Chemical Reactions and Equations
Chemical Reactions and Equations – Complete Solutions
1.1 What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (called reactants) are converted into one or more different substances (called products). During a chemical reaction, old chemical bonds are broken and new chemical bonds are formed.
1.2 Evidence of Chemical Reactions
1. Evolution or Absorption of Heat:
Some reactions release heat (exothermic) like burning of wood, while others absorb heat (endothermic) like melting of ice.
2. Change in Color:
Browning of iron (rusting) changes color from shiny grey to reddish-brown.
3. Evolution of Gas:
When zinc reacts with dilute hydrochloric acid, hydrogen gas is evolved.
4. Formation of Precipitate:
When potassium iodide solution is mixed with lead nitrate solution, a bright yellow precipitate of lead iodide forms.
5. Change in Properties:
The products have completely different properties from the reactants.
1.3 Chemical Equations
A chemical equation is a representation of a chemical reaction using the symbols and formulas of substances. It uses chemical formulas and symbols to represent the reaction.
Representation:
General Form: Reactant(s) → Product(s)
Example:
Carbon + Oxygen → Carbon dioxide
C + O₂ → CO₂
1.4 Unbalanced and Balanced Equations
Unbalanced Equation:
An equation that does not have equal number of atoms of each element on both sides is called an unbalanced equation.
Example: H₂ + O₂ → H₂O (unbalanced – 2 O on left, 1 O on right)
Balanced Equation:
An equation that has equal number of atoms of each element on both sides is called a balanced equation.
Example: 2H₂ + O₂ → 2H₂O (balanced)
Law of Conservation of Mass: Mass cannot be created or destroyed in a chemical reaction. Total mass of reactants = Total mass of products
1.5 How to Balance Chemical Equations?
Method 1: Hit and Trial Method (For Simple Equations)
Steps:
- Write the unbalanced equation with chemical formulas
- List the number of atoms of each element on both sides
- Identify which elements are not balanced
- Adjust coefficients (the numbers before the formula) to balance
- Check if the equation is balanced
Example: Balance H₂ + O₂ → H₂O
Step 1: H₂ + O₂ → H₂O
Step 2: Left side: H = 2, O = 2; Right side: H = 2, O = 1
Step 3: Oxygen is not balanced (2 on left, 1 on right)
Step 4: Add coefficient 2 to H₂O: H₂ + O₂ → 2H₂O
Step 5: Check: Left side: H = 2, O = 2; Right side: H = 4, O = 2
Still not balanced. Add coefficient 2 to H₂: 2H₂ + O₂ → 2H₂O
Check: Left side: H = 4, O = 2; Right side: H = 4, O = 2 ✓
1.6 Types of Chemical Reactions
1. Combination (Synthesis) Reaction:
General Form: A + B → AB
Two or more substances combine to form a single substance.
Examples:
- C + O₂ → CO₂
- 2H₂ + O₂ → 2H₂O
- 2Na + Cl₂ → 2NaCl
2. Decomposition Reaction:
General Form: AB → A + B
One substance breaks down into two or more substances.
Examples:
- 2H₂O₂ → 2H₂O + O₂
- CaCO₃ → CaO + CO₂ (on heating)
- AgCl → Ag + Cl (on exposure to light)
3. Displacement (Substitution) Reaction:
General Form: A + BC → AC + B
A more reactive element displaces a less reactive element from a compound.
Examples:
- Zn + CuSO₄ → ZnSO₄ + Cu
- Fe + CuSO₄ → FeSO₄ + Cu
- Cl₂ + 2NaBr → 2NaCl + Br₂
4. Double Displacement Reaction:
General Form: AB + CD → AD + CB
Two compounds exchange ions or atoms to form two new compounds.
Examples:
- AgNO₃ + NaCl → AgCl↓ + NaNO₃
- Na₂SO₄ + BaCl₂ → BaSO₄↓ + NaCl
- HCl + NaOH → NaCl + H₂O
1.7 Oxidation-Reduction (Redox) Reactions
Oxidation: Loss of electrons or gain of oxygen or loss of hydrogen
Reduction: Gain of electrons or loss of oxygen or gain of hydrogen
Example:
Zn + CuSO₄ → ZnSO₄ + Cu
Zn loses 2 electrons (oxidation)
Cu²⁺ gains 2 electrons (reduction)
1.8 Solved Examples
Example 1: Balance the equation: Fe + O₂ → Fe₂O₃
Solution:
Unbalanced: Fe + O₂ → Fe₂O₃
Count atoms: Left: Fe = 1, O = 2; Right: Fe = 2, O = 3
Balance Fe: 2Fe + O₂ → Fe₂O₃
Count atoms: Left: Fe = 2, O = 2; Right: Fe = 2, O = 3
Balance O: 4Fe + 3O₂ → 2Fe₂O₃
Balanced equation: 4Fe + 3O₂ → 2Fe₂O₃
Example 2: Identify the type of reaction: 2H₂O₂ → 2H₂O + O₂
Solution:
One substance (H₂O₂) breaks down into two substances (H₂O and O₂).
This is a Decomposition Reaction