Reactivity Series of Metals – Long Answer Questions (CBSE Class 10 Chemistry)

Medium Level (Application & Explanation)

Q1. Define the Reactivity Series of metals. How does it help predict reactions with water and acids? Illustrate with examples.

Answer: The Reactivity Series is an arrangement of metals in order of their decreasing reactivity, from Potassium at the top to Platinum at the bottom. It helps predict whether a metal will react with water, steam, or dilute acids, and whether it can displace another metal from its salt solution. Metals above hydrogen can displace hydrogen from acids, while those below do not. Very reactive metals (K, Na, Ca) react with cold water to form metal hydroxides and hydrogen gas, while metals in the middle (Mg, Zn, Fe) react with steam or acids, and unreactive metals (Cu, Ag, Au) don’t react. For example:

2Na + 2H₂O → 2NaOH + H₂ (vigorous reaction with cold water)
Mg + 2HCl → MgCl₂ + H₂ (bubbles of hydrogen)
Cu + HCl → No reaction (copper is below hydrogen).
Thus, the series serves as a predictive tool in chemistry.

Q2. Why do metals show different reactivities? Explain using electron loss, atomic size, and ionization energy with examples.

Answer: Metals differ in reactivity because of how easily they lose electrons to form positive ions. Three main factors govern this:

Ease of electron loss: Metals that lose electrons easily (like K and Na) are highly reactive.
Atomic size: As atomic size increases, the outer electrons are farther from the nucleus and are held less tightly, so they are lost more easily.
Ionization energy: Metals with low ionization energy require less energy to remove electrons, leading to greater reactivity.
Examples:
Sodium (Na) loses one electron very easily to form Na⁺, so it reacts rapidly with water.
Magnesium (Mg) holds electrons more strongly than Na, so it is less reactive; it reacts with steam but not cold water.
Gold (Au) has a high effective nuclear charge and high ionization energy; it does not lose electrons easily, so it is very unreactive.
Therefore, these atomic properties explain the order in the Reactivity Series.

Q3. What are displacement reactions? Describe the iron and copper(II) sulfate experiment with observations and conclusions.

Answer: A displacement reaction occurs when a more reactive metal pushes out a less reactive metal from its salt solution. The Reactivity Series helps predict such outcomes. In the classic iron–copper(II) sulfate experiment:

Set-up: Place a clean iron nail in CuSO₄ (blue) solution and leave for 20–30 minutes.
Observations:
- The blue solution turns light green due to formation of FeSO₄ (aq).
- A brown layer of copper forms on the iron nail.
Chemical equation: Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)
Conclusion: Iron is more reactive than copper, so it displaces copper from copper sulfate.
This principle also explains why Zn displaces copper: Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s), but copper cannot displace iron or zinc from their salts. Thus, displacement reactions visually demonstrate the relative reactivity of metals.

Q4. Explain how the Reactivity Series guides the extraction of metals from their ores. Give clear examples.

Answer: The Reactivity Series determines which extraction method is suitable:

Highly reactive metals (K, Na, Ca, Mg, Al): Their oxides are very stable and cannot be reduced by carbon. They are extracted by electrolysis of their molten salts.
- Example: Sodium from molten NaCl by electrolysis.
Moderately reactive metals (Zn, Fe, Pb): Their oxides can be reduced by carbon/CO at high temperatures.
- Examples:
  - Iron from Fe₂O₃ using CO in a blast furnace: Fe₂O₃ + 3CO → 2Fe + 3CO₂
  - Zinc from ZnO by carbon reduction: ZnO + C → Zn + CO
Less reactive/noble metals (Cu, Ag, Au): Often found in the native state and need physical methods or simple roasting and refining.
- Example: Gold is found as nuggets and separated physically.
  Thus, the series links reactivity to the feasibility of reduction methods, ensuring economic and efficient metal extraction.

Q5. What is galvanization? Using the Reactivity Series, explain how it prevents rusting of iron even if scratched.

Answer: Galvanization is the process of coating iron or steel with a thin layer of zinc to prevent rusting. It works due to the position of zinc being above iron in the Reactivity Series, making zinc more reactive.

