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Language of Chemistry - Chemical Equations

Grade 7ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A chemical equation is the symbolic representation of a chemical reaction using symbols and formulae of the substances involved.

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Reactants are the starting substances written on the left-hand side (LHS), while products are the new substances formed, written on the right-hand side (RHS).

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The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element must be equal on both sides of the equation.

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A balanced chemical equation has an equal number of atoms of each element on both the reactant and product sides.

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Valency is the combining capacity of an atom. It is used in the 'criss-cross' method to write chemical formulae. For example, Calcium (Ca2+Ca^{2+}) and Chloride (Cl1−Cl^{1-}) form CaCl2CaCl_{2}.

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Information conveyed by a chemical equation includes the names of reactants and products, their chemical formulae, the number of molecules involved, and sometimes physical states like solid (ss), liquid (ll), gas (gg), or aqueous (aqaq).

📐Formulae

2H2+O2→2H2O2H_{2} + O_{2} \rightarrow 2H_{2}O

Mg+2HCl→MgCl2+H2↑Mg + 2HCl \rightarrow MgCl_{2} + H_{2}\uparrow

CaCO3→ΔCaO+CO2↑CaCO_{3} \xrightarrow{\Delta} CaO + CO_{2}\uparrow

N2+3H2⇌2NH3N_{2} + 3H_{2} \rightleftharpoons 2NH_{3}

Zn+H2SO4→ZnSO4+H2Zn + H_{2}SO_{4} \rightarrow ZnSO_{4} + H_{2}

💡Examples

Problem 1:

Balance the following chemical equation: Fe+H2O→Fe3O4+H2Fe + H_{2}O \rightarrow Fe_{3}O_{4} + H_{2}

Solution:

3Fe+4H2O→Fe3O4+4H23Fe + 4H_{2}O \rightarrow Fe_{3}O_{4} + 4H_{2}

Explanation:

To balance the Iron (FeFe), we add a coefficient of 33 on the LHS. To balance Oxygen (OO), we add a coefficient of 44 to H2OH_{2}O on the LHS. This results in 88 atoms of Hydrogen on the LHS, so we add a coefficient of 44 to H2H_{2} on the RHS.

Problem 2:

Write the chemical formula for Aluminium Oxide using the valency method.

Solution:

Al2O3Al_{2}O_{3}

Explanation:

The valency of Aluminium (AlAl) is 33 and the valency of Oxygen (OO) is 22. By criss-crossing the valencies, the 22 from Oxygen goes to Aluminium and the 33 from Aluminium goes to Oxygen, resulting in Al2O3Al_{2}O_{3}.

Problem 3:

Identify the reactants and products in the following reaction: NaOH+HCl→NaCl+H2ONaOH + HCl \rightarrow NaCl + H_{2}O

Solution:

Reactants: NaOHNaOH (Sodium Hydroxide) and HClHCl (Hydrochloric Acid); Products: NaClNaCl (Sodium Chloride) and H2OH_{2}O (Water).

Explanation:

Substances on the left of the arrow are those that undergo change (reactants), and substances on the right are the new substances formed (products).