Which one of the following chemical reactions is not feasible?

1. Types of Chemical Reactions (basic)

In the study of chemistry, a chemical reaction is fundamentally a process of transformation. Atoms do not simply appear or disappear; instead, chemical reactions involve the breaking and making of bonds between atoms to produce new substances Science, Chapter 1, p.6. For a UPSC aspirant, understanding the 'logic' of these reactions is crucial, as it explains everything from how our bodies release energy to how industrial metals are refined.

Chemical reactions are generally categorized into four primary types based on how the atoms rearrange themselves:

Reaction Type	Description	General Form
Combination	Two or more reactants join to form a single product.	A + B → AB
Decomposition	A single reactant breaks down into two or more simpler products. This usually requires energy (heat, light, or electricity) Science, Chapter 1, p.16.	AB → A + B
Displacement	A more reactive element takes the place of a less reactive element in a compound.	A + BC → AC + B
Double Displacement	Two compounds exchange ions to form two new compounds, often resulting in a precipitate (an insoluble solid) Science, Chapter 1, p.12.	AB + CD → AD + CB

Whether a Single Displacement reaction occurs depends on the Reactivity Series—a ranking of metals from most to least reactive. For instance, Magnesium (Mg) is more reactive than Copper (Cu); therefore, Mg can displace Cu from a solution, but Cu cannot displace Mg. Additionally, we classify reactions by their energy flow: Exothermic reactions (like respiration) release heat, while Endothermic reactions absorb energy from their surroundings Science, Chapter 1, p.15.

Sources: Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.6; Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.12; Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.15; Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.16

2. Chemical Properties of Metals (basic)

When we look at the chemical properties of metals, we are essentially observing how "eager" a metal is to lose electrons and form positive ions. This eagerness isn't the same for every metal. For instance, some metals react violently with dilute acids, while others remain completely indifferent. When a reactive metal reacts with a dilute acid, it displaces hydrogen from the acid, resulting in the formation of a salt and hydrogen gas (H₂) Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20. By observing the rate at which hydrogen bubbles form, we can begin to rank metals: magnesium (Mg) reacts the most vigorously, followed by aluminium (Al), zinc (Zn), and iron (Fe), while copper (Cu) shows no reaction at all Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44.

This hierarchy of reactivity is best understood through displacement reactions. The logic is simple: a more reactive metal has the power to "kick out" or displace a less reactive metal from its salt solution. If Metal A displaces Metal B from its salt, then A is officially higher in the pecking order than B Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45. This gives rise to the Reactivity Series, which is a vertical arrangement of metals in decreasing order of their chemical activity.

Reaction Type	General Equation	Key Observation
With Dilute Acid	Metal + Acid → Salt + H₂	Bubbling/Effervescence (except for low-reactivity metals like Cu)
Displacement	A + Salt of B → Salt of A + B	Occurs only if A is more reactive than B

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45

3. Redox Reactions and Electron Transfer (intermediate)

In our journey through chemistry, the concept of Redox reactions is a cornerstone. The term itself is a portmanteau of Reduction and Oxidation. Traditionally, as noted in Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12, we define these based on oxygen: Oxidation is the gain of oxygen, while Reduction is the loss of oxygen. However, for the UPSC and intermediate chemistry, we must look deeper at the electron transfer level. In any redox reaction, one substance loses electrons and another gains them—they are two sides of the same coin because electrons cannot exist freely; they must have a destination.

To understand this better, let's look at displacement reactions. When a more reactive metal (like Zinc) is placed in a solution of a less reactive metal salt (like Copper Sulfate), a reaction occurs because the more reactive metal "wants" to be in an ionic state more than the less reactive one. This involves a transfer of electrons. The reactive metal atoms lose electrons to become ions (oxidation), while the metal ions in the solution gain those electrons to become solid metal atoms (reduction). As explained in Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.51, highly reactive metals like Sodium or Aluminium are often used as reducing agents because they are exceptionally good at giving away their electrons to reduce other metal oxides into pure metals.

