关于双水解反应

2022-02-01 08:47 作者:匆匆-cc 0人读过 | 我要投稿

双水解反应(The double hydrolysis reaction)是指弱酸阴离子和弱碱阳离子相互促进水解的反应。

事实上，并不是所有的双水解反应进行得都很彻底。

比如说，醋酸铵 $%5Cce%7BCH3COONH4%7D$ 的水解。

$%5Cce%7BCH3COONH4%7D%2B%5Cce%7BH2O%7D%5Cce%7B%3C%3D%3E%7D%5Cce%7BCH3COOH%7D%2B%5Cce%7BNH3%5Ccdot%20H2O%7D$

根据 $K%20$ 的定义，我们得到

$%5Cbegin%7Balign%7D%0AK%20%26%3D%20%5Cfrac%7Bc_%7B(%5Cce%7BCH3COOH%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BNH3%5Ccdot%20H2O%7D)%7D%7D%7Bc_%7B(%5Cce%7BCH3COO-%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BNH4%2B%7D)%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bred%7D%7Bc_%7B(%5Cce%7BCH3COOH%7D)%7D%7D%5Ccdot%20%5Ccolor%7Bblue%7D%7Bc_%7B(%5Cce%7BNH3%5Ccdot%20H2O%7D)%7D%7D%5Ccdot%20%5Ccolor%7Bgreen%7D%7Bc_%7B(%5Cce%7BH%2B%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BOH-%7D)%7D%7D%7D%7B%5Ccolor%7Bred%7D%7Bc_%7B(%5Cce%7BCH3COO-%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BH%2B%7D)%7D%7D%5Ccdot%20%5Ccolor%7Bblue%7D%7Bc_%7B(%5Cce%7BNH4%2B%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BOH-%7D)%7D%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bgreen%7D%7BK_w%7D%7D%7B%5Ccolor%7Bred%7D%7BK_%7Ba%5Cce%7B(CH3COOH)%7D%7D%7D%5Ccdot%20%5Ccolor%7Bblue%7D%7BK_%7Bb%5Cce%7B(NH3%5Ccdot%20H2O)%7D%7D%7D%7D%0A%5Cend%7Balign%7D$

查表

$%5Cbegin%7Barray%7D%7B%7Cc%7Cc%7C%7D%0A%5Chline%0A%5Ctextbf%7B%E5%BC%B1%E7%94%B5%E8%A7%A3%E8%B4%A8%7D%20%26%20%5Ctextbf%7B%E7%94%B5%E7%A6%BB%E5%B9%B3%E8%A1%A1%E5%B8%B8%E6%95%B0%7D%5C%5C%0A%5Chline%0A%5Cce%7BCH3COOH%7D%20%26%201.76%5Ctimes10%5E%7B-5%7D%5C%5C%0A%5Chline%0A%5Cce%7BNH3%5Ccdot%20H2O%7D%20%26%201.78%5Ctimes%2010%5E%7B-5%7D%5C%5C%0A%5Chline%0A%5Cend%7Barray%7D$

易得

$%5Cbegin%7Balign%7D%0AK%26%3D%5Cfrac%7BK_w%7D%7BK_%7Ba%5Cce%7B(CH3COOH)%7D%7D%5Ccdot%20K_%7Bb%5Cce%7B(NH3%5Ccdot%20H2O)%7D%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B1%5Ctimes%2010%5E%7B-14%7D%7D%7B1.76%5Ctimes%2010%5E%7B-5%7D%5Ctimes1.78%5Ctimes%2010%5E%7B-5%7D%7D%0A%5C%5C%26%5Capprox%203.19%5Ctimes%2010%5E%7B-5%7D%0A%5Cend%7Balign%7D$

水解程度很小。

来看 $%5Cce%7BAlO2-%7D$ 和 $%5Cce%7BHCO3-%7D$ 的情况。

$%5Cce%7BAlO2-%7D%2B%5Cce%7BHCO3-%7D%2B%5Cce%7BH2O%7D%5Cce%7B%3C%3D%3E%7D%5Cce%7BAl(OH)3%7D%2B%5Cce%7BCO3%5E2-%7D$

