Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are correct here? (1) mercury (II) oxide is thermally unstable and readily decomposes into mercury and oxygen on being heated above \(400^{\circ} \mathrm{C}\) (2) mercury (II) oxide is thermally stable even at high temperatures. (3) mercury (II) sulphide is precipitated from \(\mathrm{Hg}^{2+}\) solution by passing \(\mathrm{H}_{2} \mathrm{~S}\) in the presence of high concentration of hydrogen ions. (4) mercury forms two types of oxides: \(\mathrm{HgO}\) and \(\mathrm{Hg}_{7} \mathrm{O}\) (a) 1,2 and 3 (b) 1,3 and 4 (c) 2,3 and 4 (d) 2 and 4 only

Step-by-Step Solution

Answer

No provided option matches only the correct statements.

Step 1: Analyzing Statement 1

The statement suggests that mercury (II) oxide, \(\text{HgO}\), decomposes into mercury (\(\text{Hg}\)) and oxygen (\(\text{O}_2\)) when heated above \(400^{\circ} \text{C}\). This is a known chemical reaction: \(2\text{HgO} \rightarrow 2\text{Hg} + \text{O}_2\). Thus, statement 1 is correct.

Step 2: Analyzing Statement 2

Statement 2 claims that mercury (II) oxide is thermally stable even at high temperatures. This contradicts the known behavior of \(\text{HgO}\), which is thermally unstable as it decomposes above \(400^{\circ} \text{C}\). Therefore, statement 2 is incorrect.

Step 3: Analyzing Statement 3

Statement 3 indicates that mercury (II) sulfide, \(\text{HgS}\), is precipitated from \(\text{Hg}^{2+}\) solution by adding hydrogen sulfide (\(\text{H}_2\text{S}\)) in the presence of high concentration of hydrogen ions. This is a true statement as \(\text{Hg}^{2+}\) can form \(\text{HgS}\) precipitate when sulfide ions are present. Hence, statement 3 is correct.

Step 4: Analyzing Statement 4

Statement 4 suggests that mercury forms two types of oxides: \(\text{HgO}\) and \(\text{Hg}_7\text{O}\). While \(\text{HgO}\) is a well-known oxide, \(\text{Hg}_7\text{O}\) is not a commonly recognized form of mercury oxide. Thus, statement 4 is considered incorrect.

Step 5: Evaluating Options Based on Correct Statements

Based on the analysis, statements 1 and 3 are correct, while statements 2 and 4 are incorrect. Evaluating the options provided: - Option (a) includes statements 1 and 3 but incorrectly includes 2 - Option (b) includes the correct statements 1 and 3, but incorrectly includes 4 - Option (c) includes incorrect statements 2 and 4 - Option (d) includes incorrect statement 2 Thus, none of the provided options completely match only the correct statements.

Key Concepts

Decomposition Reaction

A decomposition reaction involves breaking down a compound into its simpler constituents when certain conditions are met, such as heating. An example is the decomposition of mercury (II) oxide, \(\text{HgO}\), which breaks down into mercury (Hg) and oxygen (\(\text{O}_2\)) when heated above \(400^{\circ} \text{C}\). This reaction can be written as: \[2\text{HgO} \rightarrow 2\text{Hg} + \text{O}_2\]. Understanding why such reactions happen is key. It’s all about energy.

• Higher temperatures provide enough energy to overcome the bonds holding HgO together.
• Stability is achieved through simplification into mercury and oxygen.

You can observe this by the formation of a silvery element (mercury) and the release of the gas (oxygen). These types of reactions are quite common in chemistry, especially in thermal studies and material sciences.

Mercury Oxides

Mercury oxides are compounds composed of mercury and oxygen. The most well-known version is mercury (II) oxide, \(\text{HgO}\). It’s known for decomposing relatively easily when heated, as described in decomposition reactions.
Another oxide, sometimes mentioned, is \(\text{Hg}_7\text{O}\), although it’s not as commonly recognized or encountered in practical chemistry. Understanding mercury oxides involves grasping how mercury elements bond with oxygen:

• Mercury (II) oxide, \(\text{HgO}\), is red or yellow, depending on its preparation conditions.
• It represents a typical oxidation state of mercury in compounds.

Mercury oxides have a significant role in the science of thermal stability and play into oxidation-reduction reactions. Such reactions form part of larger discussions in inorganic chemistry, offering insights into elemental behavior and stability under thermal conditions.

Precipitation Reaction

Precipitation reactions involve two substances mixing in a solution, resulting in the formation of a new, solid compound. This is commonly observed when two aqueous solutions react to form an insoluble solid, or precipitate. With mercury compounds, mercury (II) sulfide, \(\text{HgS}\), forms through such a reaction.
A detailed look at this reaction process:

• Adding \(\text{H}_2\text{S}\) gas to a solution of \(\text{Hg}^{2+}\) ions results in \(\text{HgS}\), a black precipitate.
• This is effective even with high concentrations of hydrogen ions, illustrating selectivity in reactions due to sulfide ion affinity with \(\text{Hg}^{2+}\).

Precipitation reactions like these are fundamental in laboratory settings as a method of extracting or identifying particular ions or compounds from solutions.They also serve crucial roles in water treatment and purification processes.

Mercury Compounds Chemistry

Mercury compounds exhibit a wide range of chemical properties and behaviors. Known for their toxicity and unique properties, these compounds have significant historical, industrial, and scientific importance. Mercury forms various compounds with different elements, where its common oxidation states are +1 and +2.
When discussing mercury chemistry:

• \(\text{Hg}_2\text{Cl}_2\) (Calomel) is a well-known mercury (I) compound.
• \(\text{HgO}\) is a commonly discussed mercury (II) oxide.
• Other mercury compounds include \(\text{HgS}\), often recognized for its role in pigmentation and sulfide-based reactions.

Applications range from scientific research to historical uses in medicine and extraction of gold. Understanding mercury's chemistry involves acknowledging its complexities and the environmental concerns associated with its usage.