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Difficulty: MediumDefinitions and Theories of Acids and Bases
Consider the chemical reaction represented by the equation:
BF3+NH3→F3B:NH3BF_3 + NH_3 \rightarrow F_3B:NH_3
Which of the following statements correctly describes the role of BF3BF_3 in this reaction according to acid-base theories?
  1. It acts as a Lewis acid by accepting an electron pair.Answer
  2. B
    It acts as a Brønsted-Lowry acid by donating a proton.
  3. C
    It acts as a Lewis base by donating an electron pair.
  4. D
    It acts as an Arrhenius acid by producing hydrogen ions in aqueous solution.

Answer

Boron trifluoride (BF3BF_3) acts as a Lewis acid because it accepts an electron pair from ammonia (NH3NH_3).
Boron trifluoride (BF3BF_3) has a central boron atom surrounded by six valence electrons, leaving an empty orbital. In the reaction, it accepts an unshared electron pair from nitrogen in ammonia (NH3NH_3) to form a dative covalent bond. By definition, an electron-pair acceptor is a Lewis acid.

Step-by-Step Solution

1
Analyze the electronic structure of the reactants
Boron in BF3BF_3 has 6 valence electrons (an incomplete octet) and an empty p-orbital, whereas nitrogen in NH3NH_3 has a lone pair of non-bonding electrons.
Determining electron pair availability is necessary to apply the Lewis acid-base definition.
2
Identify the type of bond formed during the reaction
Nitrogen donates its lone pair into the empty orbital of boron to form a coordinate (dative) covalent bond (F3B:NH3F_3B:NH_3).
According to the Lewis concept, an electron-pair acceptor is an acid and an electron-pair donor is a base.
3
Evaluate the applicable acid-base theory
Because no proton (H+H^+) transfer occurs and no aqueous H+H^+ ions are produced, neither Brønsted-Lowry nor Arrhenius definitions apply, making it strictly a Lewis acid-base reaction.
Lewis theory extends acid-base chemistry to reactions occurring without hydrogen or solvent participation.

Key Concept

Lewis Theory of Acids and Bases
Estimated Time:1m 0s
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Topics

Definitions and Theories of Acids and Bases

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