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在数学中,如果要证明某个命题成立,可以先假定该命题不成立,从而推论出与已知条件或事实相矛盾或相符的结论,从而得出原命题成立与否。这种思维方法在化学学科中同样也是有用的。例1.向 FeCl_2溶液中通人 H_2S 气体,会不会像CuCl_2与 H_2S 那样发生反应生成沉淀呢?可以先假定FeCl_2与 H_2S 发生了反应 FeCl_2+H_2S=FeS↓+2HCl;但是,我们熟知实验室恰恰是用 FeS 与稀 H_2SO_4或稀HCl 反应来制取 H_2S 气体的。即上述假定的反应其逆反应可以发生,故正反应不可能发生。所以向FeCl_2溶液中通入 H_2S 气体不会发生反应。向 CaCl_2溶液中通入 CO_2不会产生沉淀与上述道理一样。例2.能否用浓 H_2SO_4干燥 HF、HCl、HBr 和 HI呢?我们熟知 HF、HCl 的制取是用 CaF_2、NaCl 与浓
In mathematics, if you want to prove that a certain proposition is true, you can first assume that the proposition is not true, and then infer conclusions that are inconsistent or consistent with known conditions or facts, so as to determine whether the original proposition is true or not. This way of thinking is also useful in chemistry. Example 1. Passing H_2S gas into FeCl 2 solution, will it not react like CuCl 2 and H 2 S to form precipitate? It can be presumed that FeCl 2 reacts with H 2 S. FeCl_2+H 2 S=FeS↓+2HCl; however, we are familiar with the laboratory. It is precisely the reaction of FeS with dilute H2SO4 or dilute HCl to produce H2S gas. That is, in the above-mentioned presumed reaction, a reverse reaction may occur, so that a positive reaction cannot occur. Therefore, the introduction of H 2 S gas into the FeCl 2 solution will not react. The introduction of CO 2 into the CaCl 2 solution does not produce precipitates as described above. Example 2. Can we dry HF, HCl, HBr and HI with concentrated H_2SO_4? We know that HF and HCl are prepared with CaF_2, NaCl and