Chemical reactions

  • Chemical Reaction: A process where reactants transform into products with different properties. Bonds break and new bonds form.
  • Chemical Equation: A symbolic representation using formulae and symbols.
  • Example: \( \ce{2H2 + O2 -> 2H2O} \)

Characteristics of a Chemical Reaction

  • Evolution of a Gas: E.g., \( \ce{Zn + H2SO4 -> ZnSO4 + H2 ^} \)
  • Formation of a Precipitate: An insoluble solid. E.g., \( \ce{AgNO3 + NaCl -> AgCl v + NaNO3} \)
  • Change in Colour: E.g., Citric (colorless) + Purple Potassium Permanganate → Colorless.
  • Change in Temperature:
    • Exothermic: Releases heat. ( \( \Delta H \) is -ve). E.g., Combustion.
    • Endothermic: Absorbs heat. ( \( \Delta H \) is +ve). E.g., Photosynthesis.
  • Change of State: E.g., Burning wax (solid → liquid → gas).

Types of Chemical Reactions

1. Combination Reaction

Two or more substances combine to form a single product.

General form: \( A + B \rightarrow AB \)

Example: \( \ce{2Mg + O2 -> 2MgO} \)

2. Decomposition Reaction

A single compound breaks down into simpler substances. Often require energy (heat, light, electricity).

General form: \( AB \rightarrow A + B \)

Examples:

  • Thermal Decomposition: \( \ce{2FeSO4 ->[\Delta] Fe2O3 + SO2 + SO3} \)
  • Electrolysis: \( \ce{2H2O ->[Electricity] 2H2 ^ + O2 ^} \)
  • Photodecomposition: \( \ce{2AgBr ->[Sunlight] 2Ag + Br2} \)

3. Displacement Reaction

A more reactive element displaces a less reactive element from its compound.

General form: \( A + BC \rightarrow AC + B \)

Example (Single Displacement): \( \ce{Fe + CuSO4 -> FeSO4 + Cu v} \)

Reactivity Series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Ag > Au (Remember: Please Stop Calling Me A Zebra In The Library. Call A Guard!)

4. Double Displacement Reaction

Ions are exchanged between two reactants to form new compounds.

General form: \( AB + CD \rightarrow AD + CB \)

Subtypes:

  • Precipitation: \( \ce{BaCl2 + Na2SO4 -> BaSO4 v + 2NaCl} \)
  • Neutralization: Acid + Base → Salt + Water. \( \ce{HCl + NaOH -> NaCl + H2O} \)

5. Oxidation and Reduction (Redox)

  • Oxidation: Loss of electrons / Gain of oxygen / Loss of hydrogen.
  • Reduction: Gain of electrons / Loss of oxygen / Gain of hydrogen.
  • Oxidizing Agent: Gets reduced itself.
  • Reducing Agent: Gets oxidized itself.

Example: \( \ce{CuO + H2 -> Cu + H2O} \)
Here, \( \ce{H2} \) is oxidized (to \( \ce{H2O} \)), it is the reducing agent.
\( \ce{CuO} \) is reduced (to \( \ce{Cu} \)), it is the oxidizing agent.

6. Exothermic and Endothermic Reactions

  • Exothermic: \( \text{Reactants} \rightarrow \text{Products} + \text{Heat} \) ( \( \Delta H = -ve \) )
  • Endothermic: \( \text{Reactants} + \text{Heat} \rightarrow \text{Products} \) ( \( \Delta H = +ve \) )

Important Conditions for Reactions

  • Law of Conservation of Mass: Mass of reactants = Mass of products.
  • Balanced Equation: Number of atoms of each element on both sides must be equal.

Key Formulae & Constants

Concept Formula/Value
pH of a Solution \( \text{pH} = -\log_{10}[H^+] \)
Neutral pH 7
Acidic pH < 7
Basic (Alkaline) pH > 7
Avogadro’s Number \( N_A = 6.022 \times 10^{23} \)
Molar Mass of Water (\( \ce{H2O} \)) \( 2(1) + 16 = 18 \, \text{g/mol} \)
Molar Volume of a Gas at STP \( 22.4 \, \text{L/mol} \)
Formula for Rust \( \ce{Fe2O3·xH2O} \) (Hydrated Iron(III) Oxide)
Formula for Baking Soda \( \ce{NaHCO3} \) (Sodium Hydrogen Carbonate)
Formula for Washing Soda \( \ce{Na2CO3·10H2O} \) (Sodium Carbonate Decahydrate)

📝 Important Chemical Formulas

  • Water: \( H_2O \)
  • Carbon Dioxide: \( CO_2 \)
  • Ammonia: \( NH_3 \)
  • Hydrochloric Acid: \( HCl \)
  • Sulphuric Acid: \( H_2SO_4 \)
  • Nitric Acid: \( HNO_3 \)
  • Sodium Hydroxide: \( NaOH \)
  • Calcium Hydroxide (Slaked Lime): \( Ca(OH)_2 \)
  • Calcium Oxide (Quick Lime): \( CaO \)
  • Bleaching Powder: \( CaOCl_2 \)
  • Baking Soda: \( NaHCO_3 \)
  • Washing Soda: \( Na_2CO_3.10H_2O \)
  • Gypsum: \( CaSO_4.2H_2O \)
  • Plaster of Paris: \( CaSO_4.\frac{1}{2}H_2O \)

🧪 Important Constants & Values

  • Avogadro’s Number: \( 6.022 \times 10^{23} \)
  • Molar Volume of Gas at STP: 22.4 litres
  • STP: 273.15 K (0°C) and 1 atm pressure
  • pH of Neutral Solution: 7
  • Atomic Mass Unit (amu): \( 1.66 \times 10^{-24} \) grams
Chemical reactions