Reactivity Series of Metals – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain how the reactivity series helps in predicting the outcome of displacement reactions. Give examples.
Answer:
The reactivity series is a list of metals arranged by their ability to undergo chemical reactions. In displacement reactions, a more reactive metal can replace a less reactive metal from its compound in solution. For example, zinc is above copper in the series. When zinc metal is placed in copper sulphate solution, zinc displaces copper, forming zinc sulphate and copper (Zn + CuSO₄ → ZnSO₄ + Cu). However, if copper is put into zinc sulphate solution, no reaction occurs, since copper is less reactive. Thus, the reactivity series helps us predict which metals can displace others in such reactions.
Q2. Describe the reaction of different metals from the reactivity series with water. Give chemical equations wherever possible.
Answer:
Potassium (K) and sodium (Na) are highly reactive with water; they react explosively to form their hydroxides and release hydrogen gas (2K + 2H₂O → 2KOH + H₂↑). Calcium (Ca) reacts less violently, forming calcium hydroxide and hydrogen (Ca + 2H₂O → Ca(OH)₂ + H₂↑). Magnesium (Mg) reacts very slowly with cold water, but more rapidly with hot water or steam (Mg + 2H₂O → Mg(OH)₂ + H₂↑). Metals like zinc (Zn) and iron (Fe) react only with steam, not with cold water (Zn + H₂O (steam) → ZnO + H₂↑). Metals lower in the series, such as copper (Cu), silver (Ag), and gold (Au), do not react with water at all.
Q3. How does the position of a metal in the reactivity series determine its method of extraction? Illustrate with examples.
Answer:
Metals at the top of the reactivity series like potassium, sodium, and calcium are highly reactive. They are extracted from their compounds by electrolysis of molten salts because they cannot be reduced by carbon. Metals in the middle, such as zinc and iron, are moderately reactive and can be extracted by reduction with carbon or carbon monoxide. Less reactive metals, like silver and gold, are found in their native state, so they can be separated easily from their ores by simpler processes. Thus, the reactivity series plays an important role in deciding the extraction technique for each metal.
Q4. What is the reaction of metals with acids and how does the reactivity series help in predicting which metals can liberate hydrogen gas from acids?
Answer:
Metals above hydrogen in the reactivity series react with dilute acids to produce a salt and hydrogen gas. For example, zinc reacts with hydrochloric acid (Zn + 2HCl → ZnCl₂ + H₂↑). Metals below hydrogen, such as copper, silver, and gold, do not react with dilute acids and hence cannot liberate hydrogen. The reactivity series thus tells us that any metal higher than hydrogen will show effervescence with acids and evolve hydrogen, while those below will not react.
Q5. Why does zinc not react with iron sulphate solution, but magnesium can displace iron from iron sulphate? Justify your answer using the reactivity series.
Answer:
Zinc and iron are both found in the middle of the reactivity series, with zinc above iron. That means zinc is more reactive and can displace iron from iron sulphate (Zn + FeSO₄ → ZnSO₄ + Fe). However, if you try to react zinc with iron sulphate, no reaction occurs because iron cannot displace zinc from its compound. Magnesium is placed even above zinc, so it is more reactive than both. Magnesium can thus displace iron from iron sulphate solution (Mg + FeSO₄ → MgSO₄ + Fe). The reactivity series helps us predict which metal will be displaced.
High Complexity (Analysis & Scenario-Based)
Q6. Suppose you are given three metals: A, B, and C. Metal A reacts violently with cold water, B reacts only with steam, and C does not react even with acids. Predict the order of reactivity and identify each metal with respect to the reactivity series.
Answer:
Metal A reacts violently with cold water, suggesting it is highly reactive, like potassium (K) or sodium (Na). Metal B reacts only with steam, indicating moderate reactivity, like zinc (Zn) or iron (Fe). Metal C does not react with acids, implying it is less reactive than hydrogen, such as copper (Cu), silver (Ag), or gold (Au).
Therefore, the order of reactivity is:
A (most reactive), B (moderate reactivity), C (least reactive).
So, in terms of the reactivity series:
A = K/Na, B = Zn/Fe, C = Cu/Ag/Au.
Q7. Analyze why less reactive metals like gold and silver are found in nature as pure elements while potassium and sodium are found only as compounds.
Answer:
Gold (Au) and silver (Ag) are at the bottom of the reactivity series. They do not react easily with air, water, or acids. Thus, they remain unchanged in their surroundings and are found as native elements. On the other hand, potassium (K) and sodium (Na), at the top of the series, are very reactive. They combine easily with non-metals like oxygen and chlorine. Hence, they are always found in nature as compounds (like KCl or Na₂CO₃), not as pure metals. Nature prefers stability, so highly reactive metals get bound up, and least reactive metals remain free.
Q8. Imagine a situation where iron objects are left exposed to the environment, while a similar set is coated with zinc. Using the reactivity series, explain why the zinc-coated objects last longer.
Answer:
When iron is exposed, it reacts with oxygen and moisture to form rust. However, when iron is coated with zinc (galvanization), zinc, being more reactive than iron, acts as a protective layer. Even if the coating gets scratched, zinc will corrode in preference to iron. This is because of its higher position in the reactivity series. Thus, the zinc coating sacrifices itself, preventing iron from rusting, and making the object last longer. This principle is widely used to protect iron from corrosion.
Q9. A student mixed an unknown metal with dilute hydrochloric acid but no gas was evolved. She then mixed it with copper sulphate solution, but again no reaction occurred. What can you infer about the metal's position in the reactivity series? Explain.
Answer:
If the unknown metal does not react with dilute hydrochloric acid, it must be below hydrogen in the reactivity series. When mixed with copper sulphate solution, if it also does not react, the metal must be less reactive than copper. Therefore, the unknown metal is possibly silver (Ag), gold (Au), or even platinum (Pt). Such metals are at the bottom of the reactivity series and are known for being least reactive. This experiment proves their position as the least reactive metals.
Q10. If you had to design an experiment to determine the relative reactivity of three unknown metals, what steps would you take, and how would you use the reactivity series to interpret your results?
Answer:
First, I would react each metal with dilute hydrochloric acid to see if hydrogen gas is released. Next, I would place each metal in solutions of the other two metals' salts. For example, put metal 1 in salt solutions of metals 2 and 3, and observe if displacement happens (change in color or metal deposition). By noting which metal displaces others and which reacts fastest with acids, I can rank their reactivity. Using the reactivity series, the metal that displaces others and reacts quickest is most reactive; the one that does not react or is displaced by others is least reactive. This approach lets us build a mini-reactivity series experimentally.