Problem 77 - Chemistry: Atoms First

Question

Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each: (a) \(\mathrm{CS}_{2}+\mathrm{SH}^{-} \longrightarrow \mathrm{HCS}_{3}^{-}\) (b) \(\mathrm{BF}_{3}+\mathrm{F}^{-} \longrightarrow \mathrm{BF}_{4}^{-}\) (c) \(\mathrm{I}^{-}+\mathrm{SnI}_{2} \longrightarrow \mathrm{SnI}_{3}^{-}\) (d) \(\mathrm{Al}(\mathrm{OH})_{3}+\mathrm{OH}^{-} \longrightarrow \mathrm{Al}(\mathrm{OH})_{4}^{-}\) (e) \(\mathrm{F}^{-}+\mathrm{SO}_{3} \longrightarrow \mathrm{SFO}_{3}^{-}\)

Step-by-Step Solution

Answer

(a) CS2 is the Lewis acid and SH- is the Lewis base. (b) BF3 is the Lewis acid and F- is the Lewis base. (c) SnI2 is the Lewis acid and I- is the Lewis base. (d) Al(OH)3 is the Lewis acid and OH- is the Lewis base. (e) SO3 is the Lewis acid and F- is the Lewis base.

Step 1: Write Lewis Structures for Reactants and Products - (a)

Draw Lewis structures for CS2 showing a central C atom double bonded to two S atoms. For SH-, draw an S atom with 6 valence electrons and a single bond to an H atom with an extra lone pair representing the negative charge. For HCS3-, draw a central S atom with double bonds to two other S atoms and a single bond to an H atom, with an extra lone pair on the central S representing the negative charge.

Step 2: Identify Lewis Acid and Base - (a)

The Lewis acid is CS2 because it accepts a pair of electrons from the Lewis base, SH-, to form a bond in the product HCS3-.

Step 3: Write Lewis Structures for Reactants and Products - (b)

For BF3, draw a B atom with three single bonds to three F atoms. F- is simply an F atom with an extra lone pair representing the negative charge. In BF4-, B is the central atom with four single bonds to four F atoms and an extra lone pair on the B atom.

Step 4: Identify Lewis Acid and Base - (b)

BF3 is the Lewis acid as it accepts an electron pair from the Lewis base, F-, forming BF4-.

Step 5: Write Lewis Structures for Reactants and Products - (c)

For SnI2, draw a Sn atom with single bonds to two I atoms, and I- as an I atom with an extra lone pair. In SnI3-, Sn is central with three single bonds to three I atoms, and an extra pair of electrons represented by a negative charge.

Step 6: Identify Lewis Acid and Base - (c)

SnI2 is the Lewis acid because it accepts an electron pair from I-, the Lewis base, to form SnI3-.

Step 7: Write Lewis Structures for Reactants and Products - (d)

For Al(OH)3, draw an Al atom with three single bonds to three OH groups. OH- is an O atom with a single bond to an H atom and three lone pairs including the negative charge. In Al(OH)4-, Al is central with four single bonds to four OH groups and carries an extra lone pair as a negative charge.

Step 8: Identify Lewis Acid and Base - (d)

Al(OH)3 acts as the Lewis acid because it accepts an electron pair from the Lewis base, OH-, to form Al(OH)4-.

Step 9: Write Lewis Structures for Reactants and Products - (e)

For SO3, draw an S atom with double bonds to three O atoms. F- is an F atom with three lone pairs and an extra lone pair representing the negative charge. In SFO3-, S is central, double bonded to two O atoms, single bonded to an F atom, and single bonded to one O atom, which carries the extra negative charge.

Step 10: Identify Lewis Acid and Base - (e)

SO3 is the Lewis acid as it accepts an electron pair from the Lewis base F-, leading to the formation of SFO3-.

Key Concepts

Chemical Bonding

Understanding chemical bonding is fundamental to grasp how atoms combine and interact with each other to form molecules. Atoms bond together by sharing or transferring valence electrons, and this interaction is what dictates the structure and properties of all matter.

There are three main types of chemical bonds:

• Ionic Bonds: These occur when electrons are transferred from one atom to another, leading to the formation of positively and negatively charged ions.
• Covalent Bonds: Atoms can share pairs of electrons in covalent bonds. These can be either polar, where electrons are unequally shared, or nonpolar, where electrons are equally shared.
• Metallic Bonds: Metals bond by sharing many electrons collectively across all the atoms, creating a sea of electrons.

For instance, a compound like CS₂ involves covalent bonding where the carbon atom shares electrons with sulfur atoms to achieve a full valence shell. Similarly, molecules like BF₃ and Al(OH)₃ participate in reactions by forming covalent bonds with other atoms or ions. Understanding these bonding mechanisms allows for the prediction of compound properties and the reactions they may undergo.

Lewis Acid and Base

The Lewis acid-base concept is an essential theory in chemistry that extends the definitions of acids and bases beyond protons and hydroxide ions. A Lewis acid is a species that can accept an electron pair, while a Lewis base is a species that can donate an electron pair.

For example, in the reaction of CS₂ with SH^-, CS₂ is the Lewis acid because it accepts an electron pair, while SH^- is the Lewis base as it donates an electron pair. Analyzing reactions in terms of Lewis acid-base interactions helps chemists to understand how molecules react and how to manipulate reactions to obtain desired products.

Identifying Lewis Acids and Bases:

• Look for a compound or ion that has an incomplete octet or is capable of expanding its octet - this is likely the Lewis acid.
• Seek out negatively charged ions or neutral molecules with lone pairs of electrons - these are potential Lewis bases.

This concept also explains the behavior of molecules like BF₃ and SO₃, which act as Lewis acids by accepting electron pairs from bases such as fluoride ions (F^-) or oxygen in hydroxide ions (OH^-).

Electron Pair Acceptance

The concept of electron pair acceptance is integral to the definition of a Lewis acid. When a species accepts an electron pair, it often completes its valence shell, making it more stable. For instance, BF₃ accepts an electron pair from F^- because boron in BF₃ has only six valence electrons and by accepting more it can achieve an octet.

The electron pair acceptance phenomenon is crucial when predicting the outcome of chemical reactions. A good electron acceptor (Lewis acid) will have vacant orbitals or be able to expand its valence shell. They are often found in the middle of reactions, stabilizing transition states and intermediates, which is essential for reaction mechanisms.

Moreover, this principle does not just apply to isolated cases; it is a recurring theme in chemistry. In each of the given reactions, from CS₂ and SO₃ to Al(OH)₃, the chemical that accepts an electron pair plays a pivotal role in the direction and completion of the chemical process.