解析A中的氢氧根离子浓度为什么是减小的,不是越稀越电离吗?

2025-04-05 22:05:25
推荐回答(2个)
回答1:

越稀越电离是指电离度增加,但根据勒夏特列原理,OH-浓度一定降低,只不过电离补充出部分OH-,使得浓度降低没那麼快而已.电离平衡也是平衡,请你用化学平衡那一章所学的知识来看问题好吗?
C选项你不用管什麼中性点还是中和点的.水电离的H+和OH-都是10^-7mol/L,但是请你注意,假设溶液中HCl过量时,H+的来源还可以是HCl,而OH-会被NH4+消耗(发生水解反应),也就是说此时溶液中实际的H+>10^-7mol/L,而OH-<10^-7mol/L.我们知道pH是跟实际的H+浓度有关,跟水电离的H+浓度无关,所以C选项一定错误.
但还有一种可能性是氨水过量,溶液是NH3·H2O和NH4Cl的混合物.此时溶液呈什麼性要看二者比例.因为OH-来自於NH3·H2O的电离,因此实际OH-会增加.但OH-又会被NH4+水解消耗,浓度减小.因为电离和水解的程度不一样,当二者比例为某个值时,可以使得电离产生的OH-恰好等於水解消耗的OH-,这时候pH可以为7.那麼当不为7的时候呢?如果是电离大於水解,那麼电离出来的OH-更多,会消耗H+,也就使得溶液中H+<10^-7,OH->10^-7,溶液呈堿性.反过来那就是水解大於电离,溶液呈酸性.所以单纯给了水电离的H+=10^-7mol/L,你无法进行任何判断.

回答2:

选项A中氨水属于弱电解质,虽然越稀释电离度越大,但稀释后氨水浓度大大降低影响远大于多电离度增加,所以稀释后整体氢氧根浓度减小。
选项C,盐酸和氨水都会抑制水的电离,但两者反应后生成的氯化铵对水的电离起促进作用(铵根水解显酸性),又因为水解离出的氢离子浓度为10∧-7,说明还有剩余盐酸正好抵消了铵根对水的促进作用,所以盐酸过量,溶液显酸性。

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