated, they react to form iron sulphide.

Fe + S →Δ\xrightarrow{\enspace\Delta\enspace} FeS

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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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