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Which of the following is an example of exothermic combination reaction?
Respiration
Ammonium chloride dissolved in water
Photosynthesis
Quick lime dissolved in water
Quick lime dissolved in water
Quick lime (calcium oxide) reacting with water is a classic example of an exothermic combination reaction. In this process, calcium oxide combines with water to form slaked lime (calcium hydroxide), releasing a large amount of heat energy in the process.
Quick lime (calcium oxide) reacting with water is a classic example of an exothermic combination reaction. In this process, calcium oxide combines with water to form slaked lime (calcium hydroxide), releasing a large amount of heat energy in the process.
Adding quick lime to water is like dropping a hot piece of metal into waterтАФit rapidly heats up the solution and can even cause the water to sizzle and boil due to the intense heat released.
Remember 'EXo means EXit': EXothermic reactions let heat EXit (release heat).
CaO(s)+H2тАЛO(l)тЖТCa(OH)2тАЛ(aq)+Heat тАФ Exothermic combination reaction of quick lime with water
A combination reaction occurs when two or more substances combine to form a single product. When quick lime (CaO) reacts with water (H2тАЛO), it forms slaked lime (Ca(OH)2тАЛ). Because the bond energy released during product formation is greater than the energy required to break the reactants' bonds, the excess energy is evolved as heat, rendering the reaction exothermic: CaO(s)+H2тАЛO(l)тЖТCa(OH)2тАЛ(aq)+Heat.
Combination reaction: Two or more reactants combine to form a single product (A+BтЖТAB).
Exothermic reaction: A chemical reaction accompanied by the liberation of heat energy (╬ФH<0).
Slaked lime, Ca(OH)2тАЛ, is widely used for whitewashing walls.
Releases heat efficiently for thermal applications.
Produces slaked lime, an essential material for whitewashing and construction mortar.
The reaction is highly exothermic and can cause severe chemical burns if handled without protective gear.
Preparation of whitewashing solutions for walls.
Soil neutralization in agricultural practices.
Option A (Respiration): It is an exothermic reaction, but it is a complex decomposition/oxidation process (C6тАЛH12тАЛO6тАЛ+6O2тАЛтЖТ6CO2тАЛ+6H2тАЛO+Energy), not a combination reaction.
Option B (Ammonium chloride in water): Dissolving NH4тАЛCl in water is an endothermic process (absorbs heat).
Option C (Photosynthesis): It is an endothermic process as green plants absorb solar energy to synthesize food.
D is correct тАФ Quick lime dissolving in water combines two compounds into a single product (Ca(OH)2тАЛ) while liberating significant heat energy.
Always differentiate between reaction types (Combination vs. Oxidation/Decomposition) and energy changes (Exothermic vs. Endothermic)тАФcompetitive exams often test both classifications together in single-option MCQs.
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