Describe the changes in molecular geometry and hybridization that take place during the following reactions: (a) \(\mathrm{BF}_{3}+\mathrm{F}^{-} \longrightarrow \mathrm{BF}_{4}^{-}\) (b) \(\mathrm{PCl}_{5}+\mathrm{Cl}^{-} \longrightarrow \mathrm{PCl}_{4}^{-}\) (c) \(\mathrm{ICl}_{3}+\mathrm{Cl}^{-} \longrightarrow \mathrm{ICl}_{4}^{-}\) (d) \(\mathrm{PCl}_{3}+\mathrm{Cl}_{2} \longrightarrow \mathrm{PCl}_{5}\) (e) \(\mathrm{C}_{2} \mathrm{H}_{2}+\mathrm{H}_{2} \longrightarrow \mathrm{C}_{2} \mathrm{H}_{4}\)

Chemical reactions are processes where reactants transform into products through the breaking and forming of chemical bonds. During these transformations, not only do the substances change, but the molecular geometry and hybridization of the molecules may also change.

This change occurs because atoms reorganize their electrons to achieve the most stable configuration. The addition or removal of atoms in a chemical reaction can cause a central atom to either gain or lose electron pairs, prompting a rehybridization to accommodate the new bonding situation.

When looking at textbook problems about chemical reactions and transformations, understanding the beginning and final structures can provide insight into the intermediate steps of bonding changes.

Hybridization describes the mixing of atomic orbitals in an atom to produce new hybrid orbitals. This concept explains the observed molecular geometries with respect to the valence bond theory. When molecules undergo chemical reactions, their hybridization can change in response to the different bonding environments.

For example, boron in BF₃ undergoes an increase in coordination number due to the addition of an F^- ion, leading to a change from an sp² to an sp³ hybridized state. This is because hybridization is intimately linked with molecular geometry; a change in one often requires a simultaneous change in the other.

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Transitions in molecular geometry are typically the result of a chemical reaction that alters the number or arrangement of bonding and non-bonding electron pairs around a central atom.

The addition of new atoms or ions can place different electronic demands on the central atom, leading to a reshaping of the molecule. For instance, in the case of PCl₅ reacting with Cl^-, a lone pair replaces a bonding pair and causes a modification in geometry from trigonal bipyramidal to see-saw. Understanding these transitions helps in predicting the properties and reactivity of the resulting molecules.

Valence bond theory provides a method to visualize the electronic structure of molecules. It posits that electrons reside in quantum mechanical orbitals localized on individual atoms and that bonds form from the overlap of these atomic orbitals sharing electron density.

As per this theory, hybridization is an adaptation to explain molecular shapes that couldn't be described by the use of pure atomic orbitals alone. A central atom's hybridization can change as it forms new bonds, in reaction to the differing numbers of bonding partners or to minimize electron pair repulsions. This theory is crucial when predicting the structure of molecules in chemical reactions.