Process	Oxygen Transfer	Electron Transfer
Oxidation	Gain of Oxygen	Loss of Electrons
Reduction	Loss of Oxygen	Gain of Electrons

A classic example is the reaction between Magnesium and Hydrochloric Acid: Mg + 2HCl → MgCl₂ + H₂. Here, the Magnesium atom (Mg) loses two electrons to become a Magnesium ion (Mg²⁺), meaning it is oxidised. Simultaneously, the Hydrogen ions (H⁺) from the acid gain those electrons to form Hydrogen gas (H₂), meaning they are reduced. This flow of electrons is the "electricity" of chemical change, and it is governed by the reactivity series—a hierarchy of metals ranked by their tendency to lose electrons.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.51

4. Extraction of Metals (Metallurgy) (intermediate)

In our journey to understand chemistry, the Extraction of Metals (metallurgy) stands as a brilliant application of the Reactivity Series. Metals are rarely found in their pure form in nature because they have a tendency to react with environmental elements like oxygen and sulfur to form compounds called ores. The ease with which we can "rescue" a metal from its ore depends entirely on where it sits in the reactivity hierarchy Science, Chapter 3: Metals and Non-metals, p. 50.

To master extraction, we group metals into three distinct chemical "personality" types, each requiring a different strategy:

• Low Reactivity (The Loners): Metals like Gold, Silver, and Platinum are often found in a free state. Others, like Copper and Mercury, are found as oxides or sulfides but can be extracted simply by heating Science, Chapter 3: Metals and Non-metals, p. 49.
• Medium Reactivity (The Socialites): Metals like Zinc, Iron, and Lead are usually found as oxides, carbonates, or sulfides. We extract them through reduction, typically using Carbon (coke) to "steal" the oxygen away from the metal oxide Science, Chapter 3: Metals and Non-metals, p. 51.
• High Reactivity (The High-Energy Metals): Metals like Sodium, Magnesium, and Calcium are so reactive that Carbon cannot reduce them—they have a higher affinity for oxygen than carbon does! These must be extracted using electrolytic reduction, where electricity is used to force the metal to deposit at the cathode Science, Chapter 3: Metals and Non-metals, p. 52.

A fascinating tool in this process is the displacement reaction. A metal can be used as a reducing agent if it is more reactive than the metal currently in the compound. For example, Aluminum can displace Iron from Iron oxide because Aluminum is higher in the reactivity series. This is the core principle behind the Thermit process used to weld railway tracks Science, Chapter 3: Metals and Non-metals, p. 51-52.

Metal Category	Examples	Preferred Extraction Method
High Reactivity	Na, Mg, Ca, Al	Electrolysis of molten ore
Medium Reactivity	Zn, Fe, Pb	Reduction using Carbon (Coke)
Low Reactivity	Cu, Ag, Au	Found native or extracted by heating

Sources: Science, Chapter 3: Metals and Non-metals, p.45; Science, Chapter 3: Metals and Non-metals, p.49; Science, Chapter 3: Metals and Non-metals, p.50; Science, Chapter 3: Metals and Non-metals, p.51; Science, Chapter 3: Metals and Non-metals, p.52

5. Corrosion and Galvanization (intermediate)

Corrosion is the gradual destruction of metals when they react with environmental factors like oxygen, moisture, and acids. You can think of it as the metal trying to return to its more stable, natural state—the oxide form. While some metals like Potassium and Sodium react so violently they must be kept in kerosene, others like Magnesium, Aluminium, and Zinc are quite clever; they form a thin, tough protective oxide layer on their surface that prevents further oxidation Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.42. However, Iron is problematic because its oxide (rust: Fe₂O₃·xH₂O) is porous and flakes off, continuously exposing fresh metal to the elements until the entire structure weakens.

To combat this, we use Galvanization. This is the process of coating iron or steel with a thin layer of Zinc. Zinc is particularly effective because it is more reactive than iron. Based on the Reactivity Series, Zinc sits above Iron; therefore, it has a greater tendency to lose electrons and oxidize. This leads to a fascinating phenomenon called Sacrificial Protection. Even if the zinc coating is scratched or broken, the zinc continues to corrode preferentially, "sacrificing" itself to keep the underlying iron safe from oxidation Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.55.

