Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

A certain metal \(\mathrm{M}\) is used to prepare an antacid, which is used as a medicine for acidity. This metal accidently catches fire, which can not be extinguished by using \(\mathrm{CO}_{2}\) based extinguishers. The metal \(\mathrm{M}\) is (a) \(\mathrm{Mg}\) (b) \(\mathrm{Ba}\) (c) \(\mathrm{Be}\) (d) \(\mathrm{C}\)

Step-by-Step Solution

Answer

The metal \(\mathrm{M}\) is \(\mathrm{Mg}\) (magnesium).

Step 1: Identify Characteristics of the Metal

We are given that the metal used in the antacid is flammable and cannot be extinguished using carbon dioxide (\(\mathrm{CO}_2\)) based extinguishers. This is characteristic of certain metals. Specifically, some metals actively react with \(\mathrm{CO}_2\) under high temperatures.

Step 2: Consider the Chemistry of Antacids

Antacids are used to neutralize stomach acid. Common metals used in antacids include Group 2 elements such as magnesium (\(\mathrm{Mg}\)), barium (\(\mathrm{Ba}\)), and beryllium (\(\mathrm{Be}\)), though \(\mathrm{Be}\) is less common due to its toxicity. Carbon (\(\mathrm{C}\)) is not typically used in antacids.

Step 3: Analyze Reactivity with Carbon Dioxide

Among the options, \(\mathrm{Mg}\) is known for catching fire easily and forms magnesium oxide and carbon when it reacts with \(\mathrm{CO}_2\), making \(\mathrm{CO}_2\) ineffective in extinguishing magnesium fires. \(\mathrm{Ba}\) and \(\mathrm{Be}\), while reactive, are not typically involved in such vigorous reactions with \(\mathrm{CO}_2\). \(\mathrm{C}\) has different properties and does not behave this way in the presence of \(\mathrm{CO}_2\).

Step 4: Conclude with the Most Likely Metal

Based on its use in antacids, inability to be extinguished by Carbon dioxide extinguishers, and reactivity, the metal \(\mathrm{M}\) is \(\mathrm{Mg}\) (magnesium).

Key Concepts

Antacids

Antacids are substances that help neutralize stomach acid to relieve heartburn, indigestion, or upset stomach. They are usually compounds of bases that react with and neutralize gastric acid. Common antacids include:

• Magnesium hydroxide
• Calcium carbonate
• Sodium bicarbonate

Magnesium-based antacids, such as magnesium hydroxide, are popular due to their efficiency in neutralizing acid. They work by increasing the pH balance in the digestive tract, thus reducing acidity levels. However, in excess, they can sometimes cause diarrhea. Another noteworthy component, calcium carbonate, not only acts as an antacid but also provides calcium supplementation.

Reactivity with Carbon Dioxide

Some metals can react in unusual ways with carbon dioxide. This is especially critical for firefighting scenarios. When a fire involves certain reactive metals, using o2 as a fire extinguisher becomes ineffective or even dangerous. For example, magnesium reacts with carbon dioxide at high temperatures to form magnesium oxide and carbon. This reaction continues fueling the fire rather than extinguishing it. The general reaction can be described as:\[2 \, Mg + CO_2 \rightarrow 2 \, MgO + C\]Such reactions highlight the importance of understanding the chemical interactions involving o2 and metals. Knowing this helps in selecting the correct type of fire extinguisher in cases of metal fires.

Flammability of Magnesium

Magnesium, a light, silvery-white metal of the alkaline earth group, is known for its remarkable flammability. It can catch fire easily, especially in powdered or thinly shaved form. Once ignited, magnesium burns with a bright white flame at about 3100 °C (approximately 5600 °F). These characteristics make it difficult to extinguish using traditional fire suppressants like water or carbon dioxide. Attempting to use CO2 can be dangerous because the reaction will keep providing the magnesium fire with additional oxygen in the form of carbon dioxide, resulting in a persistent blaze. For extinguishing magnesium fires, specific dry powders or sand are recommended, which can smother the reaction effectively.

Properties of Group 2 Elements

Group 2 elements, known as alkaline earth metals, include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These elements share several distinct properties:

• They have two electrons in their outer shell.
• They exhibit higher melting points and boiling points compared to Group 1 metals.
• Though they are less reactive than alkali metals (Group 1), they can still form compounds such as oxides, hydroxides, and carbonates.
• They are typically shiny and conduct electricity well.

Of these, magnesium is particularly noted for its role in various industries, from aviation to medicine. Its application in antacids is due to its basicity, which helps in neutralizing excess acid in the stomach.