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Acids and Bases - Lewis acids and bases (HL only)

Grade 12IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A Lewis acid is defined as an electron pair acceptor, while a Lewis base is defined as an electron pair donor.

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The reaction between a Lewis acid and a Lewis base results in the formation of a coordinate covalent bond (also known as a dative bond), where both electrons in the bond originate from the Lewis base.

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The Lewis theory is more inclusive than the Brønsted-Lowry theory. While all Brønsted-Lowry bases are Lewis bases, many Lewis acids, such as BF3BF_3, AlCl3AlCl_3, and SO3SO_3, do not involve H+H^+ ions and thus are not Brønsted-Lowry acids.

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Lewis acids are often species with incomplete octets (e.g., BF3BF_3 has only 6 valence electrons around BB) or transition metal cations (e.g., Cu2+Cu^{2+}) that can accept electron pairs into empty dd-orbitals.

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Lewis bases must possess at least one non-bonding pair of electrons (lone pair). Common examples include NH3NH_3, H2OH_2O, and Cl−Cl^-.

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In organic chemistry, Lewis acids are referred to as electrophiles (electron-seeking species), and Lewis bases are referred to as nucleophiles (nucleus-seeking species).

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Ligands in complex ion chemistry act as Lewis bases by donating electron pairs to the central metal ion, which acts as the Lewis acid.

📐Formulae

A+:B→A−BA + :B \rightarrow A-B

BF3+:NH3→F3B←NH3BF_3 + :NH_3 \rightarrow F_3B\leftarrow NH_3

Cu2++6H2O→[Cu(H2O)6]2+Cu^{2+} + 6H_2O \rightarrow [Cu(H_2O)_6]^{2+}

H++:OH−→H2OH^+ + :OH^- \rightarrow H_2O

💡Examples

Problem 1:

Identify the Lewis acid and Lewis base in the following reaction: AlCl3+Cl−→AlCl4−AlCl_3 + Cl^- \rightarrow AlCl_4^-.

Solution:

Lewis Acid: AlCl3AlCl_3; Lewis Base: Cl−Cl^-.

Explanation:

The aluminum atom in AlCl3AlCl_3 has an incomplete octet and accepts an electron pair from the chloride ion (Cl−Cl^-), which possesses four lone pairs of electrons. The Cl−Cl^- acts as the donor (base) and AlCl3AlCl_3 as the acceptor (acid).

Problem 2:

Explain why CO2CO_2 can act as a Lewis acid during its reaction with OH−OH^- to form HCO3−HCO_3^-.

Solution:

The carbon atom in CO2CO_2 accepts an electron pair from the OH−OH^- ion.

Explanation:

In CO2CO_2, the carbon atom is electron-deficient due to the electronegativity of the oxygen atoms. When OH−OH^- (the Lewis base) attacks the carbon atom, one of the C=OC=O π\pi-bonds breaks, allowing the carbon to accept the new electron pair from the hydroxide ion.

Problem 3:

In the formation of the complex ion [Fe(CN)6]4−[Fe(CN)_6]^{4-}, identify the roles of Fe2+Fe^{2+} and CN−CN^-.

Solution:

Fe2+Fe^{2+} is the Lewis acid and CN−CN^- is the Lewis base.

Explanation:

The transition metal cation Fe2+Fe^{2+} has empty orbitals that accept electron pairs from the lone pairs on the cyanide ligands (CN−CN^-). Therefore, Fe2+Fe^{2+} is the electron pair acceptor (acid) and CN−CN^- is the electron pair donor (base).

Lewis acids and bases (HL only) - Revision Notes & Key Formulas | IB Grade 12 Chemistry