Chemical Reactions and Equations
This chapter explains how to identify and write chemical equations, balance them, and classify chemical reactions into types such as combination, decomposition, displacement, double displacement, and oxidation-reduction.
A balanced chemical equation has an equal number of atoms of each element on both the reactant and product sides, satisfying the Law of Conservation of Mass.
Q. What happens when a few drops of dilute hydrochloric acid are added to zinc granules? Write the chemical equation.
Zinc reacts with dilute hydrochloric acid to produce zinc chloride and hydrogen gas, with the evolution of gas bubbles: Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g).
In a combination reaction, two or more substances combine to form a single product, e.g. CaO + H2O -> Ca(OH)2.
Q. What is a displacement reaction? Give an example.
A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound. For example: Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s), where the blue colour of the solution fades as iron displaces copper.
The reaction CaCO3 heated to give CaO + CO2 is an example of:
Correct answer: (C) Thermal decomposition reaction
A decomposition reaction is the opposite of a combination reaction.
True — in a combination reaction two or more reactants form a single product, while in a decomposition reaction a single compound breaks down into two or more simpler substances.