Weak acid
From Free net encyclopedia
Template:Acids and Bases A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissolved HA still remains.
<math>\mathrm{ HA_{(aq)} \, \leftrightarrow \, H^+\,_{(aq)} +\, A^-\,_{(aq)} }</math>
The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (Ka):
<math>\mathrm{ K_a\, =\, \frac {[H^+\,][A^-\,]}{[HA]} }</math>
The greater the value of Ka, the more the formation of H+ is favored, and the lower the pH of the solution. The Ka of weak acids varies between 1.8×10-16 and 55.5. Acids with a Ka less than 1.8×10-16 are weaker acids than water. Acids with a Ka of greater than 55.5 are strong acids and almost totally dissociate when dissolved in water.
The vast majority of acids are weak acids.
Examples
- Phosphoric acid, H3PO4
- Ethanoic acid, CH3COOH
- Hydrofluoric acid, HF
- Acetylsalicylic acid, C6H4OCOCH3CO2H
- Nicotinic acid, C5H5NCOOH
- Pyruvic acid, CH3COCOOH
- Bromothymol blue, C21H15NaBr2O3Sfr:Acide faible