If the coating is intact, zinc forms a protective ZnO/ZnCO₃ layer that prevents contact of iron with air and moisture.
If the coating is scratched, zinc still protects iron through sacrificial protection: zinc oxidizes preferentially and supplies electrons to iron, preventing iron from turning into rust.
- Zn → Zn²⁺ + 2e⁻ (zinc corrodes first)
- Fe remains protected as it does not oxidize while zinc is present.
Applications: Bridges, fences, car bodies, roofing sheets.
Because zinc is higher in the Reactivity Series, it acts as a sacrificial anode, making galvanization a reliable and cost-effective anti-rust method.

High Complexity (Analytical & Scenario-Based)

Q6. You have metals Fe, Zn, and Cu and their salt solutions. Design a fair test to arrange these metals by reactivity. Include controls, expected observations, and equations.

Answer: Plan three parallel displacement tests using clean strips of Fe, Zn, Cu and solutions of CuSO₄, FeSO₄, and ZnSO₄. Keep volume, concentration, time, and temperature constant.

Set-ups and expected outcomes:
- Zn in CuSO₄: solution fades from blue, reddish Cu deposits; Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
- Fe in CuSO₄: turns light green, brown Cu coats iron; Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)
- Cu in FeSO₄ or ZnSO₄: no change (copper cannot displace Fe or Zn).
- Fe in ZnSO₄ and Zn in FeSO₄: Zn displaces Fe from FeSO₄ but Fe does not displace Zn from ZnSO₄.
Inference: The metal that displaces the others most often is most reactive. Order: Zn > Fe > Cu.
Control: Place identical metal in its own salt (e.g., Zn in ZnSO₄) to confirm no change is normal.
This systematic approach ensures a fair comparison and clear ranking by reactivity.

Q7. Magnesium reacts with dilute acids to release bubbles, but copper does not. Justify this using the Reactivity Series and write balanced equations where applicable.

Answer: In the Reactivity Series, magnesium lies above hydrogen, while copper lies below hydrogen. Metals above hydrogen can displace hydrogen from dilute acids, forming a salt and hydrogen gas. Metals below hydrogen cannot do this.

For magnesium:
- Reaction: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)
- Observation: Effervescence due to hydrogen gas; the magnesium strip gradually dissolves, and the solution warms slightly.
For copper:
- Reaction: Cu (s) + HCl (aq) → No reaction (copper is less reactive than hydrogen).
  Therefore, the position in the Reactivity Series explains the differing behaviors. Magnesium readily loses electrons to form Mg²⁺ and reduce H⁺ to H₂, while copper does not. This is consistent with the idea that a more reactive metal can displace a less reactive element (hydrogen) from its compounds, but a less reactive metal cannot.

Q8. A factory must select a metal for hot water pipelines: magnesium, iron, or copper. Analyze and recommend the best choice using the Reactivity Series and reaction behavior with water.

Answer:

Magnesium (Mg): More reactive; reacts slowly with hot water and more with steam to form MgO and H₂. It could lead to gas formation and scaling inside pipes: Mg + H₂O (steam) → MgO + H₂. Hence, not suitable.
Iron (Fe): Reacts with steam to form Fe₃O₄ and H₂ and is prone to rusting in moist air. Even with coatings, maintenance is higher: 3Fe + 4H₂O (steam) → Fe₃O₄ + 4H₂.
Copper (Cu): Lies below hydrogen; does not react with water, steam, or dilute acids (non-oxidizing). It forms a thin protective layer (patina) in moist air that resists further corrosion.
Recommendation: Copper is the best choice due to its low reactivity, corrosion resistance, and thermal conductivity. For cost-sensitive cases, galvanized iron can be used with zinc coating and additional protective measures, but copper remains superior for hot water durability.

Q9. An unknown metal X displaces iron from FeSO₄ but does not displace magnesium from MgSO₄. It reacts with steam but not cold water. Identify X with reasons and write one supporting equation.

Answer: If metal X displaces iron from FeSO₄, it must be more reactive than iron. If it cannot displace magnesium from MgSO₄, it is less reactive than magnesium. Also, if X reacts with steam but not cold water, it fits the behavior of metals in the middle of the Reactivity Series. The best match is zinc (Zn).

Justification:
- Relative position: Mg > Zn > Fe > Cu
- Displacement: Zn displaces Fe but not Mg.
- Water reaction: Zn does not react with cold water but reacts with steam to form ZnO and H₂.
Supporting equations:
- Zn (s) + FeSO₄ (aq) → ZnSO₄ (aq) + Fe (s)
- Zn (s) + H₂O (steam) → ZnO (s) + H₂ (g)
  Thus, X is zinc, based on both displacement behavior and reaction with steam.