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Weak base

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In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As Brønsted–Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH compared to strong bases. Bases range from a pH of greater than 7 (7 is neutral, like pure water) to 14 (though some bases are greater than 14). pH has the formula:

Contents

pH = − log 10 ⁡ [ H + ]

Since bases are proton acceptors, the base receives a hydrogen ion from water, H2O, and the remaining H+ concentration in the solution determines pH. Weak bases will have a higher H+ concentration because they are less completely protonated than stronger bases and, therefore, more hydrogen ions remain in the solution. If you plug in a higher H+ concentration into the formula, a low pH results. However, pH of bases is usually calculated using the OH− concentration to find the pOH first. This is done because the H+ concentration is not a part of the reaction, while the OH− concentration is.

pOH = − log 10 ⁡ [ OH − ]

By multiplying a conjugate acid (such as NH4+) and a conjugate base (such as NH3) the following is given:

K a × K b = [ H 3 O + ] [ N H 3 ] [ N H 4 + ] × [ N H 4 + ] [ O H − ] [ N H 3 ] = [ H 3 O + ] [ O H − ]

Since K w = [ H 3 O + ] [ O H − ] then, K a × K b = K w

By taking logarithms of both sides of the equation, the following is reached:

l o g K a + l o g K b = l o g K w

Finally, multiplying throughout the equation by -1, the equation turns into:

p K a + p K b = p K w = 14.00

After acquiring pOH from the previous pOH formula, pH can be calculated using the formula pH = pKw - pOH where pKw = 14.00.

Weak bases exist in chemical equilibrium much in the same way as weak acids do, with a base dissociation constant (Kb) indicating the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up:

K b = [ N H 4 + ] [ O H − ] [ N H 3 ]

Bases that have a large Kb will ionize more completely and are thus stronger bases. As stated above, pH of the solution depends on the H+ concentration, which is related to the OH− concentration by the self-ionization constant (Kw = 1.0x10−14). A strong base has a lower H+ concentration because they are fully protonated and less hydrogen ions remain in the solution. A lower H+ concentration also means a higher OH− concentration and therefore, a larger Kb.


NaOH (s) (sodium hydroxide) is a stronger base than (CH3CH2)2NH (l) (diethylamine) which is a stronger base than NH3 (g) (ammonia). As the bases get weaker, the smaller the Kb values become.

Percentage protonated

As seen above, the strength of a base depends primarily on pH. To help describe the strengths of weak bases, it is helpful to know the percentage protonated-the percentage of base molecules that have been protonated. A lower percentage will correspond with a lower pH because both numbers result from the amount of protonation. A weak base is less protonated, leading to a lower pH and a lower percentage protonated.

The typical proton transfer equilibrium appears as such:

B ( a q ) + H 2 O ( l ) ↔ H B + ( a q ) + O H − ( a q )

B represents the base.

P e r c e n t a g e   p r o t o n a t e d = m o l a r i t y   o f   H B +   i n i t i a l   m o l a r i t y   o f   B × 100 % = [ H B + ] [ B ] i n i t i a l × 100 %

In this formula, [B]initial is the initial molar concentration of the base, assuming that no protonation has occurred.

A typical pH problem

Calculate the pH and percentage protonation of a .20 M aqueous solution of pyridine, C5H5N. The Kb for C5H5N is 1.8 x 10−9.

First, write the proton transfer equilibrium:

H 2 O ( l ) + C 5 H 5 N ( a q ) ↔ C 5 H 5 N H + ( a q ) + O H − ( a q ) K b = [ C 5 H 5 N H + ] [ O H − ] [ C 5 H 5 N ]

The equilibrium table, with all concentrations in moles per liter, is

This means .0095% of the pyridine is in the protonated form of C5H5NH+.

Examples

  • Alanine
  • Ammonia, NH3
  • Methylamine, CH3NH2

    • Other weak bases are essentially any bases not on the list of strong bases.

    Simple Facts

  • An example of a weak base is ammonia. It does not contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions.
  • The position of equilibrium varies from base to base when a weak base reacts with water. The further to the left it is, the weaker the base.

    • References

    Weak base Wikipedia
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