# 7.10: Strong and Weak Acids and Acid Ionization Constant $$\left( K_\text{a} \right)$$


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## Strong and Weak Acids and Acid Ionization Constant

Acids are classified as either strong or weak, based on their ionization in water. A strong acid is an acid which is completely ionized in an aqueous solution. Hydrogen chloride $$\left( \ce{HCl} \right)$$ ionizes completely into hydrogen ions and chloride ions in water.

$\ce{HCl} \left( g \right) \rightarrow \ce{H^+} \left( aq \right) + \ce{Cl^-} \left( aq \right)$

A weak acid is an acid that ionizes only slightly in an aqueous solution. Acetic acid (found in vinegar) is a very common weak acid. Its ionization is shown below.

$\ce{CH_3COOH} \left( aq \right) \rightleftharpoons \ce{H^+} \left( aq \right) + \ce{CH_3COO^-} \left( aq \right)$

The ionization of acetic acid is incomplete, and so the equation is shown with a double arrow. The extent of ionization of weak acids varies, but is generally less than $$10\%$$. A $$0.10 \: \text{M}$$ solution of acetic acid is only about $$1.3\%$$ ionized, meaning that the equilibrium strongly favors the reactants.

Weak acids, like strong acids, ionize to yield the $$\ce{H^+}$$ ion and a conjugate base. Because $$\ce{HCl}$$ is a strong acid, its conjugate base $$\left( \ce{Cl^-} \right)$$ is extremely weak. The chloride ion is incapable of accepting the $$\ce{H^+}$$ ion and becoming $$\ce{HCl}$$ again. In general, the stronger the acid, the weaker its conjugate base. Likewise, the weaker the acid, the stronger its conjugate base.

Acid Conjugate Base
Table $$\PageIndex{1}$$: Relative Strengths of Acids and their Conjugate Bases
Strong Acids
$$\ce{HCl}$$ (hydrochloric acid) (strongest) $$\ce{Cl^-}$$ (chloride ion) (weakest)
$$\ce{H_2SO_4}$$ (sulfuric acid) $$\ce{HSO_4^-}$$ (hydrogen sulfate ion)
$$\ce{HNO_3}$$ (nitric acid) $$\ce{NO_3^-}$$ (nitrate ion)
Weak Acids
$$\ce{H_3PO_4}$$ (phosphoric acid) $$\ce{H_2PO_4^-}$$ (dihydrogen phosphate ion)
$$\ce{CH_3COOH}$$ (acetic acid) $$\ce{CH_3COO^-}$$ (acetate ion)
$$\ce{H_2CO_3}$$ (carbonic acid) $$\ce{HCO_3^-}$$ (hydrogen carbonate ion)
$$\ce{HCN}$$ (hydrocyanic acid) (weakest) $$\ce{CN^-}$$ (cyanide ion) (strongest)

Strong acids are $$100\%$$ ionized in solution. Weak acids are only slightly ionized. Phosphoric acid is stronger than acetic acid and so is ionized to a greater extent. Acetic acid is stronger than carbonic acid, and so on.

## The Acid Ionization Constant, $$K_\text{a}$$

The ionization for a general weak acid, $$\ce{HA}$$, can be written as follows:

$\ce{HA} \left( aq \right) \rightarrow \ce{H^+} \left( aq \right) + \ce{A^-} \left( aq \right)$

Because the acid is weak, an equilibrium expression can be written. An acid ionization constant $$\left( K_\text{a} \right)$$ is the equilibrium constant for the ionization of an acid.

$K_\text{a} = \frac{\left[ \ce{H^+} \right] \left[ \ce{A^-} \right]}{\left[ \ce{HA} \right]}$

The acid ionization represents the fraction of the original acid that has been ionized in solution. Therefore, the numerical value of $$K_\text{a}$$ is a reflection of the strength of the acid. Weak acids with relatively higher $$K_\text{a}$$ values are stronger than acids with relatively lower $$K_\text{a}$$ values. Because strong acids are essentially $$100\%$$ ionized, the concentration of the acid in the denominator is nearly zero and the $$K_\text{a}$$ value approaches infinity. For this reason, $$K_\text{a}$$ values are generally reported for weak acids only.

