其中以氧化数的升降作为最普遍的标准,因为并不是所有的化学反应都有氧原子的参与。同时我们要理解电子转移的过程,因为电子是带负电荷的,所以得电子氧化数降低,为还原过程,失电子氧化数升高,为氧化过程。理解了电子转移的过程,也就比较容易正确写出半反应。
那么什么是氧化数呢?氧化数就是给化合物中的每一个原子赋予一个数值,用以显示其氧化的程度。这个概念过于抽象,更重要的是理解推断氧化数的一套规则:
1. The oxidation number of any uncombined element is zero.
任何孤立的原子的氧化数为0
2. In compounds many atoms or ions have fixed oxidation numbers
在化合物中,很多原子和离子的氧化数是固定的
ü Group I elements are always +1
ü Group II elements are always +2
ü fluorine is always -1
ü hydrogen is +1 (except in NaH)
ü oxygen is -2 (except in peroxides and in F2O)
3. The oxidation number of an element in a monatomic ion is always the same as the charge.单原子离子的氧化数等于其电荷数
4. The sum of the oxidation numbers in a compound is zero.
化合物中所有原子的氧化数总和为0
5. The sum of the oxidation numbers in an ion is equal to the charge on the ion.
复合离子中,所有原子的氧化数总和等于离子的电荷数
6. In either a compound or an ion, the more electronegative element is given the negative oxidation number.在原子或离子中,电负性更大的元素氧化数为负
当我们尝试配平非氧化还原化学反应,只需要满足反应物和生成物各原子总数相等就可以了,但是在配平氧化还原反应时,还需满足氧化数升降守恒,需要遵循以下规则:
1. Write the unbalanced equation and identify the atoms which change in ox. no.
写出未配平的方程式,找出氧化数改变的原子
2. Deduce the ox. no. changes.
标明这些原子的氧化数变化
3. Balance the ox. no. changes.
加上系数,使得氧化数升降总数相等
4. Balance the atoms and charges (in ionic equations).
配平原子和电荷(针对离子反应) 二、电解反应
作为学习电解反应的基础,先要理解电解池的基本结构(教科书118页),并且理解以下一些概念:
Electrolysis is the decomposition of a compound into its elements by an electric current.电解是利用电流把化合物分解成单质。
In the electrolysis cell:
1、 the electrolyte is the compound which is decomposed; it is either a molten ionic compound or a concentrated aqueous solution of ions
电解质是被分解的化合物,可以是熔融状态的离子化合物,也可以是高浓度的离子溶液
2、 the electrodes are rods, made from either graphite or metal; the anode is the positive electrode, and the cathode is the negative electrode
电极是导电的棒,用金属或者石墨制成,与电池的正极相连的称作阳极,与负极相连的称作阴极
3、 the power supply must be direct current
电源必须是直流电
以下三种反应是经常会考察的电解反应,所以需要在理解的基础上记忆。 extracting aluminium
at the cathode: Al3+ + 3e-→Al
at the anode: 2O2-→O2 + 4e-
overall: 2Al2O3 → 4Al + 3O2
Aluminium oxide is dissolved in cryolite (Na3AlF6). Cryolite has two functions:
ü the melting point of the electrolyte is lowered
ü it improves the electrical conductivity of the electrolyte the electrolysis of brine
at the cathode: 2H+ + 2e-→H2
at the anode: 2Cl- →Cl2 + 2e-
overall: 2NaCl + 2H2O → 2NaOH + H2 + Cl2 the electrolytic purification of copper
at the cathode: Cu2++ 2e-→ Cu
at the anode: Cu→Cu2+ + 2e-