Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The INCORRECT statement is :[Main April 12, 2019 (II)] (a) Lithium is the strongest reducing agentamong the alkali metals. (b) Lithium is least reactive with water among the alkali metals. (c) LiNO, decomposes on heating to give \(\mathrm{LiNO}_{2}\) and \(\mathrm{O}_{2}\). (d) LiCl crystallises from aqueous solution as \(\mathrm{LiCl} \cdot 2 \mathrm{H}_{2} \mathrm{O}\).

Step-by-Step Solution

Answer

Statements (c) and (d) are incorrect, with the main error being in (c).

Step 1: Understanding the Question

We need to identify the incorrect statement among the four given options about lithium and its properties related to alkali metals.

Step 2: Evaluating Statement (a)

Statement (a) claims that lithium is the strongest reducing agent among alkali metals. This is true because lithium, due to its high charge density and small atomic size, has a tendency to lose electrons easily, making it a strong reducing agent.

Step 3: Evaluating Statement (b)

Statement (b) says lithium is the least reactive with water among the alkali metals. This is correct because compared to other alkali metals like sodium and potassium, lithium reacts more slowly with water.

Step 4: Evaluating Statement (c)

Statement (c) claims that lithium nitrate decomposes to form lithium nitrite and oxygen when heated. This is incorrect because when \( \text{LiNO}_3 \) is heated, it decomposes to form \( \text{Li}_2\text{O} \), \( \text{NO}_2 \), and \( \text{O}_2 \).

Step 5: Evaluating Statement (d)

Statement (d) describes LiCl crystallizing as a dihydrate \( \text{LiCl} \cdot 2\text{H}_2\text{O} \). This statement is actually false because lithium chloride crystallizes from aqueous solution as \( \text{LiCl} \cdot \text{H}_2\text{O} \), not \( \text{LiCl} \cdot 2\text{H}_2\text{O} \).

Key Concepts

Alkali Metals

Alkali metals, including lithium, potassium, and sodium, have some unique properties. They are located in Group 1 of the periodic table. These metals are very reactive and are known for their shiny appearance and softness. The reactivity of alkali metals increases as you move down the group.

• Lithium, for instance, is the least dense and least reactive with water compared to other alkali metals. It still reacts, but just at a slower pace.
• They all tend to form compounds by losing their single valence electron, which leads to strong ionic bonds.
• When handled incorrectly, especially with water, they can be dangerous due to their explosive reactions.

These characteristics make alkali metals unique and interesting for various applications, from batteries to medicine.

Reducing Agent

A reducing agent is a substance that loses electrons in a chemical reaction, allowing other substances to gain those electrons. This is known as reduction. Lithium stands out among alkali metals as a very effective reducing agent.

• The small atomic size and high charge density of lithium help it lose electrons easily.
• This property makes lithium particularly valuable in reactions where electron transfer is necessary.
• For example, lithium is used in organic reactions and in the manufacture of many products due to its strong reducing abilities.

It is fascinating how lithium's properties as a reducing agent help it facilitate diverse chemical transformations.

Crystal Structure

Crystal structure refers to the unique 3D arrangement of atoms or molecules within a crystalline solid. Lithium chloride (LiCl) forms crystals when dissolved in water and re-precipitated.

• This crystallization process involves water molecules, often resulting in a hydrate form.
• A common structure for such solids is a cubic arrangement, offering high symmetry and consistent spacing of particles.
• It's important to note that LiCl does not crystallize with two water molecules as previously stated, but rather with one, forming LiCl·H₂O.

Understanding the crystal structure is essential in fields like material science and chemistry, where the arrangement impacts physical properties and behaviors.

Thermal Decomposition

Thermal decomposition is the process where a chemical compound breaks down into simpler substances due to heating. For lithium compounds like lithium nitrate (LiNO₃), this process can yield interesting products.

• Typically, upon heating, lithium nitrate decomposes to produce lithium oxide (Li₂O), nitrogen dioxide (NO₂), and oxygen (O₂).
• This type of decomposition is an essential reaction mechanism in both industrial and laboratory settings.
• Understanding the accurate products of thermal decomposition helps chemists predict and control reactions effectively.

Accurate knowledge of such processes ensures the safe handling and use of lithium compounds in various applications.