⚗️ Chemistry — Chemical Reactions and Equations
1. Physical and Chemical Change
PHYSICAL CHANGE
- Affects only physical properties (colour, density, melting point, etc.)
- Chemical composition remains unchanged
- Can be reversed by changing conditions
- Examples: Crystallisation, sublimation, boiling, melting of ice, dissolving salt in water
CHEMICAL CHANGE
- Affects composition and chemical properties
- Results in formation of new substance
- Generally irreversible
- Examples: Burning of candle, photosynthesis, ripening of fruits, electrolysis, digestion, rusting
2. Types of Chemical Reactions
COMBINATION REACTION
- Two or more substances combine to form a single product
- Example: CaO + H₂O → Ca(OH)₂ (Calcium oxide + Water → Calcium hydroxide)
DECOMPOSITION REACTION
- A single compound splits into two or more simpler substances
- Requires energy (heat, light, or electricity)
- Types: Thermal decomposition, Photolysis, Electrolysis
- Examples: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (Thermal)
- 2AgCl → 2Ag + Cl₂ (Photolysis)
- 2NaCl → 2Na + Cl₂ (Electrolysis)
DISPLACEMENT REACTION
- A more reactive element displaces a less reactive element
- Example: Fe + CuSO₄ → FeSO₄ + Cu (Iron displaces copper)
DOUBLE DISPLACEMENT REACTION
- Exchange of ions between reactants
- Forms precipitate or insoluble product
- Example: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
EXOTHERMIC & ENDOTHERMIC REACTIONS
- Exothermic: Heat is released (e.g., Burning of fuel)
- Endothermic: Heat is absorbed
REDOX REACTIONS
- Involve both oxidation and reduction
- Oxidation: Loss of electrons, gain of oxygen, loss of hydrogen
- Reduction: Gain of electrons, loss of oxygen, gain of hydrogen
- Oxidising agent (Oxidant): Electron acceptor; Lewis acid
- Reducing agent (Reductant): Electron donor; Lewis base
3. Oxidation State
RULES FOR DETERMINING OXIDATION STATE
- Element in free state: 0
- Hydrogen: +1 (except in hydrides, where it's −1)
- Oxygen: −2 (except in peroxides, where it's −1)
- Group IA elements: +1; Group IIA: +2; Group IIIA: +3
- In neutral molecule: Sum of oxidation states = 0
- In polyatomic ion: Sum = charge on ion
- Halogens in binary compounds: −1
- Oxygen is positive in OF₂
4. Electrolysis
FARADAY'S LAWS
- First Law: W = ZIt (mass deposited ∝ charge passed)
- Second Law: W₁/W₂ = E₁/E₂ (mass ∝ chemical equivalent)
APPLICATIONS
- Purification of impure metals (e.g., copper)
- Extraction of metals (Na, Mg, Al)
- Galvanisation (coating steel with zinc)
- Preparation of organic compounds
ELECTROCHEMICAL SERIES
- K < Ca < Na < Mg < Al < Zn < Cr < Fe < Ni < H < Cu < Hg < Ag < Pd < Pt < Au
- More reactive metals at top; less reactive at bottom
5. Batteries
PRIMARY BATTERIES
- Dry Cell (Leclanche): Zinc container (anode), Carbon rod (cathode), NH₄Cl + ZnCl₂ electrolyte
- Mercury Cell: Zinc-mercury amalgam anode, HgO paste cathode, KOH + ZnO electrolyte
SECONDARY BATTERIES
- Lead Storage Battery: Lead anode, PbO₂ cathode, 38% H₂SO₄ electrolyte
- Nickel-Cadmium Cell: Cadmium anode, Nickel dioxide cathode, KOH electrolyte
FUEL CELLS
- Convert combustion energy of fuels directly into electrical energy
- H₂-O₂ fuel cell: Used in Apollo Space Programme
- Only by-product is water (non-polluting)
- Efficiency: 70-75%
- Microbial Fuel Cells (MFCs): Use anaerobic digestion to harvest energy from wastewater
6. Corrosion
EXAMPLES
