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Understanding Lewis Acids and Bases

Explore Lewis Acids and Bases in Chemistry 1011 with insights on identifying reactions and substances, bonding in complex ions, coordination numbers, ligands, and the Lewis model. Learn the difference from Bronsted-Lowry model and examples of Lewis acid-base reactions.

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Understanding Lewis Acids and Bases

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  1. Chemistry 1011 TOPIC Acids and Bases TEXT REFERENCE Masterton and Hurley Chapter 4.2 (Review), 13, 14.1, 15.1 (page 427), 21.2 (page589) Chemistry 1011 Slot 5

  2. 15.1/21.2 Lewis Acids and Bases YOU ARE EXPECTED TO BE ABLE TO: • Identify Lewis acid-base reactions • Identify substances that can behave as Lewis acids or bases (Textbook pages 427 and 589) Chemistry 1011 Slot 5

  3. Complex Ions • Transition metal ions in crystals and in solution are bonded to water molecules • Copper II Sulfate consists of Cu(H2O)42+ ions and SO42- ions • The copper ion is the central metal ion • The water molecules are ligands • The number of atoms bonded to the central metal ion is the coordination number – in this case the coordination number is 4 Chemistry 1011 Slot 5

  4. Other Ligands • If ammonia solution is added to an aqueous solution of copper II sulfate, ammonia molecules will replace the water ligands Cu(H2O)42+(aq) + 4NH3(aq) Cu(NH3)42+(aq) + 4 H2O(l) • Ammonia molecules are now the ligands. The coordination number remains 4 Chemistry 1011 Slot 5

  5. Bonding in Complex Ions • When complex ions are formed, the ligands bond to the central atom. The electrons required for the bonds come from the ligands • In Cu(H2O)42+ or Cu(NH3)42+ a lone pair of electrons from the water or ammonia molecule is shared with the central atom, which uses unfilled d-orbitals to form a bond • Any molecule or ion that has an unshared pair of electrons can donate them to a metal ion to form a covalent bond Chemistry 1011 Slot 5

  6. Lewis Acids and Bases • The Lewis model of acids and bases is an extension of the Bronsted-Lowry model • The Bronsted-Lowry model sees acid-base reactions as the exchange of hydrogen ions • The Lewis model sees acid-base reactions in terms of the giving or receiving of electron pairs • A species acts as a Lewis acid when it accepts a pair of electrons • A species acts as a Lewis base when it donates a pair of electrons Chemistry 1011 Slot 5

  7. Lewis Acids and Bases • Bronsted-Lowry bases will be Lewis bases: • The base can accept a proton because of the availability of an electron pair NH3(aq) + H2O(aq)NH4+(aq) + OH-(aq) • In complex ion formation: • Ligands such as NH3 and H2O are acting as Lewis bases • The central metal ion acts as a Lewis acid Chemistry 1011 Slot 5

  8. Lewis Acids and Bases • Substances such as transition metal ions and electron deficient molecules (eg BF3) can act as Lewis acids but will not be Bronsted-Lowry acids • In the reaction between NH3 and BF3 :NH3 + BF3 H3N:BF3 Base Acid • NH3 is the electron pair donor • BF3 is the electron pair acceptor Chemistry 1011 Slot 5

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