4.11
氧化数
在氧化还原反应中，电子在反应物种之间发生转移。电子转移由称为氧化数（或氧化态）的假设数描述。它表示使用一组规则分配的原子或元素的有效电荷。
氧化编号（氧化状态）
在离子化合物的情况下，基于在反应物种之间转移的电子数来分配氧化数。例如，在形成氯化钙（CaCl 2 ）时，钙失去两个价电子，两个氯原子各获…
在金属和非金属之间的氧化还原反应 通常都包含电子的完整转移 才能够形成离子化合物；因此，它们 很容易可以区别。但当非金属之间 只有部分电子转移时，是很难被定义为氧化还原反应的。氧化还原反应的特点是 用来表达移动电子的 原子氧化状态的改变 而一个化合物中原子的氧 化态或是氧化数就是 如果在每个异核键当中的共享电子 完全转换成更多的 负电原子时会需要充电的量。同核键则是很均匀的分割。举例来说，在气态氯化氢当中，氯就更具有负电性。如果氢的电子完全转移 给氯的话，氯就会带一负电，成为相对应的一负电的氧化态，而氢 就会带一正电后成为相对应的 一正电氧化态。在元素型态时，氧化态可以被分配给原子，而且大部分的离子和化合物都有特定的规则。以下是三个一定会遵守的原则。而剩下的原则只会等到前三个都满足了之后，才会一个一个被应用。这些原则无论二氧化 硫和碳酸钙形成反应后是否为氧化反应，都会被应用。根据第一条规定，处于自由状态的元素，它的氧化数为0，因此元素硫和氧的氧化数就 都是0。根据第三条规定，既然中性化合物的氧化数总合为0，那么在SO_2里硫和氧的氧化数 加起来也一定要是0。又依据第6条规定，在SO_2里每个氧的氧化数是负2。两个氧就代表着总和为负4。因此，硫的氧化数就必须要是正4。而硫的氧化数因此从0增加到 正4时，我们可以得知它被氧化了。另一边，当氧的氧化数从0减少到负2时，就知道它被还原了。这就是所谓的氧化还原反应。另外，在碳酸钙的例子当中，在所有的3个化合物当中，氧的氧化数为负2，而且在氧化钙和碳酸钙中钙的氧 化数为正2。根据第三条规定，碳在氧化钙和碳酸钙中必须要是正4。既然在反应当中，原子的氧化数并没有改变，那它就不是一个氧化还原反应。
View the full transcript and gain access to JoVE Core videos
Q1: What is an oxidation number and how does it relate to electron transfer?
An oxidation number is a hypothetical charge assigned to an atom in a compound that represents the effective charge if shared electrons in heteronuclear bonds were completely transferred to the more electronegative atom. Oxidation numbers track electron movement in oxidation reduction reactions electron transfer reactions, helping identify which atoms are oxidized or reduced during chemical reactions.
Q2: How do you assign oxidation numbers to elements in their free state?
According to the first rule for assigning oxidation numbers, all free elements have an oxidation number of zero, regardless of whether they are monoatomic, diatomic, or polyatomic. This applies to elemental forms like O₂, S₈, or individual atoms before they form compounds.
Q3: What are the key differences between oxidation numbers in ionic and covalent compounds?
In ionic compounds, oxidation numbers are assigned based on the number of electrons transferred between reacting species. In covalent compounds, electrons are shared rather than transferred, so oxidation numbers are assigned hypothetically by assuming complete transfer to the more electronegative atom. Both allow identification of redox reactions.
Q4: Why is oxygen typically assigned an oxidation number of −2?
Oxygen has an oxidation number of −2 in most compounds because it is highly electronegative and typically gains two electrons when bonding. The exception is in peroxides, where oxygen has an oxidation number of −1 due to the O-O bond structure.
Q5: How can you determine if a reaction is a redox reaction using oxidation numbers?
A reaction is a redox reaction if the oxidation numbers of atoms change during the reaction. If oxidation numbers remain unchanged, it is not a redox reaction. For example, in the formation of SO₂, sulfur's oxidation number increases from zero to +4 while oxygen decreases from zero to −2, indicating electron transfer and confirming it as a redox reaction.
Q6: What is the relationship between oxidation numbers and the charge of polyatomic ions?
For polyatomic ions, the sum of oxidation numbers equals the charge on the ion. For neutral compounds, the sum of oxidation numbers equals zero. This rule helps determine unknown oxidation numbers when other atoms' oxidation states are known in complex compounds.
Q7: How does electronegativity influence the assignment of oxidation numbers in covalent bonds?
In covalent bonds, the more electronegative atom is assigned the electrons in the bond when calculating oxidation numbers. For example, in hydrogen chloride, chlorine is more electronegative, so it receives hydrogen's electron, giving chlorine an oxidation number of −1 and hydrogen +1, even though the bond is covalent.