Lewis Acids & Bases (HL) (DP IB Chemistry): Revision Note

Lewis Acids & Bases

• A more general definition of acids and bases was given by G.N. Lewis:

  • A Lewis acid is an lone pair acceptor

  • A Lewis base is an lone pair donor

• This enabled a wider range of substances to be classed as acids or bases, beyond those that donate or accept protons

Lewis acid and base interactions

• The interaction between a Lewis acid and base can be shown as:

A⁺ + :B^- → A←:B

• A coordinate covalent bond forms when the Lewis base donates a lone pair of electrons to the Lewis acid

• Lewis theory is broader than Brønsted–Lowry theory because it includes reactions that do not involve proton transfer

• Some species, such as OH^- and NH₃, act as both types of base:

  • As Lewis bases they donate a lone pair of electrons

  • As Brønsted–Lowry bases they accept a proton

Diagram to show how OH^– and ammonia act as Lewis bases

Comparing Lewis and Brønsted–Lowry acid-base theory

• A Brønsted-Lowry acid is a species that can donate H⁺

• For example, hydrogen chloride (HCl) is a Brønsted-Lowry acid as it can donate a H⁺ ion

HCl (aq) → H⁺ (aq) + Cl^– (aq)

• Lewis acids are any species that are able to accept a lone pair of electrons

  • This includes H⁺

• This means Lewis theory covers a broader spectrum than Brønsted–Lowry acids

• Brønsted-Lowry theory strictly defines acids as H⁺ donors only

  • Not all acid–base reactions involve proton transfer

  • This is where Lewis theory is more broadly applicable

• A Brønsted-Lowry base is a species that can accept H⁺

  • For example, a hydroxide (OH^–) ion is a Brønsted-Lowry base as it can accept H⁺ to form water

H⁺ (aq) + OH^- (aq) → H₂O (l)

• Lewis bases and Brønsted-Lowry bases are in the same group of chemicals because they both must have a lone pair of electrons 

  • Lewis bases donate a lone pair of electrons

  • Brønsted-Lowry bases accept a proton vis their lone pair of electrons

• Examples of chemicals that can act as Lewis bases and Brønsted-Lowry bases include:

  • Hydroxide, OH^-

  • Cyanide, CN^-

  • Methylamine, CH₃NH₂