When a material combines with oxygen atoms, the material is said to be oxidized. When an oxide loses oxygen atoms, the material is said to be reduced. Note that hydrogen atoms are obtained in this reduction.
・Oxidation(materials gain oxygen atoms):\(\displaystyle 2\mathrm{Cu} + \mathrm{O}_{2} \rightarrow 2\mathrm{CuO}\)
・Reduction(materials lose oxygen atoms):\(\displaystyle \mathrm{CuO} + \mathrm{H}_{2} \rightarrow \mathrm{Cu} + \mathrm{H}_{2}\mathrm{O} \)
Chemical reactions involving both oxidation and reduction are called redox reactions.
Let’s consider the redox reactions that actually occur in active volcanoes. As we studied in earth science, sulfur, which is yellow, sometimes adheres to the neighborhood of the fumaroles of active volcanoes. Volcanic gas emitted from the fumaroles of active volcanoes contains hydrogen sulfide, which combines with oxygen in the atmosphere to produce sulfur.
In an actual chemical reaction, hydrogen sulfide loses oxygen and oxygen gains hydrogen.
・Chemical reaction to produce sulfur from hydrogen sulfide:\(\displaystyle 2\mathrm{H}_{2}\mathrm{S} + \mathrm{O}_{2} \rightarrow 2\mathrm{S} + 2\mathrm{H}_{2}\mathrm{O} \)
The above reaction is an oxidation reaction and a reduction reaction for hydrogen sulfide and oxygen, respectively.
We have considered an oxidation-reduction reaction by giving and receiving oxygen or giving and receiving hydrogen. However, it is troublesome to decide which element to focus on each time. Therefore, let’s consider the redox reaction again in terms of the number of giving-receiving electrons in each atom.
・Oxidation(The decrease in the number of electrons in an atom):\(\displaystyle 2\mathrm{H}_{2}\mathrm{S} \rightarrow 4\mathrm{H}^{+} + 2\mathrm{S}+4e^{-} \)
・Reduction(The increase in the number of electrons in an atom):\(\displaystyle \mathrm{O}_{2} + 4e^{-} \rightarrow 2\mathrm{O}^{2-} \)
・The whole chemical reaction:\(\displaystyle 2\mathrm{H}_{2}\mathrm{S} + \mathrm{O}_{2} \rightarrow 2\mathrm{S} + 2\mathrm{H}_{2}\mathrm{O}\)
Here, the electrons appearing on the both sides are canceled out. For the water molecules, note the following reaction:
・Chemical reaction to produce water molecules:\(\displaystyle 4\mathrm{H}^{+} + 2\mathrm{O}^{2-} \rightarrow 2\mathrm{H}_{2}\mathrm{O}\)
In this way, we consider redox reactions by focusing on the number of electrons.
物質が酸素と化合するとき,物質は酸化されたと言い,その物質を酸化物と呼ぶ.また酸化物が酸素を失ったとき,その酸化物は還元されたと呼ぶ.この還元では水素を得ることに注意しよう.
・酸化反応(物質が酸素を得る反応):\(\displaystyle 2\mathrm{Cu} + \mathrm{O}_{2} \rightarrow 2\mathrm{CuO}\)
・還元反応(物質が酸素を失う反応):\(\displaystyle \mathrm{CuO} + \mathrm{H}_{2} \rightarrow \mathrm{Cu} + \mathrm{H}_{2}\mathrm{O} \)
酸化反応と還元反応を合わせて酸化還元反応と呼ぶ.
実際に活火山で起こる酸化還元反応を考えてみよう.地球科学で学んだ活火山の噴気孔付近では,黄色い物質である硫黄が付着していることがある.活火山が噴気孔から排出する火山ガスは硫化水素を含んでおり,その硫化水素が大気中に含まれる酸素と化合し硫黄を生成する.
実際の化学反応では,硫化水素は酸素を失い,酸素は水素を得る.
・硫化水素から硫黄を生成する化学反応:\(\displaystyle 2\mathrm{H}_{2}\mathrm{S} + \mathrm{O}_{2} \rightarrow 2\mathrm{S} + 2\mathrm{H}_{2}\mathrm{O} \)
硫化水素にとっては酸化反応であり,酸素にとっては還元反応である.
酸素の授受または水素の授受で酸化還元反応を考えている.しかし,どの元素に着目するのかをいちいち考えることは煩わしい.そこで,各原子の電子数に着目し,酸化還元反応を再度考えよう.
・酸化反応(原子の電子数が減少する反応):\(\displaystyle 2\mathrm{H}_{2}\mathrm{S} \rightarrow 4\mathrm{H}^{+} + 2\mathrm{S}+4e^{-} \)
・還元反応(原子の電子数が増加する反応):\(\displaystyle \mathrm{O}_{2} + 4e^{-} \rightarrow 2\mathrm{O}^{2-} \)
・全体の化学反応:\(\displaystyle 2\mathrm{H}_{2}\mathrm{S} + \mathrm{O}_{2} \rightarrow 2\mathrm{S} + 2\mathrm{H}_{2}\mathrm{O}\)
ここで,両辺に現れる電子を相殺し,水分子については
・水分子の化学反応:\(\displaystyle 4\mathrm{H}^{+} + 2\mathrm{O}^{2-} \rightarrow 2\mathrm{H}_{2}\mathrm{O}\)
となることに注意する.このように電子数に着目して酸化還元反応を考えていく.