Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following pairs of reactant does not form oxygen when they react with each other? (a) \(\mathrm{Cl}_{2}, \mathrm{NaOH}\) solution (cold, dilute) (b) \(\mathrm{F}_{2}, \mathrm{NaOH}\) solution (hot, concentrated) (c) \(\mathrm{F}_{2}, \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CaOCl}_{2}, \mathrm{H}_{2} \mathrm{SO}_{4}\) (dilute)

Step-by-Step Solution

Answer

(a) \(\mathrm{Cl}_2, \mathrm{NaOH}\) solution (cold, dilute) does not produce oxygen.

Step 1: Analyze the Reaction of Cl2 with NaOH

In cold, dilute conditions, chlorine reacts with sodium hydroxide to form sodium chloride, sodium hypochlorite, and water. The reaction is: \[ \text{Cl}_2 + 2\text{NaOH} \to \text{NaCl} + \text{NaOCl} + \text{H}_2\text{O} \]. Oxygen is not produced in this reaction.

Step 2: Analyze the Reaction of F2 with NaOH

Fluorine is highly reactive and reacts with hot, concentrated NaOH to produce oxygen along with sodium fluoride and water, according to the equation: \[ 2\text{F}_2 + 2\text{NaOH} \to 2\text{NaF} + \text{H}_2\text{O} + \text{O}_2 \]. Oxygen is produced in this reaction.

Step 3: Analyze the Reaction of F2 with H2O

Fluorine's reaction with water is highly exothermic and results in the production of oxygen, hydrofluoric acid, and heat: \[ 2\text{F}_2 + 2\text{H}_2\text{O} \to 4\text{HF} + \text{O}_2 \]. Oxygen is produced in this reaction.

Step 4: Analyze the Reaction of Ca(OCl)2 with H2SO4

When calcium hypochlorite reacts with dilute sulfuric acid, it does produce oxygen. The reaction is: \[ \text{Ca(OCl)}_2 + \text{H}_2\text{SO}_4 \to \text{CaSO}_4 + \text{HCl} + \text{O}_2 \]. However, oxygen is incorrectly thought to be produced here.

Key Concepts

Chemical Reactions

Chemical reactions involve the transformation of substances through breaking and forming of chemical bonds to create new products. They occur when reactants interact under suitable conditions. Often, these reactions can lead to the production of gases, liquids, or solids.
For high school chemistry, understanding the basics of reactants and products is essential. When predicting the outcome of a chemical reaction, it is vital to consider the nature of each element involved and the conditions affecting the reaction.
Consider temperature and concentration when analyzing a reaction. The nature of the reactants, such as their readiness to gain or lose electrons, directly influences products formed. For instance, in our exercise examples, we witness unique behavior based on conditions like whether a solution is hot or cold, or its concentration.

Chlorine and Sodium Hydroxide Reaction

The reaction between chlorine (
Cl_2
) and sodium hydroxide (
NaOH
) under cold, dilute conditions is a fascinating example of how altering conditions can change the expected outcomes. In this scenario, chlorine reacts with sodium hydroxide to form sodium chloride (
NaCl
), sodium hypochlorite (
NaOCl
), and water.
Notably, no oxygen is produced. This reaction is important industrially in producing household bleach and disinfectants. The absence of oxygen production might seem surprising, but this showcases chlorine's different oxidative states. It presents a valuable learning point about chemical behavior and product formation.
This reaction occurs via a disproportionation mechanism where chlorine has dual roles: it is both reduced to
NaCl
and oxidized to
NaOCl
, demonstrating different valencies of chlorine.

Fluorine Reactivity

Fluorine (
F_2
) is known for its extreme reactivity due to its high electronegativity, making it highly efficient in forming bonds. When
F_2
reacts with a hot, concentrated
NaOH
solution, it produces oxygen.
The reaction is:

• High temperature aids in breaking chemical bonds, making the reaction highly exothermic.
• It proceeds to form sodium fluoride
  NaF
  , water, and releases oxygen
  O_2
  .

Fluorine also reacts explosively with water. The outcome of the reaction with water is the production of hydrofluoric acid (
HF
) alongside oxygen, reflecting fluorine's ability to oxidize water.

• This reaction emphasizes the importance of reaction conditions, as even water can translate into a reactive environment.

Overall, fluorine’s reactivity showcases how element's properties and reaction conditions contribute significantly to the reaction path and products.

Calcium Hypochlorite Reaction

Calcium hypochlorite (
Ca(OCl)_2
) reacts with dilute sulfuric acid (
H_2SO_4
) in a unique way. This combination is typically used for bleaching and disinfecting, where calcium sulfate (
CaSO_4
), hydrochloric acid, and oxygen (
O_2
) are expected.

• This reaction sometimes has misconceptions about the oxygen production due to variations in conditions under which reactions take place.
• Further scrutiny of conditions could reveal missing information or erroneous assumptions.

It is an excellent reminder that expectations in reactions might not always meet the theoretical ideal, thus captivating the need for critical evaluation of each reaction's details. This aids students in embracing a detailed analysis, enhancing their problem-solving skills.