6.4.4Additional lessons
Lewis Acids and Bases — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 152 of 172
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Part 1 of 4The Lewis Definitions
Lewis theory focuses on electron pairs instead of protons.
Lewis acid
- A Lewis acid is a species that accepts a pair of electrons to form a covalent bond.
Lewis base
- A Lewis base is a species that donates a pair of electrons to form a covalent bond.
Wider scope
- The theory includes reactions with no protons, such as BF₃ combining with NH₃.
Dative bond
- The new bond is a coordinate bond, because both electrons come from the base.
Part 2 of 4Examples of Each
1
Ammonia has a lone pair on nitrogen, so it is a Lewis base and also a Brønsted-Lowry base.
2
The hydroxide ion donates a lone pair, so it is both a Lewis base and a nucleophile.
3
Boron trifluoride has an empty orbital on boron, so it is a Lewis acid.
4
The proton H⁺ has an empty 1s orbital, so it is the simplest Lewis acid.
5
Metal ions such as Cu²⁺ and Fe³⁺ accept electron pairs from water or ammonia, so they are Lewis acids.
Part 3 of 4Linking the Theories
Linking the Theories
Brønsted-Lowry acid
- Every Brønsted-Lowry acid that donates H⁺ is a source of a Lewis acid, because H⁺ accepts an electron pair.
Brønsted-Lowry base
- Every Brønsted-Lowry base has a lone pair, so it is also a Lewis base.
Nucleophile
- A nucleophile is a Lewis base, since it donates an electron pair to an electron-poor centre.
Electrophile
- An electrophile is a Lewis acid, since it accepts an electron pair from a nucleophile.
| Theory | Acid | Base |
|---|---|---|
| Brønsted-Lowry | Proton donor | Proton acceptor |
| Lewis | Electron pair acceptor | Electron pair donor |
Part 4 of 4Exam Checklist & Tips
6.4.4 – Key Points for the Exam
- Lewis acid An electron-pair acceptor.
- Lewis base An electron-pair donor.
- Coordinate bond A covalent bond in which both electrons come from one atom.
- Lone pair A pair of valence electrons not involved in bonding.
Common Mistakes
- Thinking Lewis acids must contain hydrogen.
- Confusing the roles, saying that a Lewis base accepts electron pairs.
- Forgetting that every nucleophile is a Lewis base.
- Assuming Lewis theory conflicts with Brønsted-Lowry theory, when it is broader.
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