M15Q1: Arrhenius and Bronsted-Lowry Acids and Bases

Learning Objectives

  • Identify Brønsted-Lowry acids and bases and their conjugate bases and acids.
  • Identify substances that are amphiprotic.
    | Amphiprotic |
  • Write balanced chemical equations for aqueous solutions of acids or bases.

| Key Concepts and SummaryGlossary | End of Section Exercises |

Many acids and bases are found in our everyday lives, from the food that we eat to the chemicals we clean with. For example, coffee and soda are acidic solutions, while milk and vegetables are more basic. Vinegar is an acidic cleaning solution, while bleach is a basic solution. The properties of the molecules in these foods and solutions give rise to many interesting properties:

Acids Bases
Sour taste Bitter taste
Dissolve many metals Feel slippery (soapy)
Neutralized by bases Neutralized by acids
Change the color of indicator dyes Change the color of indicator dyes

In an earlier chapter on chemical reactions, we defined acids and bases as Carl Axel Arrhenius did: We identified an acid as a compound that dissolves in water to yield hydronium (H3O+) ions and a base as a compound that dissolves in water to yield hydroxide ions (OH). However, this definition is somewhat limited.

Later, we extended the definition of an acid or a base using the more general definition proposed in 1923 by the Danish chemist Johannes Brønsted and the English chemist Thomas Lowry. Their definition centers on the proton, H+. A proton is what remains when a normal hydrogen atom loses an electron. A compound that donates a proton to another compound is called a Brønsted-Lowry acid, and a compound that accepts a proton is called a Brønsted-Lowry base. An acid-base reaction is the transfer of a proton from a proton donor (acid) to a proton acceptor (base). In a subsequent chapter of this text we will introduce the most general model of acid-base behavior introduced by the American chemist G. N. Lewis.

Common Acids and Bases

Acids may be compounds such as HCl or H2SO4, organic acids like acetic acid (CH3COOH) or ascorbic acid (vitamin C), or H2O. Anions (such as HSO4, H2PO4, HS, and HCO3) and cations (such as H3O+, NH4+, and [Al(H2O)6]3+) may also act as acids.

Bases fall into the same three categories. Bases may be neutral molecules (such as H2O, NH3, and CH3NH2), anions (such as OH, HS, HCO3, CO32, F, and PO43-), or cations (such as [Al(H2O)5OH]2+]). The most familiar bases are ionic compounds such as NaOH and Ca(OH)2, which contain the hydroxide ion, OH. The hydroxide ion in these compounds accepts a proton from acids to form water:

H+ + OH → H2O

When an acid molecule donates a proton, the resulting product is referred to as the acid’s conjugate base. This species is a base because it can accept a proton (to re-form the acid):

acid  +  H2O   ⇌   hydronium ion  +  conjugate base

HF  +  H2O   ⇌   H3O+  +  F

H2SO4  +  H2O   ⇌   H3O+  +  HSO4

HSO4  +  H2O   ⇌   H3O+  +  SO4-2

NH4+  +  H2O   ⇌   H3O+  +  NH3

H2O  +  H2O   ⇌   H3O+  +  OH

Similarly, when a base accepts a proton, the resulting product is referred to as the base’s conjugate acid. This species is an acid because it can give up a proton (and thus re-form the base):

base  +  H2O   ⇌   hydroxide ion  +  conjugate acid

NH3  +  H2O   ⇌   OH  +  NH4+

S2-  +  H2O   ⇌   OH  +  HS

CO32-  +  H2O   ⇌   OH  +  HCO3

F  +  H2O   ⇌   OH  +  HF

H2O  +  H2O   ⇌   OH  +  H3O+

OH  +  H2O   ⇌   OH  +  H2O

Amphiprotic Species

Like water, many molecules and ions may either gain or lose a proton under the appropriate conditions. Such species are said to be amphiprotic. Another term used to describe such species is amphoteric, which is a more general term for a species that may act either as an acid or a base by any definition (not just the Brønsted-Lowry one). Consider for example the bicarbonate ion, which may either donate or accept a proton as shown here:

HCO3(aq)  +  H2O(l)   ⇌   CO32-(aq)  +  H3O+(aq)
HCO3(aq)  +  H2O(l)   ⇌   H2CO3(aq)  +  OH(aq)

Example 1

Representing the Acid-Base Behavior of an Amphiprotic Substance
Write separate equations representing the reaction of \text{HSO}_3^{\;\;-}

  1. as an acid with OH
  2. as a base with HI

Solution

  1. HSO3(aq)  +  OH(aq)   ⇌   SO32-(aq)  +  H2O(l)
  2. HSO3(aq)  +  HI(aq)   ⇌   H2SO3(aq)  +  I(aq)

Check Your Learning
Write separate equations representing the reaction of H2PO4

  1. as a base with HBr
  2. as an acid with OH

Answer:

(a) H2PO4(aq)  +  HBr(aq)   ⇌   H3PO4(aq)  +  Br(aq)

(b) H2PO4(aq)  +  OH(aq)   ⇌   HPO42-(aq)  +  H2O(l)

Key Concepts and Summary

After learning about equilibrium reactions, we are going to dive into one specific type of equilibrium reactions – acid-base reactions. We can use the products formed during a reaction to identify whether a molecule is an acid or a base. The definition of Arrhenius acids and bases are the most limited, but in this course, we will use the Brøntsed-Lowry acid and base definition most often, where an acid donates a proton and a base accepts a proton. After the acid donates a proton, it forms the conjugate base. After a base accepts a proton, it forms the conjugate acid.

Glossary

amphiprotic (amphoteric)
a molecule or ion that can either gain or lose a proton (H+)

Arrhenius acid
a compound that produces H3O+ when dissolved in water

Arrhenius base
a compound that produces OH when dissolved in water

Brønsted-Lowry acid
a compound that donates a proton (H+)

Brønsted-Lowry base
a compound that accepts a proton (H+)

conjugate acid
the resulting product after a base accepts a proton (H+)

conjugate base
the resulting product after an acid donates a proton (H+)

Chemistry End of Section Exercises

  1. In the following chemical equations, identify the acid, base, conjugate acid and conjugate base:
    1. C6H5COOH(aq) + CN(aq) ⇌ C6H5COO(aq) + HCN(aq)
    2. F(aq) + HNO2(aq) ⇌ NO2(aq) + HF(aq)
    3. H2O(l) + HN3(aq) ⇌ H3O+(aq) + N3(aq)
    4. CH3COO(aq) + H2O(l) ⇌ OH(aq) + CH3COOH(aq)
    5. HPO32-(aq) + H2O(l) ⇌ H2PO3(aq) OH(aq)
    6. HSO3(aq) + H2O(l) ⇌ SO32-(aq) + H3O+(aq)
  2. What is the conjugate acid of HTeO3?
  3. What is the conjugate base of HSeO4?
  4. Identify which of the following substances are amphiprotic: HC2O4, CO32-, HSO4, H2O, HCOOH, H2PO4.

Answers to Chemistry End of Section Exercises

  1. (a) acid = C6H5COOH, base = CN, conjugate acid = HCN, conjugate base = C6H5COO;
    (b) acid = HNO2, base = F, conjugate acid = HF, conjugate base = NO2;
    (c) acid = HN3, base = H2O, conjugate acid = H3O+, conjugate base = N3;
    (d) acid = H2O, base = CH3COO, conjugate acid = CH3COOH, conjugate base = OH;
    (e) acid = H2O, base = HPO32-, conjugate acid = H2PO3, conjugate base = OH;
    (f) acid = HSO3, base = H2O, conjugate acid = H3O+, conjugate base = SO32-
  2. H2TeO3
  3. SeO42-
  4. HC2O4, HSO4, H2O, H2PO4
Comments
Please use this form to report any inconsistencies, errors, or other things you would like to change about this page. We appreciate your comments. 🙂

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Chem 103/104 Resource Book by crlandis and Chem 104 Textbook Team is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.