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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. [4] 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. [5]
Water is a good polar solvent, dissolving many salts and hydrophilic organic molecules such as sugars and simple alcohols such as ethanol. Water also dissolves many gases, such as oxygen and carbon dioxide—the latter giving the fizz of carbonated beverages, sparkling wines and beers.
Water is amphoteric: it has the ability to act as either an acid or a base in chemical reactions. [87] According to the Brønsted-Lowry definition, an acid is a proton (H +) donor and a base is a proton acceptor. [88] When reacting with a stronger acid, water acts as a base; when reacting with a stronger base, it acts as an acid. [88]
Strong bases hydrolyze in water almost completely, resulting in the leveling effect." [7] In this process, the water molecule combines with a strong base, due to the water's amphoteric ability; and, a hydroxide ion is released. [7] Very strong bases can even deprotonate very weakly acidic C–H groups in the absence of water.
At equilibrium, the water contains a certain amount of alkalinity contributed by the concentration of weak acid anions. Conversely, the addition of acid converts weak acid anions to CO 2 and continuous addition of strong acids can cause the alkalinity to become less than zero. [12]
The strength of a strong acid is limited ("leveled") by the basicity of the solvent. Similarly the strength of a strong base is leveled by the acidity of the solvent. When a strong acid is dissolved in water, it reacts with it to form hydronium ion (H 3 O +). [2] An example of this would be the following reaction, where "HA" is the strong acid:
On the other hand, if a chemical is a weak acid its conjugate base will not necessarily be strong. Consider that ethanoate, the conjugate base of ethanoic acid, has a base splitting constant (Kb) of about 5.6 × 10 −10, making it a weak base. In order for a species to have a strong conjugate base it has to be a very weak acid, like water.
If, for example, water, instead of hydroxide, was used to deprotonate the carboxylic acid, the equilibrium would not favor the formation of the carboxylate salt. This is because the conjugate acid, hydronium, has a pK a of -1.74, which is lower than the carboxylic acid. In this case, equilibrium would favor the carboxylic acid.