Question

In certain polar solvents, \(\mathrm{PCl}_{5}\) undergoes an ionization reaction in which a \(\mathrm{Cl}^{-}\) ion leaves one \(\mathrm{PCl}_{5}\) molecule and attaches itself to another. The products of the ionization are \(\mathrm{PCl}_{4}^{+}\) and \(\mathrm{PCl}_{6}^{-}\). Draw a sketch showing the changes in geometric shapes that occur in this ionization (that is, give the shapes of \(\mathrm{PCl}_{5}\), \(\mathrm{PCl}_{4}^{+},\) and \(\mathrm{PCl}_{6}^{-}\) ). $$2 \mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{4}^{+}+\mathrm{PCl}_{6}^{-}$$

Short answer

The molecular geometry changes from a trigonal bipyramid in PCl5 to a tetrahedron in PCl4+ and then to an octahedron in PCl6-

Step-by-step solution

Understanding the Structures

First, we need to know each atom has a certain number of valence electrons that constantly repel each other. This means they try to maintain as much distance as possible. We know that each Phosphorus(P) atom has 5 valence electrons and each Chlorine(Cl) has 7 valence electrons. We will use this to sketch the structures.

Sketching PCl5 geometry

PCl5 follows the trigonal bipyramid geometry. So, it has 5 Cl atoms surrounding one central P atom, with 2 in axial and 3 in equatorial positions forming a trigonal bipyramid.

Sketching PCl4+ geometry

One Cl atom is missing in PCl4+, meaning that it now has only 4 atoms surrounding the central P atom. This changes the molecular shape to a Tetrahedral geometry.

Sketching PCl6- geometry

One extra Cl atom gets added in PCl6-, making a total of 6 Cl atoms around the central P atom. This changes the molecular shape to an Octahedral geometry.

Key concepts

Molecular Geometry

Understanding molecular geometry is crucial for predicting and explaining the physical properties and chemical behavior of molecules. Each molecule has a specific three-dimensional structure, the geometric arrangement of its atoms in space, which is determined by the nature of its chemical bonds and the repulsion between electron pairs around the central atom.

Through the example of the ionization reaction involving \texttt{PCl}\(_5\), we observe significant changes in molecular geometry. Initially, \texttt{PCl}\(_5\) exhibits a trigonal bipyramidal structure due to the presence of five chlorine (Cl) atoms around a central phosphorus (P) atom. However, upon ionization, one chloride is removed which results in a \texttt{PCl}\(_4$$^+\) cation adopting a tetrahedral structure since it now has four surrounding Cl atoms.

Conversely, when a Cl\(^-\) ion bonds with another \texttt{PCl}\(_5\) molecule, the \texttt{PCl}\(_6$$^-\) forms, now with six Cl atoms around the central P, offering an octahedral geometry. It's fascinating how the removal or addition of a single chloride ion can significantly alter the entire shape of a molecule.

Valence Electrons

Valence electrons play a pivotal role in chemical bonding and molecule formation. These are the outermost electrons of an atom and are involved in forming bonds with other atoms. Phosphorus (P), situated in group 15 of the periodic table, possesses five valence electrons, whereas chlorine (Cl), located in group 17, has seven valence electrons.

In the case of \texttt{PCl}\(_5\), each Cl atom shares one of its valence electrons with P to form a covalent bond, resulting in the molecule's distinctive trigonal bipyramidal geometry. During the ionization reaction, as a Cl\(^-\) ion leaves one \texttt{PCl}\(_5\) molecule and attaches to another, the electronic structure and, consequently, the valence shell occupancy of P changes. Thus, \texttt{PCl}\(_4$$^+\) has one less electron around P, while \texttt{PCl}\(_6$$^-\) gains an additional electron. These changes in the distribution of valence electrons among the molecules directly influence the molecular geometry.

Ionization Reaction

An ionization reaction involves the formation of ions from molecules or neutral atoms. In the provided exercise, \texttt{PCl}\(_5\) undergoes ionization in a polar solvent, leading to the creation of the \texttt{PCl}\(_4$$^+\) cation and the \texttt{PCl}\(_6$$^-\) anion. This reaction demonstrates how the chemical environment can induce a molecule to lose or gain electrons, thereby altering its structure.

The process can be described by the equation: \begin{align*}2 \texttt{PCl}_5 &\rightleftharpoons \texttt{PCl}_4^+ + \texttt{PCl}_6^-.end{align*}This equilibrium reaction illustrates the dynamic nature of chemical reactions in solution. Whether forming or breaking chemical bonds, these changes have profound effects on the physical shape and chemical properties of the species involved. In summary, ionization reactions are not just about the gain or loss of electrons, but they also entail a fascinating reshaping of molecules that can be visualized geometrically.