Weak Bases

What is a Weak Base?

Weak bases can be defined as basic substances that do not completely dissociate into their constituent ions when dissolved in solutions. Therefore, when dissolved in a solution, a part of the weak base dissociates into hydroxide anions and the relevant conjugate acid, and another part remains undissociated inside the solution. The ionization of a weak base is usually a type of equilibrium process in which a chemical equilibrium is established inside the solution between the concentration of the undissociated base and its constituent ions (the conjugate acid and the hydroxide anion). It is important to note that the conjugate acid of a weak base will almost always be a weak acid. Similarly, the conjugate base of a weak acid will act as a weak base.

When a weak base is dissolved in water, the following type of equilibrium arises:

B + H2O ⇌ BH+ + OH

In this equilibrium reaction, a lone pair of electrons present in the basic molecule accepts a proton from the water molecule, resulting in the formation of a hydroxide ion. The greater the concentration of the equilibrium towards the left, the weaker the base. Similarly, the greater the equilibrium concentration towards the right, the stronger the base.

Examples of Weak Bases

Ammonia (NH3)

Ammonia is a chemical compound with the formula NH3. It is a weak base which exists as a colourless gas under standard conditions for temperature and pressure. This gas is known to be characterized by its pungent smell. When ammonia is dissolved in water, the resulting solution (also referred to as ammonia water or ammonia solution) contains the ammonium cation (denoted by the chemical formula NH4+ and the hydroxide anion. However, it is important to note that only a part of the dissolved ammonia dissociates into these ions.

Trimethylamine (N(CH3)3)

Trimethylamine, often abbreviated to TMA, is a weak base with the chemical formula N(CH3)3. This compound can be prepared by reacting ammonia with methanol in the presence of a catalyst. It can also be produced from the reaction between paraformaldehyde and ammonium chloride.

Pyridine (C5H5N)

Pyridine is an organic compound with the chemical formula C5H5N. This chemical compound has a heterocyclic structure and is a weak base. The structure of pyridine is similar to that of benzene, with the exception that one of the methine groups is replaced with a single nitrogen atom. Under standard conditions for temperature and pressure, pyridine exists as a colourless liquid. It can also be noted that pyridine is a Lewis base and it has the ability to donate electron pairs to Lewis acids in order to form Lewis adducts.

Frequently Asked Questions on Weak Bases

Is sodium hydroxide a weak base? Why or why not?

Sodium hydroxide, a chemical compound with the formula NaOH, is known to be a strong base. This is because sodium hydroxide undergoes almost complete ionization when it is dissolved in water. Weak bases are the basic substances that do not completely ionize in water. An example of a weak base is ammonia. When NH3 is dissolved in water, a part of it dissociates into ammonium cation and hydroxide anions by interacting with the water molecules. However, some ammonia remains unionized in the solution.

List 3 examples of weak bases

Three common examples of weak bases are listed below.

  • Ammonia
  • Pyridine
  • Methylamine

Is water a weak base?

Yes, pure water acts as a weak base. In fact, pure water acts as both a weak acid and a weak base. This is because a small amount of water dissociates into protons and hydroxide anions, binding with the remaining water molecules to form hydronium ions and hydroxide ions.

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