✔ 最佳答案
what's meant by "neutralization" ?
in secondary schools, we're taught "acid and base react to give salt and water only", which is not very accurate, especially when one considers the existence of equilibrium of salts in water.
i'd like to put it in this way:
neutralization is combination of species which has same no. of equivalent of acid and base.
take sodium ethanoate (sodium acetate) as example.
acetic acid (ethanoic acid) is a weak acid:
CH3COOH <======> CH3COO(-) + H(+)
when sodium acetate is dissolved in water, sodium ion is freely moving in water. however, acetate ion is not very stable in water, and can react with hydrogen ion in water, giving back unionized acetic acid.
that's the question: where does H(+) come from?
H2O <======> H(+) + OH(-)
water itself can ionize to give hydrogen ion and hydroxide ion.
so we can write this:
CH3COOH + OH(-) <======> CH3COO(-) + H2O
acetate ion is unstable, so some of the ions react to give molecular acid and hydroxide ion.
hydroxide ion is stable, so now the solution contains following:
H2O, CH3COO(-), CH3COOH, Na(+), OH(-), (and trace amount of H(+)).
since one reacted acetate ion gives one acetic acid molecule and one hydroxide ion, there's still same no. of equivalent of acid and base. the solution is still neutralized.
the point is, there's some excess hydroxide ion, so the solution becomes alkaline.
alkaline solution does not mean that it contains no acid; it just says that the acid in the solution is not strong enough to react completely with the base.
ions of weak acids and bases, such as carbonate, hydrogencarbonate, phosphate (conjugate acid is hydrogenphosphate HPO4(2-)), fluoride, ammonium etc. will exhibit such effect.
solutions of these ions may not be neutral in pH, even they're neutralized in sense of stoichiometry.