易得

$%5Cbegin%7Balign%7D%0AK%20%26%3D%20%5Cfrac%7B%5C%23%20c_%7B(%5Cce%7BAl(OH)3%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BCO3%5E2-%7D)%7D%7D%7Bc_%7B(%5Cce%7BAlO2-%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BHCO3-%7D)%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bred%7D%7Bc_%7B(%5Cce%7BAl(OH)3%7D)%7D%7D%5Ccdot%20%5Ccolor%7Bblue%7D%7Bc_%7B(%5Cce%7BCO3%5E2-%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BH%2B%7D)%7D%7D%7D%7B%5Ccolor%7Bred%7D%7Bc_%7B(%5Cce%7BAlO2-%7D)%7D%5Ccdot%20c_%7B(%5Cce%7BH%2B%7D)%7D%7D%5Ccdot%20%5Ccolor%7Bblue%7D%7Bc_%7B(%5Cce%7BHCO3-%7D)%7D%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bblue%7D%7BK_%7Ba(%5Cce%7BHCO3-%7D)%7D%7D%7D%7B%5Ccolor%7Bred%7D%7BK_%7Ba(%5Cce%7BAl(OH)3%7D)%7D%7D%7D%0A%5Cend%7Balign%7D$

## 这里本来不应该把 $%5Ccolor%7Bgray%7D%7B%5Cce%7BAl(OH)3%7D%7D$ 写进表达式的，很遗憾的是笔者没有找到相应的 $%5Ccolor%7Bgray%7D%7BK_%7Bsp_a%7D%7D$ 值，只好凑了一个酸式电离常数进去。

查表

$%5Cbegin%7Barray%7D%7B%7Cc%7Cc%7C%7D%0A%5Chline%0A%5Ctextbf%7B%E5%BC%B1%E7%94%B5%E8%A7%A3%E8%B4%A8%7D%20%26%20%5Ctextbf%7B%E5%B9%B3%E8%A1%A1%E5%B8%B8%E6%95%B0%7D%5C%5C%0A%5Chline%0A%5Cce%7BHCO3-%7D%20%26%205.6%5Ctimes10%5E%7B-11%7D%5C%5C%0A%5Chline%0A%5Cce%7BAl(OH)3%7D%20%26%206.3%5Ctimes%2010%5E%7B-13%7D%5C%5C%0A%5Chline%0A%5Cend%7Barray%7D$

容易计算出

$%5Cbegin%7Balign%7D%0AK%20%26%3D%5Cfrac%7BK_%7Ba(%5Cce%7BHCO3-%7D)%7D%7D%7BK_%7Ba(%5Cce%7BAl(OH)3%7D)%7D%7D%0A%5C%5C%26%3D%20%5Cfrac%7B5.6%5Ctimes10%5E%7B-11%7D%7D%7B6.3%5Ctimes%2010%5E%7B-13%7D%7D%0A%5C%5C%26%5Capprox%2088.9%0A%5Cend%7Balign%7D$

常数不大，说明反应可以发生，可认为不大量共存。

再来看 $%5Cce%7BAl%5E3%2B%7D$ 和 $%5Cce%7BHCO3-%7D$ 的情况。

$%5Cce%7BAl%5E3%2B%7D%2B3%5Cce%7BHCO3-%7D%5Cce%7B%3C%3D%3E%7D%5Cce%7BAl(OH)3%7D%2B3%5Cce%7BCO2%20%5E%7D$

