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Weak Acids and Weak Bases

weak acids and bases example

Core Concepts

In this tutorial, you will learn about weak acids and weak bases, including why they are considered weak and examples of each. We’ll also show you a list of weak acids, and the weakest bases.

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What makes an Acid or Base ‘Weak’?

Weak Acids

When thinking about what makes about an acid weak, think about its dissociation property. When they are dissolved in a solution, weak acids do not completely dissociate into their different constituent ions. Furthermore, when a weak acid is dissolved in water, an equilibrium is developed between the concentrations of the weak acid itself and its constituent ions. A weak acid is not completely ionized in solution.

For example, hydrofluoric acid, HF, is a weak acid. When dissolved in water, HF ion exist in equilibrium with H+, which reacts with water to form hydronium, and F ions. Since the acid does not completely dissociate into its ionic components, it is a weak acid.

HA + H2O ⇌ H3O+ + A

weak acids and bases example

There are only several strong acids. Once you learn and memorize the strong acids, you may assume all other acids are weak acids. Furthermore, you can distinguish a strong acid from a weak one by looking at the acid dissociation constant, Ka, or pKa values. Strong acids will tend to have a high Ka and low pKa value while weak acids have small Ka and high pKa values.

Weak Bases

The concept is very similar when thinking about a weak base; however, in this case, a weak base is a base that cannot fully ionize or fully accept hydrogen ions in a solution. The base needs to dissociate in solution in order for it to react with the acid to form an acid base pair; if the base does not break into its constituent ions, it is not able to react the way it should. When dissolved in water, the solution contains a small amount of hydroxide ions and a large amount of the undissociated base. A weak base does not completely ionize.

For example, with ammonium hydroxide, NH4OH, the counterpart’s of the base are NH4+ and OH; the NH4+ supposedly accepts the acidic hydrogen in solution; however, in a weak base these ion do not fully dissociate so there is a mixture of NH4OH and the acid base pair.

B + H2O ⇌ BH+ + OH

You can distinguish a strong base from a weak one by its base dissociation constant, Kb. This variable mathematically represents the base strength; the weaker the base, the smaller the Kb value.

Example – Is Acetic Acid Strong or Weak?

Acetic acid has a Ka value of 1.8 x 10-5. Since the Ka is quite small, acetic acid is a weak acid, and it does not completely ionize in water.

Examples of Weak Acids and Bases

List of Weak Acids

Weak acids remain mainly as a complete molecule in solution. Only a small fraction of the acid molecule “breaks up”, or dissociates, into ions. However, these “weak acids” can still be highly corrosive and dangerous. This is a short list of weak acid examples – but there are many more!

  • Formic Acid (HCOOH)
  • Acetic Acid (CH3COOH)
  • Benzoic Acid (C6H5COOH)
  • Hydrofluoric Acid (HF)
  • Phosphoric Acid (H3PO4)
  • Sulfurous Acid (H2SO3)
  • Carbonic Acid (H2CO3)
  • Nitrous Acid (HNO2)
  • Hydrocyanic Acid (HCN)
  • Hydrosulfuric Acid (H2S)
  • Citric Acid (C6H8O7)

List of Weak Bases

Here is a list of some weak base examples. Many weak bases are simply slightly soluble hydroxides, like magnesium hydroxide, while others are organic compounds.

  • Ammonium Hydroxide (NH4OH)
  • Aniline (C6H5NH2)
  • Ammonia (NH3)
  • Methylamine (CH3NH2)
  • Ethylamine (CH3CH2NH2)
  • Aluminum hydroxide (Al(OH)3)
  • Magnesium Hydroxide (Mg(OH)2)
  • Pyridine (C5H5N)
  • Sodium Bicarbonate (NaHCO3)

What is the weakest base?

Water can be considered both a weak acid and a weak base, because it ionizes slightly to give both hydrogen ions and hydroxide ions. It is definitely one of the weakest bases!

Salts of strong acids and strong bases create solutions are considered neutral. Examples are sodium perchlorate, sodium chloride, and sodium nitrate. We would classify these compounds as “neutral” rather than as weak bases.

Weak Acids and Bases in Real World Applications

  • Weak acids and bases are commonly used in organic chemistry and biochemistry to create a buffer solution, in titrations, or to catalyze a specific reaction.
  • Acetic Acid is an active component seen in vinegar.
  • Benzoic Acid occurs naturally in many plants and is usually used for food preservation.
  • Ammonia is seen in household cleaning supplies, bug repellents, fertilizer and more.
weakest bases

Strong vs. Concentrated & Weak vs. Dilute

You should be careful when thinking about a strong acid or base versus a concentrated one, and a weak acid or base versus a dilute one. A concentrated acid means a solution without much water and a diluted acid means a solution with a high amount of solvent. Just because a base has a high concentration does not mean it is strong; and just because an base is very dilute, does not not mean it is a weak and non dangerous base.

Further Reading

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