Question

The pair(s) of reagents that yields paramagnetic species is/are (\Lambda) Na and excess \(\mathrm{NH}_{3}\). (B) \(\mathrm{K}\) and excess \(\mathrm{O}_{2}\). (C) Cu and dilute \(\mathrm{HNO}_{3}\). (D) \(\mathrm{O}_{2}\) and 2 -ethylanthraquinol.

Short answer

The pairs of reagents that yield paramagnetic species are (B) K and excess O2 and (C) Cu and dilute HNO3.

Step-by-step solution

Understanding the Concept of Paramagnetism

Paramagnetic species have one or more unpaired electrons which make them attracted to magnetic fields. The formation of paramagnetic species depends on the electronic configuration of the elements involved and the nature of the reagents.

Analyzing Each Pair of Reagents

We'll consider each pair of reagents and determine if they can yield a paramagnetic species:(A) Na reacts with excess NH3 to form sodium amide (NaNH2) which is not paramagnetic.(B) Potassium (K) reacts with excess oxygen (O2) to form superoxides like KO2, which contain the superoxide anion (O2-) with an unpaired electron, thus paramagnetic.(C) Cu reacts with dilute HNO3 to usually produce Cu(II) ions, which have an unpaired electron and are paramagnetic.(D) O2 reacts with 2-ethylanthraquinol to form hydrogen peroxide (H2O2) and anthraquinone, none of which are paramagnetic.

Identifying Paramagnetic Products

Based on the analysis in Step 2, the pairs of reagents that will produce paramagnetic species are:(B) K and excess O2, forming KO2 (a paramagnetic species), and(C) Cu and dilute HNO3, forming Cu(II) ions which are also paramagnetic.

Key concepts

Understanding Paramagnetism

Paramagnetism is a fascinating aspect of chemistry involving magnetic properties in certain materials. But what makes a species paramagnetic? The answer lies in the presence of unpaired electrons in the atomic or molecular orbitals. Atoms or molecules with unpaired electrons are not completely paired off with opposite spins, which allows them to interact with magnetic fields. When placed in a magnetic field, these unpaired electrons will align with the field, causing the species to be attracted to the magnet.

Let's take a closer look at the exercise provided. We are tasked to identify which pair(s) of reagents form paramagnetic species. The process requires us to examine the electronic configuration of the reaction products. For example, superoxides, such as those formed when potassium reacts with excess oxygen, are paramagnetic due to the presence of the superoxide anion \(O_2^-\), which contains an odd number of electrons and, hence, at least one unpaired electron. This unpaired electron responds to magnetic fields, confirming the paramagnetic nature of the species.

The Role of Unpaired Electrons

Unpaired electrons are the primary indicators of paramagnetism in a chemical species. These electrons exist due to an atom's or a molecule's electronic structure, often resulting from incompletely filled electron orbitals. In chemistry, the electronic configuration dictates the arrangement of electrons in atomic or molecular orbitals, and when these arrangements leave one or more electrons without an opposite paired electron, paramagnetism arises.

Regarding our exercise, we not only look at the initial elements but consider how they change upon reaction. For example, when copper (Cu) reacts with dilute nitric acid \(HNO_3\), it typically forms Cu(II) ions. Cu(II) ions have an electronic configuration that includes unfilled d-orbitals, specifically d9, resulting in unpaired electrons. Such unpaired electrons make Cu(II) ions paramagnetic. On the other hand, substances like sodium amide \(NaNH_2\) possess no such unpaired electrons and are therefore not paramagnetic.

Chemical Reactivity and Paramagnetism

Chemical reactivity plays a crucial role in the formation of paramagnetic species. It refers to the tendency of a chemical species to engage in a chemical reaction. The reactivity is governed by several factors, including the electronic configuration of the reactants and the nature of the chemical reagents involved. For instance, the reaction condition, whether a reagent is in excess or dilute, can greatly influence the type of product formed and if unpaired electrons will be present.

Reactivity and Formation of Paramagnetic Species

Returning to the exercise, we can examine how potassium's reactivity with excess oxygen leads to the formation of a superoxide, which is paramagnetic due to its reactive electronic structure. Likewise, copper's reactivity with dilute HNO3 produces Cu(II) ions, which are also paramagnetic. It's crucial to note that the specific conditions of the reagents, like 'excess' or 'dilute,' are keys to predicting the formation of paramagnetic species.