1. Copyright © Cengage Learning. All rights reserved.15 | 1 Brønsted–Lowry Concept of Acids and Bases The Brønsted–Lowry concept expands the compounds that can be considered acids and bases. It defines acids and bases based on what happens in a reaction. Any reaction involving H + (proton) that transfers from one molecule to another is an acid–base reaction, regardless of whether it occurs in aqueous solution or if there is OH − present. All reactions that fit the Arrhenius definition also fit the Brønsted–Lowry definition.

2. Copyright © Cengage Learning. All rights reserved.15 | 2 Brønsted–Lowry acid –the species donating a proton in a proton- transfer reaction; it is a proton donor. –Any material that has H can potentially be a Brønsted–Lowry acid. Brønsted–Lowry base –the species accepting a proton in a proton- transfer reaction; it is a proton acceptor.

3. Copyright © Cengage Learning. All rights reserved.15 | 3 Conjugate Acid–Base pair –Substances in the acid–base reaction that differ by the gain or loss of a proton, H +. The acid is called the conjugate acid of the base. The base is called a conjugate base of the acid. Acid BaseConjugate acid Conjugate base

4. Copyright © Cengage Learning. All rights reserved.15 | 4 Brønsted–Lowry Acids When HCl dissolves in water, the HCl is the acid because HCl transfers an H + to H 2 O, forming H 3 O + ions. –Water acts as base, accepting H +. When NH 3 dissolves in water, the NH 3 (aq) is the base because NH 3 accepts an H + from H 2 O, forming OH – (aq). –Water acts as acid, donating H +. HCl(aq) + H 2 O(l) → Cl – (aq) + H 3 O + (aq) acidbase

5. Copyright © Cengage Learning. All rights reserved.15 | 5 Amphoteric Substances Amphoteric substances can act as either an acid or a base because they have both a transferable H and an atom with lone pair electrons. Water acts as base, accepting H + from HCl. HCl(aq) + H 2 O(l) → Cl – (aq) + H 3 O + (aq) Water acts as acid, donating H + to NH 3. NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + OH – (aq)

6. Copyright © Cengage Learning. All rights reserved.15 | 6 What is the conjugate acid of H 2 O? What is the conjugate base of H 2 O? The conjugate acid of H 2 O has gained a proton. It is H 3 O +. The conjugate base of H 2 O has lost a proton. It is OH -.

7. Copyright © Cengage Learning. All rights reserved.15 | 7 Label each species as an acid or base. Identify the conjugate acid-base pairs. a.HCO 3 - (aq) + HF (aq) H 2 CO 3(aq) + F - (aq) b.HCO 3 - (aq) + OH - (aq) CO 3 2- (aq) + H 2 O (l) BaseAcid Conjugate base Conjugate acid AcidBase Conjugate acid Conjugate base

8. Copyright © Cengage Learning. All rights reserved.15 | 8 Conjugate Acid–Base Pairs

9. Copyright © Cengage Learning. All rights reserved.15 | 9 Brønsted-Lowry Concept of Acids and Bases In the Brønsted-Lowry concept: 1.A base is a species that accepts protons; OH - is only one example of a base. 2.Acids and bases can be ions as well as molecular substances. 3.Acid-base reactions are not restricted to aqueous solution. 4.Some species can act as either acids or bases depending on what the other reactant is.