- Rusting of iron: Fe₂O₃·H₂O (brown flaky substance)
- Tarnishing of silver: Silver sulphide (Ag₂S)
- Green coating on copper/bronze: Basic copper carbonate
PREVENTION
- Painting, oiling, greasing
- Galvanisation (coating with zinc)
- Chrome plating
7. Catalysis
CHARACTERISTICS
- Alters rate of reaction without being consumed
- Remains unchanged in mass and composition
- Does not initiate a reaction
- More efficient in finely divided form
- Decreases activation energy
INDUSTRIAL CATALYSTS
| Process | Catalyst | Promoter |
|---|---|---|
| Haber Process (Ammonia) | Iron | Molybdenum |
| Contact Process (H₂SO₄) | Vanadium pentoxide | — |
| Ostwald Process (HNO₃) | Platinum gauze | — |
| Deacon Process (Cl₂) | Cupric chloride | — |
ENZYME CATALYSIS
- Enzymes are biocatalysts (proteins)
- Highly specific in action
- Optimum temperature: 25-37°C
- Affected by pH change
- Examples: Starch → Maltose (Diastase), Maltose → Glucose (Maltase), Urea → NH₃ + CO₂ (Urease)
Summary: Chemical Reactions at a Glance
| Topic | Key Point |
|---|---|
| Combination | Two or more → One product |
| Decomposition | One → Two or more products |
| Displacement | More reactive replaces less reactive |
| Redox | Oxidation + Reduction simultaneously |
| Oxidising Agent | Accepts electrons; Lewis acid |
| Reducing Agent | Donates electrons; Lewis base |
KEY TAKEAWAYS FOR UPSC
- Chemical change: New substance formed; irreversible
- Decomposition: Thermal, photolysis, electrolysis
- Redox: Oxidation = loss of electrons; Reduction = gain of electrons
- Oxidising agent: Electron acceptor; Reducing agent: Electron donor
- Fuel cells: H₂-O₂ fuel cell used in Apollo mission; efficiency 70-75%
- Corrosion: Rusting of iron, tarnishing of silver
- Catalyst: Decreases activation energy; remains unchanged
- Enzymes: Biocatalysts; proteins; optimum temperature 25-37°C
High-Yield Interactive UPSC Quiz (Chemistry — Chemical Reactions and Equations)
Q1. Which type of reaction is: CaO + H₂O → Ca(OH)₂?
Correct Answer: B. CaO + H₂O → Ca(OH)₂ is a combination reaction where two substances combine to form a single product.
Q2. 2AgCl → 2Ag + Cl₂ is an example of:
Correct Answer: B. 2AgCl → 2Ag + Cl₂ is photolysis (decomposition by sunlight).
Q3. In a redox reaction, the substance that accepts electrons is called:
Correct Answer: B. An oxidising agent accepts electrons (is reduced) in a redox reaction.
Q4. What is the by-product of H₂-O₂ fuel cell?
Correct Answer: B. H₂-O₂ fuel cell produces only water as a by-product, making it non-polluting.
Q5. The catalyst used in the Haber process is:
Correct Answer: B. Iron is the catalyst used in the Haber process for ammonia synthesis, with molybdenum as promoter.
Q6. Which of the following is an example of corrosion?
Correct Answer: B. Rusting of iron is a common example of corrosion.
Q7. The efficiency of fuel cells is approximately:
Correct Answer: C. Fuel cells have an efficiency of approximately 70-75%.
Q8. Enzymes are most active at which temperature range?
Correct Answer: B. Enzymes have optimum temperature range of 25-37°C.
Q9. What is the oxidation state of oxygen in OF₂?
Correct Answer: C. In OF₂, oxygen has oxidation state of +2 (since fluorine is more electronegative).
Q10. Which process uses vanadium pentoxide as a catalyst?
Correct Answer: B. Vanadium pentoxide (V₂O₅) is used as a catalyst in the Contact Process for manufacturing sulphuric acid.