易得

$%5Cbegin%7Balign%7D%0AK%20%26%3D%20%5Cfrac%7Bc%5E3_%7B(%5Cce%7BH2CO3%7D)%7D%7D%7Bc_%7B(%5Cce%7BAl%5E3%2B%7D)%7D%5Ccdot%20c%5E3_%7B(%5Cce%7BHCO3-%7D)%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bblue%7D%7Bc%5E3_%7B(%5Cce%7BH2CO3%7D)%7D%5Ccdot%20%5Ccolor%7Bgreen%7D%7Bc%5E3_%7B(%5Cce%7BH%2B%7D)%7D%5Ccdot%20%7Bc%5E3_%7B(%5Cce%7BOH-%7D)%7D%7D%7D%7D%7D%7B%5Ccolor%7Bred%7D%7Bc_%7B(%5Cce%7BAl%5E3%2B%7D)%7D%5Ccdot%7Bc%5E3_%7B(%5Cce%7BOH-%7D)%7D%7D%7D%20%5Ccdot%20%5Ccolor%7Bblue%7D%7Bc%5E3_%7B(%5Cce%7BH%2B%7D)%7D%5Ccdot%20c%5E3_%7B(%5Cce%7BHCO3-%7D)%7D%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bgreen%7D%7BK%5E3_w%7D%7D%7B%5Ccolor%7Bred%7D%7BK_%7Bsp_b(%5Cce%7BAl(OH)3%7D)%7D%7D%5Ccdot%20%5Ccolor%7Bblue%7D%7BK%5E3_%7Ba(%5Cce%7BH2CO3%7D)%7D%7D%7D%0A%5Cend%7Balign%7D$

查表

$%5Cbegin%7Barray%7D%7B%7Cc%7Cc%7C%7D%0A%5Chline%0A%5Ctextbf%7B%E5%BC%B1%E7%94%B5%E8%A7%A3%E8%B4%A8%7D%20%26%20%5Ctextbf%7B%E5%B9%B3%E8%A1%A1%E5%B8%B8%E6%95%B0%7D%5C%5C%0A%5Chline%0A%5Cce%7BH2CO3%7D%20%26%204.5%5Ctimes10%5E%7B-7%7D%5C%5C%0A%5Chline%0A%5Cce%7BAl(OH)3%7D%20%26%201.1%5Ctimes%2010%5E%7B-33%7D%5C%5C%0A%5Chline%0A%5Cend%7Barray%7D$

容易计算出

$%5Cbegin%7Balign%7D%0AK%26%3D%5Cfrac%7BK%5E3_w%7D%7BK_%7Bsp_b(%5Cce%7BAl(OH)3%7D)%7D%5Ccdot%20K%5E3_%7Ba(%5Cce%7BH2CO3%7D)%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B(1%5Ctimes%2010%5E%7B-14%7D)%5E3%7D%7B1.1%5Ctimes%2010%5E%7B-33%7D%5Ctimes%20(4.5%5Ctimes%2010%5E%7B-7%7D)%5E3%7D%0A%5C%5C%26%5Capprox%201.0%5Ctimes10%5E8%20%0A%5Cend%7Balign%7D$

常数较大，可认为不共存。

再来看 $%5Cce%7BAl%5E3%2B%7D$ 和 $%5Cce%7BAlO2-%7D$ 的情况。

$%5Cce%7BAl%5E3%2B%7D%2B3%5Cce%7BAlO2-%7D%2B6%5Cce%7BH2O%7D%5Cce%7B%3C%3D%3E%7D4%5Cce%7BAl(OH)3%7D$

易得

$%5Cbegin%7Balign%7D%0AK%20%26%3D%20%5Cfrac%7B%5C%23%20c%5E3_%7B(%5Cce%7BAl(OH)3%7D)%7D%7D%7Bc_%7B(%5Cce%7BAl%5E3%2B%7D)%7D%5Ccdot%20c%5E3_%7B(%5Cce%7BAlO2-%7D)%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bblue%7D%7Bc%5E3_%7B(%5Cce%7BAl(OH)3%7D)%7D%5Ccdot%20%5Ccolor%7Bgreen%7D%7Bc%5E3_%7B(%5Cce%7BH%2B%7D)%7D%5Ccdot%20%7Bc%5E3_%7B(%5Cce%7BOH-%7D)%7D%7D%7D%7D%7D%7B%5Ccolor%7Bred%7D%7Bc_%7B(%5Cce%7BAl%5E3%2B%7D)%7D%5Ccdot%7Bc%5E3_%7B(%5Cce%7BOH-%7D)%7D%7D%7D%20%5Ccdot%20%5Ccolor%7Bblue%7D%7Bc%5E3_%7B(%5Cce%7BH%2B%7D)%7D%5Ccdot%20c%5E3_%7B(%5Cce%7BAlO2-%7D)%7D%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B%5Ccolor%7Bgreen%7D%7BK%5E3_w%7D%7D%7B%5Ccolor%7Bred%7D%7BK_%7Bsp_b(%5Cce%7BAl(OH)3%7D)%7D%7D%5Ccdot%20%5Ccolor%7Bblue%7D%7BK%5E3_%7Ba(%5Cce%7BAl(OH)3%7D)%7D%7D%7D%0A%5Cend%7Balign%7D$

