9.7 Strong and Weak Bases

Defining Base Strength

The strength of a base is defined by its ability to accept a proton ( $H^{+}$ ) from a solvent, typically water. The more readily a base accepts a proton, the stronger it is.

Strong bases are substances that ionize completely in an aqueous solution to produce hydroxide ions ( $O H^{-}$ ). They are typically hydroxides of alkali metals (Group 1) and alkaline earth metals (Group 2).

Examples of strong bases include:

Sodium Hydroxide ( $N a O H$ ): $N a O H_{(a q)} \to N a_{(a q)}^{+} + O H_{(a q)}^{-}$
Potassium Hydroxide ( $K O H$ ): $K O H_{(a q)} \to K_{(a q)}^{+} + O H_{(a q)}^{-}$

The hydroxide ion ( $O H^{-}$ ) produced is a strong Brønsted-Lowry base because it readily accepts a proton to form water: $O H_{(a q)}^{-} + H_{(a q)}^{+} \to H_{2} O_{(l)}$

Weak Bases and Equilibrium

Weak bases do not ionize completely in water. Instead, they establish an equilibrium where only a fraction of the base molecules accept a proton from water.

For a generic weak base, B, the equilibrium reaction in water is: $B_{(a q)} + H_{2} O_{(l)} ⇌ B H_{(a q)}^{+} + O H_{(a q)}^{-}$

The Base Ionization Constant ( $K_{b}$ )

The extent of ionization for a weak base is quantified by the base ionization constant ( $K_{b}$ ). It is the equilibrium constant for the reaction of the base with water.

The expression for $K_{b}$ is: $K_{b} = \frac{[ B H ^{+} ] [ O H ^{-} ]}{[ B ]}$

A large $K_{b}$ value indicates a high degree of ionization, meaning the base is relatively strong.
A small $K_{b}$ value indicates a low degree of ionization, meaning the base is weak.

The $p K_{b}$ Value

For convenience, the strength of a base is often expressed using the $p K_{b}$ , which is the negative logarithm of the base ionization constant. $p K_{b} = - lo g K_{b}$

A small $p K_{b}$ value corresponds to a large $K_{b}$ , indicating a stronger base.
A large $p K_{b}$ value corresponds to a small $K_{b}$ , indicating a weaker base.

$K_{b}$ and $p K_{b}$ Values of Some Common Bases

The table below shows the $K_{b}$ and $p K_{b}$ values for several common bases at standard conditions.

Name of Base	Formula	$K_{b}$	$p K_{b}$
Diethylamine	$(C_{2} H_{5})_{2} NH$	$9.6 \times 1 0^{- 4}$	3.02
Ethylamine	$C_{2} H_{5} NH_{2}$	$5.6 \times 1 0^{- 4}$	3.25
Methylamine	$CH_{3} NH_{2}$	$4.5 \times 1 0^{- 4}$	3.34
Ammonia	$NH_{3}$	$1.7 \times 1 0^{- 5}$	4.76
Pyridine	$C_{5} H_{5} N$	$5.6 \times 1 0^{- 9}$	8.25
Aniline	$C_{6} H_{5} NH_{2}$	$4.3 \times 1 0^{- 10}$	9.37

Analysis:

Based on the $p K_{b}$ values, the order of base strength from strongest to weakest is: Diethylamine > Ethylamine > Methylamine > Ammonia > Pyridine > Aniline.
Diethylamine is the strongest base listed in the table (smallest $p K_{b}$ ).
Aniline is the weakest base listed (largest $p K_{b}$ ).

Name of Base

Formula

K_{b}

p K_{b}

Diethylamine

(C_{2} H_{5})_{2} NH

9.6 \times 1 0^{- 4}

3.02

Ethylamine

C_{2} H_{5} NH_{2}

5.6 \times 1 0^{- 4}

3.25

Methylamine

CH_{3} NH_{2}

4.5 \times 1 0^{- 4}

3.34

Ammonia

NH_{3}

1.7 \times 1 0^{- 5}

4.76

Pyridine

C_{5} H_{5} N

5.6 \times 1 0^{- 9}

8.25

Aniline

C_{6} H_{5} NH_{2}

4.3 \times 1 0^{- 10}

9.37

Defining Base Strength

Strong Bases

Weak Bases and Equilibrium

The Base Ionization Constant ( $K_{b}$ )

The $p K_{b}$ Value

$K_{b}$ and $p K_{b}$ Values of Some Common Bases

9.7 Strong and Weak Bases

Defining Base Strength

Strong Bases

Weak Bases and Equilibrium

The Base Ionization Constant ( $K_{b}$ )

The $p K_{b}$ Value

$K_{b}$ and $p K_{b}$ Values of Some Common Bases