Problem 130 - A Complete Resource Book in Chemistry for JEE Main

Question

A greenish yellow gas reacts with an alkali metal hydroxide to form a halate which can be used in fire works and safety matches. The gas and halate respectively are (a) \(\mathrm{Br}_{2}, \mathrm{KBrO}_{3}\) (b) \(\mathrm{Cl}_{2}, \mathrm{KClO}_{3}\) (c) \(\mathrm{I}_{2}, \mathrm{NaIO}_{3}\) (d) \(\mathrm{Cl}_{2}^{2}, \mathrm{NaClO}_{3}\)

Step-by-Step Solution

Answer

The gas is \(\mathrm{Cl}_{2}\) and the halate is \(\mathrm{KClO}_{3}\) from option (b).

Step 1: Identify the Characters Involved

First, let's understand the key components involved in the reaction: a greenish yellow gas and an alkali metal hydroxide that form a halate. A halate is an ion with the formula \( ext{XO}_3^-\) where \(X\) is a halogen.

Step 2: Recognize the Greenish Yellow Gas

Among the given options, chlorine gas \( (\mathrm{Cl}_{2}) \) is known for its greenish yellow appearance. Bromine is reddish-brown, and iodine is violet; hence, \(\mathrm{Cl}_{2}\) is the correct gas that fits the description.

Step 3: Understand the Chemical Reaction

The reaction between chlorine gas \(\mathrm{Cl}_{2}\) and an alkali metal hydroxide such as potassium hydroxide \(\mathrm{KOH}\) forms potassium chlorate \(\mathrm{KClO}_{3}\), among other products. This corresponds to a chlorate ion and is a known use in fireworks and safety matches.

Step 4: Identify the Option

Of the options presented, \(\mathrm{Cl}_{2}\) and \(\mathrm{KClO}_{3}\) from option (b) represent the greenish yellow gas and halate required by the question's criteria, aligning with known characteristics and chemical reactions.

Key Concepts

Chlorine Gas

Chlorine gas, with the chemical symbol \(\mathrm{Cl}_2\), is one of the most recognizable members of the halogen family. This gas is easily identified due to its distinct greenish-yellow color. Chlorine is widely used in various industries, including water purification and the production of household cleaning products.
Chlorine gas is highly reactive because it is a molecular halogen. This means it can readily combine with other elements and compounds. When chlorine gas is introduced to alkali metal hydroxides, such as sodium hydroxide \(\mathrm{NaOH}\) or potassium hydroxide \(\mathrm{KOH}\), they undergo a chemical reaction to form various chlorinated compounds, one of which is potassium chlorate.
It's essential to handle chlorine gas with care, as it is toxic and can cause irritation to the respiratory system. Despite its dangers, chlorine is extremely useful in multiple applications, making it a valuable chemical in many sectors.

Potassium Chlorate

Potassium chlorate, described by the chemical formula \(\mathrm{KClO}_3\), is a chemical compound that results from the reaction between chlorine gas and potassium hydroxide. This compound belongs to a category known as chlorates. Chlorates have the general formula \(\mathrm{XO}_3^-\), where \(X\) is typically a halogen.
Potassium chlorate is particularly significant in the realm of pyrotechnics. Due to its ability to release oxygen when decomposed, \(\mathrm{KClO}_3\) acts as a potent oxidizing agent. As an oxidizer, it provides a source of oxygen that enables combustion, a critical component in igniting fireworks and propelling them into the sky.
Additionally, potassium chlorate is used in safety matches. When the match head, often coated with \(\mathrm{KClO}_3\), is struck against a rough surface, the friction activates the chemical reaction needed to ignite the match. This showcases the versatility of \(\mathrm{KClO}_3\) in various explosive and flammable environments.

Halates in Fireworks

Halates, a group of chemical compounds, include substances like potassium chlorate \(\mathrm{KClO}_3\). These compounds are crucial in the manufacturing of fireworks due to their strong oxidizing properties. An oxidizing agent works by providing the oxygen needed for the combustion of other substances, which is a critical factor in the explosiveness and visual effects of fireworks.
In pyrotechnics, the visual display of fireworks relies heavily on the chemical reactions fueled by halates. Halates help in sustaining the combustion once the fireworks are ignited. The additional oxygen released by the decomposition of halates intensifies the burning of the firework's combustible materials, allowing for bright and vivid displays in the night sky.
The production of different colored lights in fireworks is also often dependent on the halates used. By combining halates with various metal salts, different colors can be produced. For example, strontium salts produce red colors, and copper compounds create blue hues. Halates play a critical role in ensuring that these vibrant colors are achieved.