## 同样，这里本来不应该把 $%5Ccolor%7Bgray%7D%7B%5Cce%7BAl(OH)3%7D%7D$ 写进表达式的，很遗憾的是笔者没有找到相应的 $%5Ccolor%7Bgray%7D%7BK_%7Bsp_a%7D%7D$ 值，只好凑了一个酸式电离常数进去。

查表

$%5Cbegin%7Barray%7D%7B%7Cc%7Cc%7Cc%7C%7D%0A%5Chline%0A%5Ctextbf%7B%E5%BC%B1%E7%94%B5%E8%A7%A3%E8%B4%A8%7D%20%26%20%5Ctextbf%7B%E5%B9%B3%E8%A1%A1%E5%B8%B8%E6%95%B0%7D%20%26%20%5Ctextbf%7B%E5%A4%87%E6%B3%A8%7D%5C%5C%0A%5Chline%0A%5Cce%7BAl(OH)3%7D%20%26%206.3%5Ctimes10%5E%7B-13%7D%20%26%20K_a%5C%5C%0A%5Chline%0A%5Cce%7BAl(OH)3%7D%20%26%201.1%5Ctimes%2010%5E%7B-33%7D%20%26%20K_%7Bsp_b%7D%5C%5C%0A%5Chline%0A%5Cend%7Barray%7D$

容易计算出

$%5Cbegin%7Balign%7D%0AK%20%26%3D%5Cfrac%7BK%5E3_w%7D%7BK_%7Bsp_b(%5Cce%7BAl(OH)3%7D)%7D%5Ccdot%20K%5E3_%7Ba(%5Cce%7BAl(OH)3%7D)%7D%7D%0A%5C%5C%26%3D%5Cfrac%7B(1%5Ctimes%2010%5E%7B-14%7D)%5E3%7D%7B1.1%5Ctimes%2010%5E%7B-33%7D%5Ctimes%20(6.3%5Ctimes%2010%5E%7B-13%7D)%5E3%7D%0A%5C%5C%26%5Capprox%203.6%5Ctimes%2010%5E%7B27%7D%0A%5Cend%7Balign%7D$

反应几乎完全进行，应认为不共存。

除了以上提及的部分，下列双水解也应引起注意。

$%5Cbegin%7Barray%7D%7B%7Cc%7Cc%7C%7D%0A%5Chline%0A%5Cce%7BAl%5E3%2B%7D%20%26%20%5Cce%7BS%5E2-%7D%2C%5Cce%7BHS-%7D%2C%5Cce%7BCO3%5E2-%7D%2C%5Cce%7BSiO3%5E2-%7D%2C%5Cce%7BClO-%7D%5C%5C%0A%5Chline%0A%5Cce%7BAlO2-%7D%20%26%20%5Cce%7BFe%5E3%2B%7D%2C%5Cce%7BNH4%2B%7D%5C%5C%0A%5Chline%0A%5Cce%7BFe%5E3%2B%7D%20%26%20%5Cce%7BCO3%5E2-%7D%2C%5Cce%7BHCO3-%7D%2C%5Cce%7BSiO3%5E2-%7D%2C%5Cce%7BClO-%7D%5C%5C%0A%5Chline%0A%5Cend%7Barray%7D$