Chemical Reactions and Equation: Class 10 NCERT Guide

What Are Chemical Reactions and Equations?

A chemical reaction is a process where substances called reactants are transformed into new substances called products. This isn't just mixing things together; it involves the breaking of old chemical bonds and the formation of new ones. How can you tell a reaction is happening? Look for these signs:

• Change in State: A solid might form from liquids (precipitation), or a gas might evolve.
• Change in Colour: A solution might change colour, like when iron nails are dipped in blue copper sulphate solution, which turns green.
• Evolution of a Gas: You might see bubbles, like when zinc reacts with acid to produce hydrogen gas.
• Change in Temperature: Some reactions release heat (exothermic), making the container warm, while others absorb heat (endothermic), making it feel cold.

A chemical equation is a shorthand way of representing a chemical reaction. Instead of writing long sentences, we use symbols and formulas. For example, 'Hydrogen gas reacts with oxygen gas to form water' can be written as: 2H₂(g) + O₂(g) → 2H₂O(l). Here, H₂ and O₂ are the reactants, and H₂O is the product. The arrow indicates the direction of the reaction ('yields' or 'produces').

Balancing Chemical Equations: A Step-by-Step Guide

1. Step 1: Write the Skeletal Equation — Write down the chemical formulas for all reactants and products. Let's balance the reaction of iron with steam (water) to form iron(III) oxide and hydrogen gas. Skeletal equation: Fe + H₂O → Fe₃O₄ + H₂
2. Step 2: List the Number of Atoms — Count the atoms of each element on both the reactant side (LHS) and product side (RHS). | Element | LHS (Reactants) | RHS (Products) | |---|---|---| | Fe | 1 | 3 | | H | 2 | 2 | | O | 1 | 4 |
3. Step 3: Start Balancing — Begin with the compound that has the most atoms, which is Fe₃O₄. First, balance the oxygen atoms. There are 4 on the RHS and 1 on the LHS. Place a coefficient '4' before H₂O. Equation becomes: Fe + 4H₂O → Fe₃O₄ + H₂
4. Step 4: Balance the Other Elements — Now, the hydrogen atoms on the LHS are 4 x 2 = 8, while on the RHS there are 2. Place a coefficient '4' before H₂. Equation becomes: Fe + 4H₂O → Fe₃O₄ + 4H₂. Next, balance iron. There are 3 Fe on the RHS and 1 on the LHS. Place a '3' before Fe. Equation becomes: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
5. Step 5: Final Verification — Count the atoms again to ensure they are equal on both sides. | Element | LHS (Reactants) | RHS (Products) | |---|---|---| | Fe | 3 | 3 | | H | 8 | 8 | | O | 4 | 4 | The equation is now balanced: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g). It's good practice to add the state symbols.

Key Types of Chemical Reactions

Combination Reaction

Two or more reactants combine to form a single product. Example: Burning of magnesium ribbon. 2Mg(s) + O₂(g) → 2MgO(s)

Decomposition Reaction

A single compound breaks down into two or more simpler substances. It's the opposite of combination. Example: Heating limestone. CaCO₃(s) → CaO(s) + CO₂(g)

Displacement Reaction

A more reactive element displaces a less reactive element from its salt solution. Example: Iron displacing copper. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Double Displacement Reaction

Two compounds react by an exchange of ions to form two new compounds. Often results in the formation of a precipitate. Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Oxidation and Reduction (Redox)

Reactions involving simultaneous oxidation (gain of oxygen or loss of hydrogen) and reduction (loss of oxygen or gain of hydrogen). Example: CuO(s) + H₂(g) → Cu(s) + H₂O(l). Here, CuO is reduced to Cu, and H₂ is oxidized to H₂O.

Exam Tips: Avoid These Common Errors!

Don't Change the Formula! A very common mistake is changing the subscripts in a chemical formula to balance an equation (e.g., changing H₂O to H₂O₂). This is incorrect as it changes the substance itself! Always balance by changing the coefficients (the numbers in front of the formulas).

Memorize the Reactivity Series: For displacement reactions, you MUST know which metal is more reactive. Questions often ask if a reaction will occur. Without knowing the series (e.g., Potassium > Sodium > Calcium > ... > Gold), you can't answer correctly.

State Symbols Matter: Unless specified otherwise, always include the physical states of reactants and products: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water). These can carry marks.

Redox Confusion: Remember the mnemonic OIL RIG – Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). For Class 10, think of it in terms of oxygen and hydrogen: Oxidation is Gain of Oxygen / Loss of Hydrogen. Reduction is the opposite.

Practice Questions with Solutions

• Q: Balance the following chemical equation: Fe₂O₃(s) + CO(g) → Fe(s) + CO₂(g) A: Step 1: Balance Fe atoms. Fe₂O₃ + CO → 2Fe + CO₂. Step 2: Balance O atoms by adjusting CO and CO₂. If we have 3 CO, we need 3 CO₂. Fe₂O₃ + 3CO → 2Fe + 3CO₂. Step 3: Check all atoms. Fe: 2 on left, 2 on right. C: 3 on left, 3 on right. O: (3+3) = 6 on left, (3*2) = 6 on right. All balanced. Final answer: Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)
• Q: When a green coloured compound 'X' is heated, it decomposes to form a brown solid 'Y' and a gas 'Z' which has the smell of burning sulphur. Identify X, Y, Z and the type of reaction. A: Step 1: The green compound 'X' is typically Ferrous Sulphate (FeSO₄·7H₂O). Heating causes dehydration and then decomposition of FeSO₄. Step 2: FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g). The brown solid 'Y' is Ferric Oxide (Fe₂O₃). The gas 'Z' is Sulphur Dioxide (SO₂) and/or Sulphur Trioxide (SO₃), which smell of burning sulphur. Step 3: A single compound breaking down into simpler substances is a decomposition reaction. Final answer: X = Ferrous Sulphate (FeSO₄), Y = Ferric Oxide (Fe₂O₃), Z = Sulphur Dioxide (SO₂) and Sulphur Trioxide (SO₃). Type of reaction: Decomposition reaction.
• Q: In the reaction MnO₂(s) + 4HCl(aq) → MnCl₂(aq) + 2H₂O(l) + Cl₂(g), identify the substance oxidized and the oxidizing agent. A: Step 1: Identify changes in oxidation states. In HCl, Cl is -1. In Cl₂, Cl is 0. Since the oxidation state of Cl increases from -1 to 0, HCl is oxidized. Step 2: The substance that gets oxidized is the reducing agent. The substance that causes oxidation (and itself gets reduced) is the oxidizing agent. Step 3: In MnO₂, Mn is +4. In MnCl₂, Mn is +2. Since the oxidation state of Mn decreases from +4 to +2, MnO₂ is reduced, hence it is the oxidizing agent. Final answer: Substance oxidized: HCl. Oxidizing agent: MnO₂.
• Q: What happens when an iron nail is dipped in a copper sulphate solution? Write the balanced chemical equation and state the type of reaction. A: Step 1: Iron is more reactive than copper. Therefore, iron will displace copper from its salt solution. Step 2: The blue colour of the copper sulphate solution fades and turns light green due to the formation of iron(II) sulphate. A reddish-brown deposit of copper metal forms on the surface of the iron nail. Step 3: The balanced chemical equation is Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). This is a displacement reaction. Final answer: The blue solution turns light green, and a reddish-brown deposit forms on the nail. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Type: Displacement reaction.

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