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Oxidation and Reduction – Long Answer Questions

Medium Level (Application & Explanation)

Q1. Using the copper–copper oxide experiment, explain what is meant by oxidation and reduction. Why is this called a redox reaction?

Answer:

  • When copper is heated in air, it combines with oxygen to form a black coating of copper(II) oxide (CuO). This is written as: 2Cu + O₂ → 2CuO. Since copper gains oxygen, it undergoes oxidation.
  • If hydrogen gas is then passed over hot CuO, a reverse change happens: CuO + H₂ → Cu + H₂O. Here, CuO loses oxygen and is converted back to brown metallic copper, so CuO is reduced.
  • These two changes happen together: one substance is oxidized while the other is reduced. Hence, the process is called a redox reaction.
  • In the second step, H₂ is oxidized to H₂O (it gains oxygen), so hydrogen acts as a reducing agent. CuO acts as an oxidizing agent because it supplies oxygen to hydrogen.

Q2. Describe the activity for oxidation of copper in a china dish. State materials, steps, observations, and inferences, including safety aspects.

Answer:

  • Materials: 1 g copper powder, china dish, Bunsen burner, safety goggles.
  • Steps:
    1. Place copper powder in a clean china dish. 2) Wear safety goggles. 3) Heat gently using a Bunsen burner.
  • Observations: The reddish-brown copper gradually forms a black coating. The black substance is copper(II) oxide (CuO). The reaction is: 2Cu + O₂ → 2CuO.
  • Inference: Copper combines with oxygen from air and is oxidized.
  • Extension: When hydrogen gas is passed over the hot black CuO, the coating turns brown again as CuO + H₂ → Cu + H₂O. This shows reduction (loss of oxygen by CuO) and the simultaneous oxidation of hydrogen to water.
  • Safety: Use goggles, avoid inhaling fumes, and handle the burner carefully to prevent burns.

Q3. In the reaction ZnO + C → Zn + CO, identify what is oxidized and what is reduced. Explain with observations and reasons.

Answer:

  • The reaction is: ZnO + C → Zn + CO.
  • Here, zinc oxide (ZnO) is converted to zinc (Zn) by losing oxygen. Thus, ZnO is reduced.
  • Carbon (C) combines with oxygen (from ZnO) to form carbon monoxide (CO). Since carbon gains oxygen, carbon is oxidized.
  • Therefore, carbon acts as a reducing agent (it removes oxygen from ZnO), and ZnO acts as an oxidizing agent (it provides oxygen to carbon).
  • Observation: The white powder of ZnO changes to shiny, metallic zinc upon heating with carbon, indicating reduction of the oxide.
  • This is a clear example of a redox reaction where oxidation and reduction occur together, and it also shows how carbon is commonly used to obtain metals from their oxides.

Q4. Explain the redox changes in MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂. What do you observe and how do you infer oxidation and reduction?

Answer:

  • The balanced reaction is: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂.
  • Manganese dioxide (MnO₂) is converted to manganese(II) chloride (MnCl₂). Since MnO₂ loses oxygen, it is reduced.
  • Hydrochloric acid (HCl) produces chlorine gas (Cl₂), which means the chloride part from HCl is oxidized to chlorine. So, HCl is oxidized.
  • Thus, MnO₂ acts as an oxidizing agent (it causes HCl to lose electrons/turn to Cl₂ by accepting oxygen), while HCl acts as a reducing agent.
  • Observations: Effervescence with the release of greenish-yellow chlorine gas and formation of a solution of MnCl₂; water is also formed.
  • Safety: Chlorine is irritating; perform the experiment in a well-ventilated area or fume hood and wear goggles and a mask.

Q5. Define corrosion and rancidity. Describe their causes, signs, and everyday examples, connecting them to oxidation.

Answer:

  • Corrosion is the deterioration of metals due to chemical reactions with the environment, commonly involving oxidation. Example: rusting of iron in moist air forms iron oxide, seen as a reddish-brown powder on iron gates, bridges, and car bodies. Signs include loss of shine, color change, and flaky deposits.
  • Rancidity is the spoilage of fats and oils due to oxidation, causing bad smell and taste. Example: Opened cooking oil or fried snacks developing an off odor.
  • Causes: Presence of oxygen, moisture, heat, and light accelerate oxidation.
  • Prevention: Use of antioxidants and nitrogen-filled packing for foods like chips slows oxidation. Keeping metals dry and using protective coverings helps reduce corrosion.
  • Both processes are everyday examples of oxidation reactions affecting materials and food quality.

High Complexity (Analytical & Scenario-Based)

Q6. A black coating on hot copper turns brown when a gas is passed over it. Identify the gas and explain the sequence of redox changes with equations and observations.

