a) What is meant by the term "combustion reaction"? [1] b) Write the examples of combustion reactions. [2] c) What are the characteristics of combustion reactions? [2]

Model Answer

a) Meaning of Combustion Reaction

Combustion reaction is a chemical reaction in which a substance reacts with oxygen (usually from air) to produce heat and light. This type of reaction is commonly known as burning. In combustion, a fuel (such as wood, coal, petrol, or natural gas) combines with oxygen to form oxides (like carbon dioxide and water) while releasing energy in the form of heat and light.

Carbon dioxideCarbon dioxide

b) Examples of Combustion Reactions

Two common examples of combustion reactions are:

1. Burning of Methane (Natural Gas) $$\mathrm{CH}{}_4+2\mathrm{O}{}_2\to \mathrm{CO}{}_2+2\mathrm{H}{}_2\mathrm{O}+\mathrm{Heat}+\mathrm{Light}$$ Here, methane ($\mathrm{CH}{}_4$) reacts with oxygen ($\mathrm{O}{}_2$) to produce carbon dioxide ($\mathrm{CO}{}_2$), water ($\mathrm{H}{}_2\mathrm{O}$), and releases energy.

2. Burning of Wood (Cellulose, $\mathrm{C}{}_6\mathrm{H}{}_{10}\mathrm{O}{}_5$) $$\mathrm{C}{}_6\mathrm{H}{}_{10}\mathrm{O}{}_5+6\mathrm{O}{}_2\to 6\mathrm{CO}{}_2+5\mathrm{H}{}_2\mathrm{O}+\mathrm{Heat}+\mathrm{Light}$$ In this reaction, cellulose (a component of wood) burns in oxygen to form carbon dioxide, water, and releases heat and light.

c) Characteristics of Combustion Reactions

Combustion reactions have the following key characteristics:

1. Release of Energy

   • Combustion reactions are exothermic, meaning they release heat and light energy.
   • Example: Burning of fuels like petrol, kerosene, and wood produces heat and light.

2. Formation of Oxides

   • The products of combustion are usually oxides (such as $\mathrm{CO}{}_2$, $\mathrm{H}{}_2\mathrm{O}$, $\mathrm{SO}{}_2$, etc.).
   • Example: Burning of magnesium in air produces magnesium oxide ($\mathrm{MgO}$).

3. Requires Oxygen

   • Combustion cannot occur without oxygen (or an oxidizing agent).
   • Example: A candle extinguishes when covered with a glass (cutting off oxygen supply).

4. Rapid Reaction

   • Combustion reactions occur quickly, often with visible flames or sparks.
   • Example: Burning of paper or wood produces flames.

5. Formation of Ash (Incomplete Combustion)

   • If combustion is incomplete (due to insufficient oxygen), carbon (soot) or carbon monoxide ($\mathrm{CO}$) may form instead of $\mathrm{CO}{}_2$.
   • Example: Poorly burning wood produces black smoke (soot).

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Note: Always ensure proper ventilation and safety precautions when dealing with combustion reactions.