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Chemical Reactions and Equations - Write and balance chemical equations for reactions

Grade 10CBSE

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

🔑Concepts

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A Balanced Chemical Equation is a representation where the number of atoms of each element on the reactant side is equal to the number of atoms on the product side.

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The balancing of chemical equations is based on the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. Hence, the total mass of elements present in the products must be equal to the total mass of elements present in the reactants.

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To balance an equation, we use the Hit and Trial Method, where we multiply the chemical formulae by the smallest possible whole number coefficients.

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During balancing, the chemical formulae of the substances (subscripts) must never be changed; only the coefficients (numbers in front of the formulae) can be adjusted.

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Physical states are often mentioned for completeness: (s)(s) for solid, (l)(l) for liquid, (g)(g) for gas, and (aq)(aq) for aqueous (dissolved in water).

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Essential conditions such as temperature, pressure, or a catalyst are written above or below the reaction arrow (→\rightarrow).

📐Formulae

Total mass of Reactants=Total mass of Products\text{Total mass of Reactants} = \text{Total mass of Products}

aA+bB→cC+dDaA + bB \rightarrow cC + dD

2H2(g)+O2(g)→2H2O(l)2H_2(g) + O_2(g) \rightarrow 2H_2O(l)

💡Examples

Problem 1:

Balance the following chemical equation: Fe(s)+H2O(g)→Fe3O4(s)+H2(g)Fe(s) + H_2O(g) \rightarrow Fe_3O_4(s) + H_2(g)

Solution:

3Fe(s)+4H2O(g)→Fe3O4(s)+4H2(g)3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)

Explanation:

First, balance the element with the maximum number of atoms, which is Oxygen. There are 4 Oxygen atoms in Fe3O4Fe_3O_4, so we put coefficient 4 before H2OH_2O on the LHS. Now, there are 8 Hydrogen atoms on the LHS, so we put coefficient 4 before H2H_2 on the RHS. Finally, there are 3 Iron atoms on the RHS, so we put coefficient 3 before FeFe on the LHS.

Problem 2:

Write the balanced chemical equation for: Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.

Solution:

NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)

Explanation:

Counting the atoms: NaNa: 1 (LHS), 1 (RHS); OO: 1 (LHS), 1 (RHS); HH: 2 (LHS), 2 (RHS); ClCl: 1 (LHS), 1 (RHS). Since the number of atoms for every element is already equal, the equation is already balanced.

Problem 3:

Balance the reaction between Aluminium and Copper chloride: Al+CuCl2→AlCl3+CuAl + CuCl_2 \rightarrow AlCl_3 + Cu

Solution:

2Al(s)+3CuCl2(aq)→2AlCl3(aq)+3Cu(s)2Al(s) + 3CuCl_2(aq) \rightarrow 2AlCl_3(aq) + 3Cu(s)

Explanation:

Start with Chlorine (ClCl). It has 2 atoms on the LHS and 3 on the RHS. To balance, find the LCM (6). Multiply CuCl2CuCl_2 by 3 and AlCl3AlCl_3 by 2. Now balance Aluminium (AlAl) by adding 2 on the LHS and Copper (CuCu) by adding 3 on the RHS.