Key Concepts
- 1What is a balanced chemical equation?
- 2Types of chemical reactions?
Important Formulas & Facts
A chemical equation with equal number of atoms of each element on both sides. Law of Conservation of Mass — matter is neither created nor destroyed.
Combination (A+B→AB), Decomposition (AB→A+B), Displacement (A+BC→AC+B), Double displacement (AB+CD→AD+CB), Redox (simultaneous oxidation + reduction).
Must-Know Questions
Q1A brown substance X on heating in air forms a substance Y. When hydrogen gas is passed over heated Y, it turns back into X. Identify X and Y.
X is copper (Cu) which is brown. On heating in air, it forms copper oxide (CuO) which is black (substance Y). When H₂ is passed over heated CuO, it reduces back to Cu. CuO + H₂ → Cu + H₂O.
Q2What is a balanced chemical equation? Why should chemical equations be balanced?
A balanced chemical equation has an equal number of atoms of each element on both sides of the equation. Chemical equations must be balanced to satisfy the law of conservation of mass, which states that matter can neither be created nor destroyed in a chemical reaction. The total mass of reactants must equal the total mass of products.
Q3Translate the following statement into a balanced chemical equation: Barium chloride and sodium sulphate react to give insoluble barium sulphate and a solution of sodium chloride.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq). This is a double displacement reaction. BaSO₄ is the white insoluble precipitate formed.
Q4What happens when dilute hydrochloric acid is added to iron filings?
Iron reacts with dilute HCl to form ferrous chloride and hydrogen gas. Fe + 2HCl → FeCl₂ + H₂↑. This is a displacement reaction. Hydrogen gas is evolved with brisk effervescence.
Q5Why is respiration considered an exothermic reaction?
Respiration is an exothermic reaction because during respiration, glucose (C₆H₁₂O₆) combines with oxygen to produce carbon dioxide, water, and energy. The reaction releases energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy. This energy is used by the body for various life processes.
Practice Chemical Reactions and Equations
Reinforce what you just revised with practice questions