Problem 144 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{Mg}\) and \(\mathrm{Zn}\) have following resemblance: (a) Both of them can be used as electrodes (b) \(\mathrm{MgO}\) and \(\mathrm{ZnO}\) are amphoteric. (c) Both of them can be used to prevent corrosion (d) \(\mathrm{MgCO}_{3}, \mathrm{ZnCO}_{3}\) both on heating give their oxides.

Step-by-Step Solution

Answer

Options (a), (c), and (d) are true.

Step 1: Analyze Option (a)

Both magnesium (Mg) and zinc (Zn) can serve as electrodes. Magnesium is commonly used in electrochemical cells as a sacrificial anode to protect other metals from corrosion. Zinc, similarly, is also used as an electrode, especially in batteries such as the zinc-carbon battery. Therefore, option (a) is true.

Step 2: Analyze Option (b)

Option (b) claims that both \( \mathrm{MgO} \) and \( \mathrm{ZnO} \) are amphoteric. Amphoteric oxides can act as both acids and bases. \( \mathrm{ZnO} \) is indeed amphoteric as it reacts with both acids and bases. However, \( \mathrm{MgO} \) is not amphoteric; it is a basic oxide. Thus, option (b) is false.

Step 3: Analyze Option (c)

Option (c) states that both can prevent corrosion, which is true. Magnesium acts as a sacrificial anode, and zinc is widely used for galvanization, coating iron to prevent rusting. Thus, both metals help prevent corrosion, making this option true.

Step 4: Analyze Option (d)

Option (d) states that upon heating \( \mathrm{MgCO_3} \) and \( \mathrm{ZnCO_3} \), they decompose to form \( \mathrm{MgO} \) and \( \mathrm{ZnO} \) respectively. This is true, as both carbonates decompose upon heating to form their respective oxides and carbon dioxide gas. Therefore, option (d) is correct.

Key Concepts

Electrode Use

Metals like magnesium (Mg) and zinc (Zn) play a significant role in electrochemical applications due to their properties as electrodes. An electrode is a conductor through which electricity enters or leaves an object, substance, or region.

• For magnesium, it is frequently employed as a *sacrificial anode*. This means it can give up electrons easily to protect another metal from corroding by itself corroding first. This process is crucial in applications like marine environments where metal structures are prone to corrosion.
• Zinc is another versatile electrode, particularly known for its use in standard batteries like zinc-carbon batteries. It functions as an efficient electron donor, facilitating the current flow within the battery.

Understanding how metals serve as electrodes helps us appreciate their value in both protecting structures from corrosion and powering electronic devices.

Corrosion Prevention

Corrosion is a natural process that breaks down metals, leading to deterioration and failure of structures. Fortunately, metals like magnesium and zinc offer effective solutions to combat this.

• Magnesium acts as a protective measure by being a sacrificial anode. In this role, magnesium corrodes in place of more critical metal infrastructures, halting their degradation.
• Zinc is extensively employed as a protective layer in a process called galvanization. This involves coating iron or steel with zinc to safeguard it against rust. The zinc layer corrodes over time, but in doing so, it protects the underlying metal by forming a barrier.

These applications of magnesium and zinc are pivotal in enhancing the longevity of metallic structures in various environments.

Amphoteric Oxides

Amphoteric substances have the unique ability to react both as acids and bases. This fascinating characteristic is present in some oxides, such as zinc oxide (ZnO).

• Zinc oxide can react with both acidic and basic substances. For example, it will react with hydrochloric acid to form zinc chloride and water, and with sodium hydroxide to form sodium zincate, exhibiting its dual nature.
• However, not all metal oxides share this characteristic. Magnesium oxide (MgO), for instance, is a basic oxide and does not exhibit amphoteric behavior. It typically reacts with acids but not bases.

Understanding which oxides are amphoteric is crucial for industrial and chemical applications, as it determines how they behave in different chemical environments.

Thermal Decomposition of Carbonates

The thermal decomposition of carbonates is a process where carbonate compounds break down upon heating, usually resulting in the formation of metal oxides and the release of carbon dioxide gas. This is a common reaction for many metal carbonates, including those of magnesium and zinc.

• When magnesium carbonate (MgCO₃) is heated, it decomposes to form magnesium oxide (MgO) and carbon dioxide (CO₂).
• Similarly, zinc carbonate (ZnCO₃), upon heating, breaks down into zinc oxide (ZnO) and CO₂.

This reaction is essential in various fields, from material science to manufacturing processes, where specific oxides are required for further application. Understanding the conditions and products of thermal decomposition helps in predicting and utilizing chemical behavior in practical applications.