Acids and bases are classifications of chemical compounds based on reactivity properties. The goal is to be able to predict the reaction products when chemical species are brought together.
The nature of an acid/base reaction is definition dependent: we define what an acid is, what a base is, and what the acid/base reaction is. Then, working within the given definition, we hope to be able to identify an acid and a base and, if they are mixed together, we want to be able to predict the products of the reaction and the extent of reactivity.
We will discuss two different acid/base definitions: Brønsted–Lowry and Lewis (briefly).
Acid: a substance that can donate a hydrogen ion (H+ donor).
Base: a substance that can accept a hydrogen ion (H+ acceptor).
Acid/Base reaction: transfer of a hydrogen ion (H+ transfer).
This definition is not solvent restricted or phase restricted but we will primarily focus on the chemistry of aqueous solutions. The largest restriction on this definition is the exclusive focus on the H+ ion.
To account for equilibrium reactions, we define two new terms:
Conjugate acid: the H+ donor written on the products side of the reaction.
Conjugate base: the H+ acceptor written on the products side of the reaction.
The autoionization of water is a Brønsted–Lowry acid/base reaction:
H2O(l) + H2O(l) → ← H3O+(aq) + OH–(aq)
H–Ö:| H ← H–Ö: | H H–Ö–H+ | H :Ö:– | H
Base Acid Conjugate Acid Conjugate Base
Water acts as both the acid and the base.
The hydronium ion (H3O+) is the conjugate acid and hydroxide ion (OH–) is the conjugate base
Our goal in the course is to considerably improve our understanding of Brønsted–Lowry acids and bases.
The autoionization of water is an equilibrium so we can write a mass action expression:
Kc = [H3O+]e[OH–]e = Kw
Kw is the equilibrium constant for the autoionization of water.
At 25 °C, Kw = 1.0×10–14
Memorize this forever!
If [H3O+] > [OH–] then the solution is referred to as acidic.
If [H3O+] < [OH–] then the solution is referred to as basic.
If [H3O+] = [OH–] then the solution is referred to as neutral.
Kw allows us to quantitatively determine the amount of one ion if we know the concentration of the other.
The hydronium ion concentration in vinegar is typically around 1×10–3 M. What is the hydroxide ion concentration in vinegar? Is vinegar acidic or basic?
Use Kw: [OH–] = Kw/[H3O+] = Kw = 1.0×10–14/1×10–3 = 1×10–11 M
The solution is acidic since the hydronium ion concentration is larger than the hydroxide ion concentration.
The range of hydronium ion or hydroxide ion concentrations is large, typically from 10–1 to 10–14 M. All these exponents are annoying, so we have defined a scale that measures hydronium ion concentration in terms of a smaller range of numbers. This is the pH scale:
pH = –log[H3O+]
In pure water:
[H3O+] = 1.0×10–7 so the pH is
pH = –log[H3O+] = –log(1.0×10–7) = 7.00
Values of the pH below 7 indicate an excess of hydronium ion over hydroxide ion. This is referred to as an acidic solution.
Values of the pH greater than 7 indicate an excess of hydroxide ion over hydronium ion. This is referred to as a basic solution.
Equivalently, we can define pOH:
pOH = –log[OH–]
At 25 °C, pH and pOH are related by
pH + pOH = 14.00 (only at 25 °C)