Question

White phosphorus on reaction with \(\mathrm{NaOH}\) gives \(\mathrm{PH}_{3}\) as one of the products. This is a (A) dimerization reaction (B) disproportionation reaction (C) condensation reaction (D) precipitation reaction

Short answer

The reaction of white phosphorus with NaOH that produces PH3 as one of the products is a (B) disproportionation reaction.

Step-by-step solution

Understand the Types of Reactions

To solve the exercise, we need to understand what each type of reaction means. A dimerization reaction involves two identical molecules combining to form a larger molecule. A disproportionation reaction is a specific type of redox reaction in which an element in one oxidation state is simultaneously oxidized and reduced, forming at least two different products with the element in different oxidation states. A condensation reaction involves two molecules combining with the loss of a small molecule, often water. A precipitation reaction involves the formation of an insoluble solid from the reaction of two soluble substances.

Analyze the Given Reaction

Now, let's analyze the given reaction where white phosphorus reacts with NaOH to produce phosphine (PH3) and other products. In the reaction, phosphorus is present in two different oxidation states in the products: in PH3, it has a lower oxidation state, while in the other products (not specified), it may have a higher oxidation state. This indicates a transfer of electrons where the same element is being oxidized and reduced.

Identify the Reaction Type

From the analysis in step 2, it is clear that the same element, phosphorus, is undergoing both oxidation and reduction in one reaction. This fits the definition of a disproportionation reaction.

Key concepts

Redox Reaction

In chemistry, a redox reaction, short for reduction-oxidation reaction, is a type of chemical process involving the transfer of electrons between two species.

It encompasses two simultaneous sub-processes: oxidation, where an atom or molecule loses one or more electrons, and reduction, in which an atom or molecule gains electrons. To remember this, the phrase 'OIL RIG' can be helpful - Oxidation Is Loss, Reduction Is Gain.

During redox reactions, the oxidation states of atoms are changed. These reactions are integral to numerous processes, including cellular respiration, combusting fuels, and metallurgy. In the context of educational syllabi like the JEE Advanced, understanding redox reactions is crucial, not only for theoretical subjects but for practical applications and experimental understanding.

Consider a galvanic cell, where a spontaneous redox reaction generates an electric current, or corrosion, which is a redox reaction where metal is oxidized by oxygen and water. Recognizing these reactions and being able to balance them is a fundamental skill tested in advanced competitive exams like JEE.

Oxidation States

Oxidation states, also referred to as oxidation numbers, are a concept in chemistry that provide a hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic.

These numerical values serve as a bookkeeping method, helping us track the transfer of electrons during redox reactions. This is imperative because the conservation of charge is a key principle in chemistry - electrons are never created or destroyed, but simply transferred.

Understanding how to assign oxidation states to elements in compounds or ions is foundational for analyzing the process of redox reactions. In a molecule like water (H₂O), oxygen has an oxidation state of -2, while hydrogen has an oxidation state of +1. Any change in these numbers in a chemical reaction indicates that oxidation or reduction has occurred. For JEE Advanced aspirants, mastering the rules for assigning oxidation numbers is a must, as it directly ties into predicting the feasibility and direction of chemical reactions as well as balancing redox reactions.

Chemical Reactions in JEE Advanced

The Joint Entrance Examination (JEE) Advanced is notable for its challenging and comprehensive chemistry section, demanding a deep understanding of various chemical reactions, including disproportionation reactions.

In disproportionation reactions, a single element undergoes both oxidation and reduction, transforming into at least two different products with varying oxidation states. This type of reaction is particularly interesting and often appears in JEE Advanced chemistry problems because it encapsulates the complexity of redox processes.

To excel in JEE Advanced and similar competitive exams, students must have a thorough grasp of not just the basic types of chemical reactions like synthesis, decomposition, or combustion, but also more complex ones such as disproportionation. They are expected to identify the reaction type from the reactants and products, predict the outcome, and balance the overall reaction. The understanding of disproportionation reactions, similar to the exercise concerning white phosphorus and NaOH, is a testament to a student's proficiency in applying the principles of redox chemistry to solve intricate problems.