Acid-Base Intro.

Acids and Bases

Bronsted-Lowry Definitions

• A Bronsted-Lowry acid is a proton donor.

• A Bronsted-Lowry base is a proton acceptor.

Let's review what a proton is:

• A neutral hydrogen atom (most common isotope) contains:

  • One proton in the nucleus

  • One electron outside the nucleus

• Removing the electron leaves only the proton (the nucleus of a hydrogen atom), which is equivalent to H+.

When dealing with Bronsted-Lowry definitions, think about one proton (H+) difference between the acid and base.

Example: HCl + H2O

1. HCl donates a proton to water.

2. A lone pair of electrons on the oxygen in water picks up the proton. The electron from hydrogen is left behind and goes to the chlorine.

3. Products:

   • Hydronium ion (H3O+): The oxygen is now bonded to three hydrogens, giving it a +1 formal charge. The two electrons from the oxygen form the new bond with the proton.

   • Chloride anion (Cl−): Chlorine gains an additional lone pair, giving it a negative charge.

4. Therefore:

   • HCl is a Bronsted-Lowry acid because it donates a proton.

   • Water is a Bronsted-Lowry base because it accepts a proton.

Conjugate Acid-Base Pairs

Let's think about the possibility of the reverse reaction in the previous example.

• The chloride anion (Cl−) would have to function as a base and pick up a proton from the hydronium ion (H3O+).

• Cl− is the conjugate base to HCl.

• H3O+ functions as an acid. This would be the conjugate acid to water.

Definition of Conjugate Pairs

Examples of Conjugate Pairs

🧲 Lewis Definitions

• A Lewis acid is an electron pair acceptor.

  • A good way to remember this is with the mnemonic device Acid and electron pair Acceptor

• A Lewis base is an electron pair donor.

  • A good way to remember this is with the mnemonic device Lewis Base, if you flip the B you get a D for electron pair Donor

Example: Boron Trifluoride (BF3BF3​) + Water (H2OH2​O)

1. Boron in BF3 does not have an octet (only six electrons around it).

2. Boron is sp^2 hybridized, meaning it has an empty p orbital that can accept a pair of electrons. Therefore, BF3 functions as a Lewis acid.

3. Water (H2O) has a lone pair of electrons on the oxygen that it can donate. Therefore, water functions as a Lewis base.

4. The lone pair of electrons on oxygen is donated into the empty p orbital of boron, forming a bond between the oxygen and the boron.

5. This gives the oxygen a +1 formal charge and the boron a -1 formal charge.

6. There is no H+ transfer in this reaction, so the Bronsted-Lowry definition does not apply.

Lewis Definitions are More Broad

The Lewis definition is broader than the Bronsted-Lowry definition and can be applied to reactions where there is no H+ transfer.

• Water can be a Lewis base: Water can donate a pair of electrons.

• HCl can be a source of a Lewis acid: The proton (H+) accepts a pair of electrons.