Chemical Reactions And Equations Class 10 NCERT Guide

Understanding Chemical Changes and the Law of Conservation of Mass

A chemical reaction is a process where the original substances lose their nature and identity to form new chemical substances with entirely different properties. In a chemical reaction, rearrangement of atoms takes place while the chemical bonds between atoms are broken and formed. We can observe a chemical reaction through indicators like change in state, change in colour, evolution of gas, or change in temperature. To represent these reactions easily on paper, we use chemical equations containing chemical formulas of reactants (on the left) and products (on the right).

Why must we balance these equations? The fundamental rule governing any chemical process is the Law of Conservation of Mass. According to this law, matter can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of elements present in the products must be exactly equal to the total mass of elements present in the reactants. To satisfy this law, the number of atoms of each element must remain equal on both the LHS and RHS sides of a balanced chemical equation.

Step-by-Step Guide to Balancing Chemical Equations

1. Write the Unbalanced Skeleton Equation — Identify reactants and products and write their chemical formulas. For example, when iron reacts with steam, it forms iron (II, III) oxide and hydrogen gas: Fe + H2O -> Fe3O4 + H2.
2. Count the Atoms of Each Element — List the number of atoms of each element on the Reactant (LHS) and Product (RHS) side. For Fe + H2O -> Fe3O4 + H2, we have: LHS [Fe=1, H=2, O=1] and RHS [Fe=3, H=2, O=4].
3. Balance the Element with Maximum Atoms First — Oxygen has 4 atoms on RHS and 1 on LHS. To balance Oxygen, multiply H2O on LHS by a coefficient of 4. This updates the LHS to [Fe=1, H=8, O=4].
4. Balance the Remaining Elements (Hydrogen & Iron) — Now, balance Hydrogen: LHS has 8 H atoms, so multiply H2 on RHS by 4. Next, balance Iron: RHS has 3 Fe atoms, so multiply Fe on LHS by 3. This gives us: 3Fe + 4H2O -> Fe3O4 + 4H2.
5. Verify and Add Physical States — Count again to ensure LHS and RHS atoms are equal: Fe (3=3), H (8=8), O (4=4). Now add the states: 3Fe (s) + 4H2O (g) -> Fe3O4 (s) + 4H2 (g).

Types of Chemical Reactions Explained

Combination Reaction

A reaction in which a single product is formed from two or more reactants. Example: Burning of coal, C (s) + O2 (g) -> CO2 (g).

Decomposition Reaction

A reaction in which a single reactant breaks down into two or more simpler products. This can be thermal (using heat), electrolytic (using electricity), or photolytic (using sunlight).

Displacement Reaction

A reaction in which a more reactive element displaces a less reactive element from its salt solution. Example: Fe (s) + CuSO4 (aq) -> FeSO4 (aq) + Cu (s).

Double Displacement Reaction

A reaction in which there is an exchange of ions between the reactants to form new compounds. Often results in the formation of an insoluble precipitate.

Oxidation and Reduction (Redox)

Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. When both occur simultaneously, it is called a Redox reaction.

Pro Exam Tips: Avoid Common Chemistry Mistakes

• Never change chemical subscripts: When balancing equations, only change the coefficients in front of formulas (e.g., write 2H2O, never change water to H2O2).
• Exothermic vs Endothermic: Always remember that respiration is an exothermic reaction because glucose combines with oxygen to release energy. Decomposition reactions are almost always endothermic because they require energy input to break chemical bonds.
• Identify Oxidized/Reduced Substances from Reactants: When asked to identify the oxidizing/reducing agent or substance oxidized/reduced, look only at the reactant side. Students often incorrectly pick products as answers!

Practice Questions with Solutions

• Q: Balance the following chemical equation step-by-step: HNO3 + Ca(OH)2 -> Ca(NO3)2 + H2O. A: Step 1: Count the atoms on both sides. LHS: H = 3 (1 from HNO3 + 2 from Ca(OH)2), N = 1, O = 5 (3 + 2), Ca = 1 RHS: H = 2, N = 2, O = 7 (6 + 1), Ca = 1 Step 2: Balance Nitrogen by multiplying HNO3 on LHS by 2. 2HNO3 + Ca(OH)2 -> Ca(NO3)2 + H2O Now LHS: H = 4 (2 + 2), N = 2, O = 8 (6 + 2), Ca = 1 Step 3: Balance Hydrogen by multiplying H2O on RHS by 2. 2HNO3 + Ca(OH)2 -> Ca(NO3)2 + 2H2O Now RHS: H = 4, N = 2, O = 8, Ca = 1 Final answer: 2HNO3 + Ca(OH)2 -> Ca(NO3)2 + 2H2O (All atoms are balanced!).
• Q: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions. A: Step 1: Define Combination Reaction. In a combination reaction, two or more substances combine to form a single substance (e.g., H2 + O2 -> H2O). Step 2: Define Decomposition Reaction. In a decomposition reaction, a single compound breaks down into two or more simpler substances (e.g., H2O -> H2 + O2). Step 3: Show contrasting equations. Combination: 2H2 (g) + O2 (g) -> 2H2O (l) Decomposition: 2H2O (l) -(Electricity)-> 2H2 (g) + O2 (g) Final answer: They are opposites because combination builds a single compound from elements, while decomposition breaks a compound down into elements/compounds.
• Q: Why does the colour of copper sulphate solution change when an iron nail is dipped in it? A: Step 1: Identify the type of reaction. This is a displacement reaction. Step 2: Explain the reactivity difference. Iron (Fe) is more reactive than Copper (Cu) according to the reactivity series. Step 3: Write the reaction chemical equation. Fe (s) + CuSO4 (aq) [Blue] -> FeSO4 (aq) [Light Green] + Cu (s) [Brown deposit] Final answer: The blue colour of copper sulphate solution fades and changes to light green because iron displaces copper to form iron sulphate, and brown copper metal gets deposited on the iron nail.
• Q: Identify the substance oxidized, reduced, oxidizing agent, and reducing agent in this reaction: MnO2 + 4HCl -> MnCl2 + 2H2O + Cl2. A: Step 1: Trace Oxygen and Hydrogen transfers. - MnO2 loses oxygen to become MnCl2. Loss of oxygen is Reduction. Thus, MnO2 is reduced. - HCl loses hydrogen and gains chlorine to form Cl2. Loss of hydrogen is Oxidation. Thus, HCl is oxidized. Step 2: Determine oxidizing and reducing agents. - The substance reduced (MnO2) acts as the Oxidizing Agent. - The substance oxidized (HCl) acts as the Reducing Agent. Final answer: Oxidized: HCl; Reduced: MnO2; Oxidizing Agent: MnO2; Reducing Agent: HCl.

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