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Definition and Examples of Weak Electrolytes

Electrolytes are substances that, once dissolved in water, break down into cations and anions. Cations are positively charged ions and anions are negatively charged ions. When an electrolyte dissolves in water, it is said to be ionized.

There are two groups of electrolytes: strong electrolytes and weak electrolytes. The first are completely ionized, that is, 100%. The seconds are partially ionized, between 1 and 10%. The main species in solution for strong electrolytes are ions. Instead, the main species in solution for weak electrolytes is the non-ionized compound itself.

In simple words: weak electrolytes are electrolytes that hardly dissociate (do not break down into cations and anions) in an aqueous solution.

Examples of weak electrolytes

Weak acids such as HF (hydrofluoric acid), HC 2 H 3 O 2 (acetic acid), H 2 CO 3 (carbonic acid) and H 3 PO 4 (phosphoric acid) and weak bases such as NH 3 ( ammonia) and C 5 H 5 N (pyridine) are weak electrolytes. Most nitrogen-containing molecules are also weak electrolytes.

It is important to keep in mind that salt can have low solubility in water and yet be a strong electrolyte. This is because the amount of dissolved salt, even if limited, is fully ionized in the water. Some authors consider that water is a weak electrolyte. The reason is that water partially dissociates into H+ and OH- ions. However, others consider it a non-electrolyte. This is because only a very small amount of water dissociates or breaks down into ions.

Difference Between Dissociate and Dissolve

The importance of a substance dissolving in water has been mentioned. However, whether or not a substance dissolves in water is not a decisive factor in determining the strength of an electrolyte. In other words, dissociation and dissolution are not the same.

Thus, dissociation refers to the moment in which one compound disintegrates into another. Instead, dissolution occurs when a liquid compound is diluted within an aqueous solution.

Acetic acid as a weak electrolyte

Acetic acid, found in vinegar, is a fairly water-soluble compound. That is, this compound does not dissociate; however, it does dissolve. This acid is a weak electrolyte because its dissociation constant is small, which means there will be few ions in the mixture to conduct electricity.

Most of the acetic acid remains intact as its parent molecule instead of its ionized form, ethanoate (CH 3 COO – ). Because of this, acetic acid dissolves in water and ionizes into ethanoate and the hydronium ion, but its equilibrium position is to the left of the dissociation equation, making the reactants favored. That is, when ethanoate and hydronium are formed, they easily return to acetic acid and water:

CH 3 COOH + H 2 O ⇆ CH 3 COO –  + H 3 O +

Note : The small amount of ethanoate makes acetic acid a weak electrolyte, rather than a strong one.