Problem 78 - General Chemistry: Principles and Modern Applications

Question

The following very strong acids are formed by the reactions indicated: $$ \mathrm{HF}+\mathrm{SbF}_{5} \longrightarrow \mathrm{HSb} \mathrm{F}_{6} $$ (called "super acid," hexafluoroantimonic acid) $$ \mathrm{HF}+\mathrm{BF}_{3} \longrightarrow \mathrm{HBF}_{4} $$ (tetrafluoroboric acid) (a) Identify the Lewis acids and bases. (b) To which atom is the H atom bonded in each acid?

Step-by-Step Solution

Answer

(a) Lewis Acids: \( \mathrm{SbF}_{5} \), \( \mathrm{BF}_{3} \); Lewis Bases: \( \mathrm{HF} \) (b) In \( \mathrm{HSbF}_{6} \), H is bonded to Sb; in \( \mathrm{HBF}_{4} \), H is bonded to B.

Step 1: Identify Lewis Acids And Bases

In the given reactions, \( \mathrm{HF} \) is the Lewis base in both reactions as it donates an electron pair. In the first reaction, \( \mathrm{SbF}_{5} \) is the Lewis acid as it accepts an electron pair. In the second reaction, \( \mathrm{BF}_{3} \) is the Lewis acid as it also accepts an electron pair.

Step 2: Identify Atoms Bonded to Hydrogen

To find which atom is bonded to the H atom in each acid, analyze the molecular structure of the products from the initial reactions. In \( \mathrm{HSbF}_{6} \), the H atom is bonded to Sb atom. Similarly, in \( \mathrm{HBF}_{4} \), the H atom is bonded to the B atom.

Key Concepts

Superacids

Superacids are a fascinating category of acids that are stronger than 100% sulfuric acid. They have unusual properties, making them important in chemistry. Hexafluoroantimonic acid ( HSbF_6 ) is a classic example. It forms from a reaction between hydrofluoric acid ( HF ) and antimony pentafluoride ( SbF_5 ). These acids are potent due to their ability to donate protons exceedingly well. They are used in cases where a very strong proton donor is needed.
The strong acidity results primarily from their ability to stabilize the negative charge created when they give up a hydrogen ion ( H^+ ). This stabilization often involves a highly electronegative atom that strongly binds to the remaining structure.

• They aid in polymerization processes.
• Help in hydrocarbon cracking in the petrochemical industry.

Understanding superacids involves recognizing their capacity to be stronger than the usual strong acids encountered.

Lewis Acids

Lewis acids are substances that can accept a pair of electrons. In the equations provided, antimony pentafluoride ( SbF_5 ) and boron trifluoride ( BF_3 ) are excellent examples of Lewis acids.
They accept an electron pair from the molecule they react with. These species are electron-deficient, which drives their behavior as electron pair acceptors.
The structural characteristic of many Lewis acids, like BF_3 , is the presence of an incomplete octet, which makes them eager to accept electrons to attain stability.

• Lewis acids are different from Brønsted-Lowry acids, which donate protons.
• They often form complex structures through coordination with electron-rich species.

Lewis acids play crucial roles in many reactions where the sharing or transferring of electrons between entities occurs.

Lewis Bases

Lewis bases are substances that donate an electron pair. In the reactions given, hydrofluoric acid ( HF ) acts as a Lewis base. Though it's often seen as a weak acid, in terms of Lewis theory, HF effectively donates an electron pair to both SbF_5 and BF_3 . When HF donates electrons, it forms new, stable compounds as observed in the reactions.

• Lewis bases tend to have lone pairs of electrons.
• They are often nitrogen or oxygen-containing compounds, but other elements can also be bases.

The ability of a compound to donate electrons determines its efficacy as a Lewis base. This fundamental concept explains many reaction patterns and compound formations.

Molecular Structure Analysis

Molecular structure analysis involves understanding how atoms within a molecule are bonded. In examining the products HSbF_6 and HBF_4 , it becomes evident what atoms the hydrogen ( H ) is bonded to. It involves delving into the structural configuration of compounds formed post-reaction. For HSbF_6 , hydrogen is bonded to antimony ( Sb ), while in HBF_4 , it's attached to boron ( B ).
In these analyses:

• Predict structures using bonding theories.
• Highlight the significance of hybridization and geometry in determining bonding.

Molecular structure analysis is pivotal for predicting reactions and understanding physical properties. This knowledge gives insights into reaction mechanisms and stability of compounds.