Chemical reactions and equations
Short Questions
1. What is the significance of the arrow in a chemical equation?
Answer : It indicates the direction of the reaction from reactants to products.
2. Why is a chemical equation balanced according to the law of conservation of mass?
Answer : The total mass of atoms in reactants must equal that in products to conserve mass.
3. What distinguishes a combination reaction from other reaction types?
Answer : Two or more reactants combine to form a single product.
4. How does a decomposition reaction differ from a combination reaction?
Answer : A single reactant breaks down into two or more products.
5. What characterizes a displacement reaction?
Answer : A more reactive element displaces a less reactive one from its compound.
6. Why is the reaction between AgNO₃ and NaCl classified as a double displacement reaction?
Answer : The ions (Ag⁺ and Na⁺, NO₃⁻ and Cl⁻) exchange partners to form a precipitate.
7. What indicates an exothermic reaction in a chemical equation?
Answer : Heat is released, often written as “+ Heat” on the product side.
8. Why is the decomposition of CaCO₃ an endothermic reaction?
Answer : It requires heat input to break bonds, indicated by Δ above the arrow.
9. How does the size of reactant particles affect the rate of a chemical reaction?
Answer : Smaller particles increase surface area, speeding up the reaction rate.
10. What role does a catalyst play in a chemical reaction?
Answer : It increases the reaction rate without undergoing permanent chemical change.
11. Why is MnO₂ used in the decomposition of KClO₃?
Answer : It acts as a catalyst, accelerating the release of O₂ gas.
12. What defines an oxidation reaction in terms of electron transfer?
Answer : It involves the loss of electrons by a reactant.
13. How is reduction defined in a redox reaction?
Answer : It involves the gain of electrons or loss of oxygen by a reactant.
14. Why is the reaction CuO + H₂ → Cu + H₂O a redox reaction?
Answer : CuO loses oxygen (reduction), and H₂ gains oxygen (oxidation).
15. What is the chemical formula of rust?
Answer : Fe₂O₃·xH₂O (hydrated iron(III) oxide).
16. Why does rusting occur faster in the presence of salt?
Answer : Salt enhances the electrochemical reaction by increasing conductivity.
17. What causes rancidity in oils and fats?
Answer : Air oxidation produces foul-smelling compounds, altering taste and odor.
18. How do antioxidants prevent rancidity in food?
Answer : They inhibit oxidation reactions, slowing the spoilage process.
19. Why is concentrated H₂SO₄ added slowly to water during dilution?
Answer : To control the exothermic reaction and prevent splashing or evaporation.
20. What is nascent oxygen, and why is it reactive?
Answer : It is a highly reactive single oxygen atom ([O]) formed before O₂ molecule formation.
Long Questions
1. Explain the steps to balance the chemical equation N₂(g) + H₂(g) → NH₃(g).
Answer : Start by writing N₂ + H₂ → NH₃, noting 2 N atoms on the left and 1 on the right, so place a 2 before NH₃: N₂ + H₂ → 2NH₃. Balance H by placing a 3 before H₂ (6 H atoms on both sides): N₂ + 3H₂ → 2NH₃. Verify that 2 N and 6 H atoms are equal on both sides, confirming the balanced equation.
2. Why is the reaction between HCl and NaOH classified as exothermic, and how is it represented?
Answer : The reaction HCl + NaOH → NaCl + H₂O releases heat due to the formation of strong bonds in products. This is indicated by “+ Heat” on the product side in the chemical equation. It’s a neutralization reaction, typical of strong acid-base interactions.
3. Analyze the role of Zn in the displacement reaction with CuSO₄ solution.
Answer : Zinc, being more reactive, displaces Cu²⁺ ions from CuSO₄, forming ZnSO₄ and Cu metal. The reaction is Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s), with Zn oxidizing to Zn²⁺. This demonstrates the reactivity series, where Zn ranks higher than Cu.
4. How does the double displacement reaction between BaSO₄ and K₂CrO₄ form a precipitate?
Answer : BaSO₄ (insoluble) reacts with K₂CrO₄, exchanging ions to form BaCrO₄ (yellow precipitate) and K₂SO₄ (soluble). The equation is BaSO₄(aq) + K₂CrO₄(aq) → BaCrO₄↓ + K₂SO₄(aq). The insoluble BaCrO₄ drives the reaction, as precipitates lower solubility.
5. Evaluate the significance of the law of conservation of mass in balancing chemical equations.
Answer : The law ensures that the number of atoms of each element remains constant before and after a reaction. Balancing equations, like 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O, adjusts coefficients to equalize atom counts. This reflects the principle that mass is neither created nor destroyed.
6. Why is the decomposition of sugar an example of a decomposition reaction, and what are its products?
Answer : Heating sugar (C₁₂H₂₂O₁₁) breaks it into carbon (C) and water (H₂O): C₁₂H₂₂O₁₁ → 12C + 11H₂O. This single reactant yields multiple products, defining a decomposition reaction. The process requires heat, indicating an endothermic nature.
7. Discuss how the concentration of reactants affects the rate of reaction with an example.
Answer : Higher concentration increases collision frequency, speeding up reactions, as seen when concentrated HCl reacts faster with CaCO₃ than dilute HCl. In CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O, concentrated HCl produces CO₂ more rapidly. This is due to more frequent effective collisions.
8. Explain the electrochemical mechanism of rust formation on iron surfaces.
Answer : Iron oxidizes at the anode (Fe → Fe²⁺ + 2e⁻), while oxygen reduces at the cathode (O₂ + 4H⁺ + 4e⁻ → 2H₂O). Fe²⁺ ions form Fe³⁺, which react with water to produce rust (Fe₂O₃·xH₂O). This redox process is accelerated by moisture and salts.
9. How does the presence of a catalyst like MnO₂ affect the decomposition of H₂O₂?
Answer : MnO₂ lowers the activation energy, speeding up H₂O₂ decomposition: 2H₂O₂ → 2H₂O + O₂. It remains unchanged chemically, enabling faster O₂ release without altering the reaction’s products. This demonstrates catalysis in enhancing reaction rates.
10. Why is the reaction between vegetable oil and H₂ a reduction reaction, and what conditions are required?
Answer : Vegetable oil gains hydrogen to form vanaspati ghee, a reduction process: Vegetable oil + H₂ → Vanaspati ghee. It requires 60°C and a Ni catalyst to facilitate hydrogen addition. This hydrogenation reduces unsaturation in the oil’s molecular structure.
11. Analyze the redox reaction 2H₂S + SO₂ → 3S + 2H₂O, identifying the oxidant and reductant.
Answer : H₂S loses hydrogen (oxidation), acting as the reductant, while SO₂ gains hydrogen (reduction), acting as the oxidant. The reaction forms sulfur and water, balancing electron transfer. This simultaneous oxidation-reduction defines it as a redox reaction.
12. How do endothermic and exothermic reactions differ in terms of energy exchange, with examples?
Answer : Endothermic reactions absorb heat (e.g., CaCO₃ → CaO + CO₂ requires heat), breaking bonds. Exothermic reactions release heat (e.g., CaO + H₂O → Ca(OH)₂ + heat), forming stable bonds. The energy change determines their classification and practical applications.
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