Method	Mechanism	Examples
Barrier Protection	Physically blocking air/moisture from reaching the metal.	Painting, Oiling, Greasing
Sacrificial Protection	Using a more reactive metal to corrode in place of the base metal.	Galvanization (Zinc), Magnesium blocks on ships

In the industrial world, managing corrosion is vital for sustainability. In India, recycling metals like Iron and Aluminium is a major focus to minimize waste and preserve resources Science-Class VII, NCERT(Revised ed 2025), The World of Metals and Non-metals, p.54. Understanding these chemical principles allows us to build bridges, ships, and infrastructure that last for decades rather than years.

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.42; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.55; Science-Class VII, NCERT(Revised ed 2025), The World of Metals and Non-metals, p.54

6. The Metal Activity (Reactivity) Series (exam-level)

In the world of chemistry, not all metals are created equal. Some, like Potassium, react so violently with water that they must be stored under oil, while others, like Gold, remain untarnished for centuries. The Activity Series (or Reactivity Series) is a specialized list where metals are arranged in the order of their decreasing chemical reactivity Science, Class X (NCERT 2025 ed.), Chapter 3, p. 45. This series is an essential tool for predicting how metals will behave when they encounter acids, water, or salt solutions of other metals.

The fundamental rule of the Activity Series is the Displacement Principle: a metal higher in the series is more "powerful" (more reactive) and can displace a metal lower in the series from its compound. For instance, if you place an Iron (Fe) nail into a blue solution of Copper(II) sulphate (CuSO₄), the Iron will displace the Copper because Iron is more reactive. The solution eventually turns green as Iron sulphate (FeSO₄) forms, and the displaced Copper deposits on the nail Science, Class X (NCERT 2025 ed.), Chapter 1, p. 11. However, the reverse is impossible; Copper cannot displace Iron because it lacks the chemical "strength" to do so.

From an exam perspective, understanding this hierarchy is crucial for understanding metal extraction. Metals at the top of the series (like Sodium and Magnesium) are so reactive they are never found in nature as free elements; they are always bound in compounds. Conversely, metals at the very bottom (like Gold and Silver) are often found in their native, free state because they do not easily react with oxygen or moisture Science, Class X (NCERT 2025 ed.), Chapter 3, p. 49. Chemically, this reactivity is linked to reduction potentials: metals with more negative potentials are stronger reducing agents and sit higher on the list.

Reactivity Level	Metals	Observation with Dilute HCl
High	K, Na, Ca, Mg	Violent or rapid evolution of H₂ bubbles Science, Class X (NCERT 2025 ed.), Chapter 3, p. 44
Medium	Al, Zn, Fe, Pb	Moderate reaction; rate decreases down the series
Low	Cu, Hg, Ag, Au	No reaction (no H₂ gas formed)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.45; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.44; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.11; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.49

7. Solving the Original PYQ (exam-level)

You have just explored how metals compete to form compounds, and this question is a direct application of the Reactivity Series. In a single displacement reaction, a metal can only displace another from its salt solution if it is more reactive (higher in the series). This concept, discussed in Science, class X (NCERT 2025 ed.), essentially treats chemical reactions as a hierarchy where the "stronger" element displaces the "weaker" one. To solve this, you must apply the hierarchy: Mg > Zn > Fe > Pb > Cu.

Walking through the logic, we look for the outlier where a less reactive metal is trying to displace a more reactive one. In option (C) Cu + PbCl2, Copper (Cu) is attempting to displace Lead (Pb). However, according to the activity series, Copper is positioned below Lead. Because Copper has a higher reduction potential and is less likely to lose electrons than Lead, it cannot force Lead out of its compound. Therefore, this reaction is not feasible, making (C) the correct choice. Your mental check should always be: Is the lone metal higher on the list than the metal in the compound?

The UPSC often includes options like (A), (B), and (D) to test your memorization of the middle section of the series. In these distractors, Iron (Fe), Zinc (Zn), and Magnesium (Mg) are all more reactive than Copper, so those reactions occur easily. The common trap is misordering the transition metals near the bottom of the series. Aspirants often forget that Lead (Pb) sits just above Hydrogen, while Copper (Cu) sits below it. By remembering that Copper is one of the least reactive common metals, you can quickly identify it as the one incapable of displacement in this set.