DP IB Chemistry: HL

Revision Notes

Syllabus Edition

First teaching 2014

Last exams 2024

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18.1.1 Lewis Theory

Lewis Theory of Acids & Bases

  • A more general definition of acids and bases was given by G.N. Lewis who defined them as:
    • A Lewis acid is an electron pair acceptor
    • A Lewis base is an electron pair donor

Lewis acid and Lewis base general, downloadable IB Chemistry revision notes

General mechanism for Lewis acids and bases

  • This enabled a wider range of substances to be classed as acids or bases
  • This can be shown in the following examples in which a hydroxide ion, OH-, and ammonia, NH3, donate a pair of electrons to a hydrogen ion
18-1-lewis-acid-and-lewis-base-example

The OH- ion and ammonia act as Lewis bases in both examples by donating an electron pair

Brønsted-Lowry Acids and Bases

  • Brønsted-Lowry acid is a species that can donate a proton
    • For example, hydrogen chloride (HCl) is a Brønsted-Lowry acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl) ion

HCl (aq) → H+ (aq) + Cl– (aq)

  • A Brønsted-Lowry base is a species that can accept a proton
    • For example, a hydroxide (OH) ion is a Brønsted-Lowry base as it can accept a proton to form water

OH (aq) + H+ (aq) → H2O (l)

Weak acids dissociating

  • In an equilibrium reaction, the products are formed at the same rate as the reactants are used
  • This means that at equilibrium, both reactants and products are present in the solution
  • For example, ethanoic acid (CH3COOH) is a weak acid that partially dissociates in solution
  • When equilibrium is established there are CH3COOH, H2O, CH3COO and H3Oions present in the solution

Lewis Acid, Lewis Base, Brønsted-Lowry acid or Brønsted-Lowry base

A point to consider when thinking about Lewis acids and bases as well as Brønsted-Lowry acids and bases is the donating and accepting of protons

  • Brønsted-Lowry acid and base theory considers acids as proton donors only
  • This does not of course occur in every reaction
  • For example:
    • The lone pair on the nitrogen atom in ammonia, NH3 , can be donated to the boron atom in boron trifluoride, BF3, creating a molecule of NH3BF3
    • In this case, neither compound reacts as an Brønsted-Lowry acid or Brønsted-Lowry base as no protons (H+ ions) are being donated or accepted
    • Only electron pairs are being donated and accepted

NH3BF3 example, downloadable IB Chemistry revision notes

Ammonia donates a lone pair of electrons to form a coordinate bond 

  • Here boron forms three sp2 hybridised orbitals leaving a vacant 2pz orbital which allows the lone pair on the nitrogen atom to form a dative covalent bond

Hybridisation of boron, downloadable IB Chemistry revision notes

Hybridisation of the boron atom 

  • The following molecules can behave as either Lewis bases and Brønsted-Lowry bases
    • Lewis bases as they can donate an electron pair
    • Brønsted-Lowry base as they can accept a proton

Bronsted and Lewis examples, downloadable IB Chemistry revision notes

Examples of molecules that can behave both as Lewis bases and Brønsted-Lowry base

Identifying Lewis Acids & Bases

  • In the case of a complex ion, such as hexaaquacopper(II), the water molecule is acting as a Lewis base and the metal ion is acting as a Lewis acid
    • Copper(II), like other transition metals, can form a complex due to a partially occupied d subshell
    • Cu2+ (aq) + 6H2O (l) → [Cu(H2O)6]2+ (aq)

Copper complex, downloadable IB Chemistry revision notes

Hexaaquacopper(II) complex 

  • Given that Lewis acids can accept a pair of electrons, they can be classed as electrophiles
    • An electrophile is a electron-deficient species that can accept a lone pair from a nucleophile, in the same way the Cu2+ ion accepts electron pairs from water

  • The cyanide ion, -CN , water, H2O , ammonia , NH3 , are examples of Lewis bases and they can also act as nucleophiles
    • Nucleophiles are electron rich species with at least on pair of lone electrons

Worked example

Identify the Lewis acid and Lewis base in the following reaction

Worked example methanoate ion, downloadable IB Chemistry revision notes

Methanoate ion reacting with water

Answer

    • The Lewis acid is water, H2O
      • The hydrogen in the water molecule is accepting a pair of electrons leaving an OH- ion

    • The Lewis base is the methanoate ion, HCOO-
      • The lone pair of electrons in the methanoate ion forms a coordinate bond with one of the hydrogens from the water molecule

We have seen previously that water can act as a Brønsted-Lowry acid or base, so it should be no surprise that water can act as both a Lewis acid or base depending on how it is interacting with other species

 

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