The table below is a listing of acid ionization constants for several acids. Note that polyprotic acids have a distinct ionization constant for each ionization step, with each successive ionization constant being smaller than the previous one.

Name of Acid Ionization Equation $$K_\text{a}$$ p$$K_\text{a}$$
Table $$\PageIndex{2}$$: Acid Ionization Constants at $$25^\text{o} \text{C}$$
Sulfuric acid

$$\ce{H_2SO_4} \rightleftharpoons \ce{H^+} + \ce{HSO_4^-}$$

$$\ce{HSO_4} \rightleftharpoons \ce{H^+} + \ce{SO_4^{2-}}$$

very large

$$1.3 \times 10^{-2}$$

pKa1 >>>>1

pKa2 =1.89

Oxalic acid

$$\ce{H_2C_2O_4} \rightleftharpoons \ce{H^+} + \ce{HC_2O_4^-}$$

$$\ce{HC_2O_4} \rightleftharpoons \ce{H^+} + \ce{C_2O_4^{2-}}$$

$$6.5 \times 10^{-2}$$

$$6.1 \times 10^{-5}$$

pKa1 = 1.18

pKa2 = 4.21

Phosphoric acid

$$\ce{H_3PO_4} \rightleftharpoons \ce{H^+} + \ce{H_2PO_4^-}$$

$$\ce{H_2PO_4^-} \rightleftharpoons \ce{H^+} + \ce{HPO_4^{2--}}$$

$$\ce{HPO_4^{2-}} \rightleftharpoons \ce{H^+} + \ce{PO_4^{3-}}$$

$$7.5 \times 10^{-3}$$

$$6.2 \times 10^{-8}$$

$$4.8 \times 10^{-13}$$

pKa1 = 2.12

pKa2 = 7.20

pKa3 = 12.32

Hydrofluoric acid $$\ce{HF} \rightleftharpoons \ce{H^+} + \ce{F^-}$$ $$7.1 \times 10^{-4}$$

pK= 3.15

Nitrous acid $$\ce{HNO_2} \rightleftharpoons \ce{H^+} + \ce{NO_2^-}$$ $$4.5 \times 10^{-4}$$ pK= 3.35
Benzoic acid $$\ce{C_6H_5COOH} \rightleftharpoons \ce{H^+} + \ce{C_6H_5COO^-}$$ $$6.5 \times 10^{-5}$$ pK= 4.18
Acetic acid $$\ce{CH_3COOH} \rightleftharpoons \ce{H^+} + \ce{CH_3COO^-}$$ $$1.8 \times 10^{-5}$$ pK= 4.74
Carbonic acid

$$\ce{H_2CO_3} \rightleftharpoons \ce{H^+} + \ce{HCO_3^-}$$

$$\ce{HCO_3^-} \rightleftharpoons \ce{H^+} + \ce{CO_3^{2-}}$$

$$4.2 \times 10^{-7}$$

$$4.8 \times 10^{-11}$$

pKa1 = 6.40

pKa2 = 10.32

Hydrocyanic acid $$\ce{HCN} \rightleftharpoons \ce{H^+} + \ce{CN^-}$$ $$4.9 \times 10^{-10}$$ pK= 9.31

## pKa values

Because Ka values are usually very small numbers, sometimes chemists prefer to work with pKa values, that is:

pKa = -log Ka

Table $$\PageIndex{2}$$ shows the pKa values for several bases. Notice that the stronger the acid, the higher the Ka values but the lower its pKa

## Summary

• Strong and weak acids are defined.
• The acid ionization constant $$\left( K_\text{a} \right)$$ and pKa are defined.
• The stronger the acid, the higher the Ka values but the lower its pKa.

7.10: Strong and Weak Acids and Acid Ionization Constant $$\left( K_\text{a} \right)$$ is shared under a CC BY-NC license and was authored, remixed, and/or curated by LibreTexts.