In CBSE Class 10 Science, Chapter 1 "Chemical Reactions and Equations," a combination
reaction is one of the types of chemical reactions that are discussed.
A combination reaction, also known as a synthesis reaction, is a chemical reaction in which
two or more substances combine to form a single product. In other words, it is the opposite
of a decomposition reaction where a single compound breaks down into two or more simpler
substances.
The general equation for a combination reaction is:
A + B → AB
Here, A and B represent the reactants, which can be elements or compounds, and AB
represents the product, which is a compound. During a combination reaction, the reactants
combine chemically to form a new compound with different properties than the original
substances.
For example, the reaction of hydrogen gas (H2) with oxygen gas (O2) to form water (H2O) is
a combination reaction:
2H2 + O2 → 2H2O
In this reaction, two molecules of hydrogen gas combine with one molecule of oxygen gas to
form two molecules of water. The resulting water molecule has different properties than
hydrogen or oxygen, such as its ability to exist as a liquid at room temperature and its ability
to extinguish fire.
Combination reactions are commonly observed in various natural and synthetic processes.
They play a vital role in the formation of compounds and the synthesis of new substances in
chemistry.
Exothermic reactions:
- Exothermic reactions are chemical reactions that release energy in the form of
heat.
- In these reactions, the energy of the products is lower than the energy of the
reactants.
- Common examples include combustion reactions where fuels burn and give off
heat.
- The temperature of the surroundings usually increases during exothermic reactions.
Endothermic reactions:
, - Endothermic reactions are chemical reactions that absorb energy from the
surroundings.
- In these reactions, the energy of the products is higher than the energy of the
reactants.
- Common examples include thermal decomposition reactions and certain reactions
in photosynthesis.
- The temperature of the surroundings usually decreases during endothermic
reactions.
Examples-
Quicklime (calcium oxide, CaO) and slaked lime (calcium hydroxide, Ca(OH)2) are
commonly used substances that can participate in combination reactions.
1. Combination reaction involving Quicklime:
Quicklime, also known as burnt lime, reacts with water (H2O) in a combination reaction to
form slaked lime. The reaction can be represented as follows:
CaO + H2O → Ca(OH)2
In this reaction, calcium oxide (quicklime) combines with water to produce calcium hydroxide
(slaked lime). This reaction is highly exothermic and is often used in applications such as
construction, agriculture, and water treatment.
2. Combination reaction involving Slaked Lime:
Slaked lime can also participate in a combination reaction with carbon dioxide (CO2) from
the atmosphere. This reaction is commonly known as carbonation and is responsible for the
process of carbonating beverages. The reaction can be represented as follows:
Ca(OH)2 + CO2 → CaCO3 + H2O
In this reaction, calcium hydroxide (slaked lime) reacts with carbon dioxide to form calcium
carbonate (chalk) and water.
These combination reactions involving quicklime and slaked lime showcase the ability of
these substances to combine with other compounds or elements to form new compounds.
Certainly! Here are examples of combination reactions related to the burning of coal, burning
of natural gas, and respiration:
3. Burning of Coal:
The burning of coal involves a combination reaction where coal (carbon) reacts with oxygen
(O2) in the air to produce carbon dioxide (CO2) and release energy in the form of heat. The
balanced equation for the combustion of coal can be represented as follows:
C + O2 → CO2
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