- A combination reaction is a reaction where two or more substances combine to form a single product.
Example: CaO + H₂O → Ca(OH)₂ - Combination reactions are often exothermic, meaning they release heat.
Example: Reaction of quick lime with water. - Respiration is an exothermic reaction where glucose combines with oxygen to release energy.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy - One product is formed in combination reactions, regardless of the number of reactants (usually 2 or more).
Definitions [12]
Define the following terms
Chemical bond
A chemical bond is the force which holds the atoms of a molecule together as in a compound.
Define the following terms:
Effervescence
Formation of gas bubbles in a liquid during a reaction is called effervescence.
Define the term – double decomposition – neutralization reaction.
When an acid reacts with a base salt and water are formed and this is called Neutralisation.
Double-decomposition – Neutralisation reaction :
“Is the chemical reaction between two compounds (acid and base) to interchange radicals and produce salt and water.”
Definition: Combination Reaction
A reaction in which a single product is formed from two or more reactants is known as a combination reaction.
OR
When two or more reactants combine in a reaction to form a single product, it is a combination reaction.
Definition: Thermal Decomposition
When a decomposition reaction is carried out by heating, it is called thermal decomposition.
Definition: Decomposition Reaction
A single reactant breaks down to give simpler products. This is a decomposition reaction.
OR
The chemical reaction in which two or more products are formed from a single reactant is a decomposition reaction.
Definition: Single Displacement Reaction
The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.
Definition: Double Displacement Reaction
Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.
OR
The reaction in which the ions in the reactants are exchanged to form a precipitate is a double displacement reaction.
Definition: Endothermic Reaction
A chemical reaction in which heat is absorbed from the surroundings or must be continuously supplied is called an endothermic reaction.
Definition: Endothermic Process
The process in which heat is absorbed from the outside, is called endothermic process.
Definition: Exothermic Reaction
A chemical reaction in which heat is released when reactants are converted to products is called an exothermic reaction.
Definition: Exothermic Process
The processes in which heat is given out are called exothermic processes.
Chemica Equations [1]
Chemical Equations: Combination Reaction
- C + O₂ → CO₂
- 2H₂ + O₂ → 2H₂O
- 2Mg + O₂ → 2MgO
Key Points
Key Points: Combination Reaction
Key Points: Decomposition Reaction
- Thermal decomposition involves heating a compound to break it into simpler substances, e.g.,
2Pb(NO₃)₂ ⟶ 2PbO + 4NO₂ + O₂ (brown fumes of NO₂ observed). - Electrolytic decomposition uses electricity to decompose compounds, e.g.,
2H₂O ⟶ 2H₂ + O₂ (Electrolysis of water gives hydrogen and oxygen gases). - Photodecomposition occurs when light energy breaks down a compound, e.g.,
2AgCl ⟶ 2Ag + Cl₂ (white silver chloride turns grey in sunlight). - Thermal decomposition of calcium carbonate produces lime (CaO) and CO₂ gas, which turns lime water milky.
- Decomposition reactions are usually endothermic and require heat, light, or electricity to occur (e.g., the decomposition of ferrous sulphate, calcium carbonate, or sugar).
Key Points: Single Displacement Reactions
- A more reactive metal can replace a less reactive metal from its compound in solution.
- Zinc, iron, and lead can displace copper from copper sulphate or copper chloride solutions.
- During displacement, the displaced metal appears in elemental form, while the replacing metal forms its salt.
- In the reaction Fe + CuSO₄ → FeSO₄ + Cu, the iron nail turns brownish due to deposited copper, and the blue solution fades.
- Heat may be released during displacement reactions, indicating that the reaction is exothermic.