Answer:

  • When copper is heated in air, it forms a black coating of copper(II) oxide (CuO): 2Cu + O₂ → 2CuO. This is oxidation of copper (gain of oxygen).
  • If a gas is passed over hot CuO and the black layer turns brown, the gas is hydrogen (H₂). The reaction is: CuO + H₂ → Cu + H₂O.
  • In this step, CuO is reduced to Cu (it loses oxygen), and H₂ is oxidized to H₂O (it gains oxygen).
  • Observations include a color change from black to brown metallic copper and possible condensation of water on cooler parts of the apparatus.
  • Thus, both steps together illustrate a redox cycle: first oxidation (Cu to CuO), then reduction (CuO to Cu), with H₂ acting as a reducing agent.

Q7. Food companies flush snack packets with nitrogen. Analyze how this prevents rancidity and suggest conditions that promote or slow this oxidation.

Answer:

  • Rancidity is due to the oxidation of fats and oils, which produces unpleasant smells and tastes. Oxygen in air reacts with oil molecules, especially at higher temperatures and in light.
  • Flushing packets with nitrogen removes most of the oxygen, creating an inert atmosphere that greatly slows oxidation. This keeps snacks crisp and fresh for longer.
  • Conditions that promote rancidity: Exposure to air (oxygen), heat, light, and long storage. Partially used oil bottles kept open also go rancid faster.
  • Ways to slow rancidity: Use airtight containers, store in cool, dark places, and add antioxidants (commonly used by manufacturers). Minimizing contact with air and heat is key.
  • This approach directly targets the oxidation step, proving that controlling the reactant (oxygen) controls the rate of rancidity.

Q8. After rain, an iron gate develops a red-brown powdery layer. Explain why rusting accelerates in this situation and discuss likely consequences and simple checks to monitor it.

Answer:

  • The red-brown powder is rust (iron oxide) formed when iron reacts with oxygen in the presence of moisture. Rain provides the necessary water that speeds up oxidation of iron, so rusting is faster after rainfall.
  • The chemical change is an oxidation reaction, where iron loses electrons to oxygen, forming hydrated iron oxides. Visibly, shine is lost, and powdery flakes appear.
  • Consequences: Over time, corrosion causes weakening of structures, rough surfaces, and may lead to safety issues for gates, railings, bridges, and vehicle bodies.
  • Simple checks: Regularly observe for color change, powdery deposits, and pitting. After rains, inspect joints and corners where water stagnates.
  • Basic care: Keep surfaces dry when possible and maintain protective coverings. Early detection prevents extensive damage by addressing oxidation promptly.

Q9. A student heats ZnO with carbon but notices incomplete conversion to metal. Analyze what might have happened and how to confirm the redox changes using observations.

Answer:

  • The intended reaction is: ZnO + C → Zn + CO, where ZnO is reduced (loses oxygen) and carbon is oxidized to CO (gains oxygen).
  • If conversion is incomplete, possible reasons include: insufficient carbon, inadequate heating, or short reaction time, reducing the extent of oxygen removal from ZnO.
  • Observations for success: Appearance of shiny metallic zinc indicates reduction; formation of CO is not directly visible, but vigorous heating helps drive the reaction forward.
  • If white ZnO remains mixed with the product, it signals partial reduction. Increasing temperature, ensuring good mixing, and providing enough carbon improves completion.
  • This analysis shows how reaction conditions affect the redox balance, and that visible metallic luster is a practical indicator of successful reduction.

Q10. For the reactions 2Cu + O₂ → 2CuO, CuO + H₂ → Cu + H₂O, ZnO + C → Zn + CO, and MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, identify oxidized and reduced substances and justify the choice of oxidizing and reducing agents.

Answer:

  • 2Cu + O₂ → 2CuO: Copper is oxidized (gains oxygen). O₂ is reduced in the sense that it is consumed to form oxide. Here, O₂ acts as the oxidizing agent because it causes oxidation by supplying oxygen.
  • CuO + H₂ → Cu + H₂O: CuO is reduced to Cu (loses oxygen). H₂ is oxidized to H₂O (gains oxygen). Thus, H₂ is the reducing agent, and CuO behaves as an oxidizing agent toward hydrogen.
  • ZnO + C → Zn + CO: ZnO is reduced to Zn; carbon is oxidized to CO. Therefore, carbon is the reducing agent, while ZnO is the oxidizing agent.
  • MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂: MnO₂ is reduced to MnCl₂; HCl is oxidized to Cl₂. Hence, HCl is the reducing agent, and MnO₂ is the oxidizing agent.
  • In each case, identify oxidation by gain of oxygen and reductio...

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