Weak base: Encyclopedia - Weak base

Weak base

Acid-base reaction theories
pH
Self-ionization of water
Buffer solutions
Systematic naming
Redox reactions
Electrochemistry
Acids:

• Strong acids
• Weak acids

Bases:

• Strong bases
• Weak bases

In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As 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 level 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). The pH level has the formula:

Since bases are proton acceptors, the base receives a hydrogen ion from water, H₂O, and the remaining H⁺ concentration in the solution determines the pH level. 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 level results. However, the pH level of bases is usually calculated using the OH^- concentration to find the pOH level first. This is done because the H⁺ concentration is not a part of the reaction, while the OH^- concentration is.

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

Since K_w = [H₃O ⁺ ][OH ⁻ ] then,

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

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

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

Weak bases exist in chemical equilibrium much in the same way as weak acids do, with a Base Ionization Constant (K_b) (or the Base Dissociation Constant) indicating the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up:

Bases that have a large K_b will ionize more completely and are thus stronger bases. As stated above, the pH of the solution depends on the H⁺ concentration, which is related to the OH^- concentration by the Ionic Constant of water (K_w = 1.0x10^-14) (See article Self-ionization of water.) 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 K_b.

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 K_b values become. The pie-chart representation is as follows:

• purple areas represent the fraction of OH- ions formed
• red areas represent the cation remaining after ionization
• yellow areas represent dissolved but non-ionized molecules.

Weak base - Percentage protonated

As seen above, the strength of a base depends primarily on the pH level. 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 level 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 represents the base.

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

Weak base - A typical pH problem

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

First, write the proton transfer equilibrium:

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

This means .0095% of the pyridine is in the protonated form of C₅H₆N⁺.

Weak base - Examples

• Alanine, C₃H₅O₂NH₂
• Ammonia, NH₃
• Magnesium hydroxide, Mg(OH)₂
• Methylamine, CH₃NH₂
• Pyridine, C₅H₅N

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

Adapted from the Wikipedia article "Weak base", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki