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我们来看看其他

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组分原子或分子的氧化态

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那么假设有氧化镁(MgO)

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氧化镁MgO

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我用不同的颜色表示氧

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00:00:16,580 --> 00:00:18,970
那么它们的氧化态是什么？

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你可能已经知道了

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但还是来看看元素周期表吧

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因为再熟悉一下也无伤大雅

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呐 镁(Mg)

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镁有两个价电子

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它是第二主族元素

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它很乐意抛弃这两个电子

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而氧(O) 地球人都知道

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它是电负性最强的原子之一

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它电负性太强了

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以致于“氧化”都是以氧命名的

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然后我们知道氧喜欢得到两个电子

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所以这简直是天作之合

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这玩意儿想丢掉两个电子

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而这玩意儿想得到两个

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那么接下来会怎么样？

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镁会失去两个电子

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它本来是中性的

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那么假设它会带上2单位正电荷

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然后 氧就带上2单位负电荷了

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因为它拿走那两个电子了

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所以在氧化镁分子里

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镁的氧化态是+2

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而氧的氧化态是-2

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我们现在来做难一点的

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假设是氢氧化镁 Mg(OH)2

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氢氧根是(OH)2

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这里氢氧根有两个

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好啦 我的思路还是… 嗯

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镁很乐意失去它的电子 两个电子

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这样它就会带上正电

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这种假设下它的价态是正的

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那么我的思路是

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嘿 镁是+2

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我先写下来

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记住啦 如果这是个电中性化合物

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为了使一切和谐

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全部价态加起来要等于0

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那我们来看看行不行

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呐 氧

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我武断认为它的氧化态

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应该是-2

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那我写下来吧

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是-2

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而氢 如果它和氧成键…

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记住 这情况下 氢会先和氧

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成键

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然后一起和镁成键

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所以氢和氧成键

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氢 如果它和镁成键

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你可能会觉得 噢

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可能氢会拿走那两个电子

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然后它就变成负价的了

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不过氢和氧成键的时候

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它是放弃电子那个

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它只有一个电子可以放弃

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所以它是+1价

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嗯… 开始你可能会说 好了

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我把氧化态都加起来

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+2-2就是0 还有+1

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我就得到氧化态是+1

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Sal 这没道理啊

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这是个电中性化合物啊

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你要留意的是 噢 错了

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你可是有两个氢氧根的啊

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那你应该先算出

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氢氧根的总氧化态

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那是-2+1

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所以整个氢氧根

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总共是-1价

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然而你有两个 对吧？

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你这有两个氢氧根

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所以每个氢氧根对整个化合物的

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氧化态的贡献是-1

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而这有两个

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所以这是-2 再加上镁的+2

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全部加起来是0

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搞定了

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接下来 我想顺便做点别的

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我想重新回到某些问题上

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不过我想顺便讲讲

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一些术语

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因为我已经某程度上用了“氧化态”、

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“被氧化” 或者“被还原”

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然而 我们讲了那么多

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水解离的问题…

93
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那 2份水

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反应处于平衡

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生成1份的H3O+和OH-

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很明显 这些都在溶液里

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现在 先看水分子

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这里水的各个氧化态是多少？

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呐 上集我们做过这个了

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氧的氧化态是-2

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因为它从两个氢那里勾搭了两个电子

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每个氢都贡献了1个电子

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所以氢的氧化态是+1

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那么我们来看这个分子

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都加起来

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因为有两个+1的氢

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所以是+2

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+2加上氧的-2 等于0

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水是中性化合物

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那这边 氧化态又是多少？

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也就是其中一个氢离开了它的水分子

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然后粘上了另一个水分子

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但它没带走电子

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所以它把电子留这里了

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那么这个氧的氧化态还是-2

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而这个氢还是+1

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所以你要用-2+1

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就得到-1

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这次就没问题

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因为氢氧根的确是带负电的

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那这个的价态怎么样？

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每个氢都是+1

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然后这个氧是-2

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那么如果你看整个离子的电荷

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3个氢分别是+1 那就是+3

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只是加起来了

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还有-2

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所以就是+3-2 所以整个离子是

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+1价的 而事实正是如此

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现在 我想问你们

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有哪个原子的氧化态变了吗？

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这儿全部的氢…

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或者说这两份水的

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或者是两个水分子的

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那我就有4个氢了

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对吧？ 而它们的氧化态都是+1

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等式右边 有4个氢

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全部的氧化态都是+1

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那么它们都是+1价 这个反应里…

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无论是哪个反应方向

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氢都没有被氧化

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它的氧化态没有变

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可能氢在前面形成水的反应里

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被氧化了

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不过这个反应里 它没有

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同理 氧…

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有两个氧分子 是原子 这儿

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每个都是-2价

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这里有两个氧原子

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每个都是-2价

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对于这个反应 至少

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从氧化态看 没有电子转手

153
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所以这不是一个氧化还原反应

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我会在下集里详细地讲

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不过我要讲清楚

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这儿没东西被氧化或还原

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因为它们的氧化态保持不变

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因为有时我会说 嘿 看

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镁的氧化态是+2

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而氧的氧化态是-2

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镁被氧化了

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它失去了两个电子

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而氧被还原

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它得到了两个电子

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那么我指的是某个生成它的反应

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但是也不总是这样的

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你可以假设一个

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不必发生的反应

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但是镁的氧化态

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一定是+2

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而氧的氧化态

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或者说氧化态 是-2

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不过我相信我说镁被氧化时

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你懂我的意思

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00:07:40,310 --> 00:07:43,080
其实就是 镁通过失去两个电子

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从一个中性的镁

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变成了一个带正电的镁

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所以它被氧化了

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接下来 我们来做些难题

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过氧化氢(H2O2)

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我已经说过几次了

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氧是趋向于变成-2价的

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这个是-1

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我想你看出特点了 这些元素都是+1

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氢是+1或-1

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这些都是+2

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我想你明白这规律了

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这就是你希望得到或失去电子

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你会说 呐

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水的氧通常是-2价的

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所以这应该…

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好啦

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氢是+1 因为它和水成键

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而氧是-2

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如果你这样想 马上就会碰到难题了

196
00:08:31,310 --> 00:08:32,750
这是个中性分子…

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嗯 两个氢就是+2

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00:08:35,430 --> 00:08:36,880
两个-2的氧

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00:08:36,890 --> 00:08:37,920
是-4

200
00:08:37,940 --> 00:08:39,470
所以整个氧的氧化态

201
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最终是-4

202
00:08:41,530 --> 00:08:42,860
肯定不是这样的

203
00:08:42,870 --> 00:08:44,120
因为这个不带电

204
00:08:44,140 --> 00:08:45,790
这儿就有难题了

205
00:08:45,800 --> 00:08:47,480
而有难题是因为…

206
00:08:47,490 --> 00:08:50,410
如果你仔细观察过氧化氢的结构

207
00:08:50,440 --> 00:08:53,550
氧之间是成键的

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00:08:53,570 --> 00:08:55,170
这就是“过氧”

209
00:08:55,180 --> 00:08:59,630
而每个氧又和氢成键

210
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所以这时

211
00:09:01,730 --> 00:09:03,960
特别是第一年的化学课上

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00:09:03,970 --> 00:09:07,370
过氧根 尤其是过氧化氢

213
00:09:07,390 --> 00:09:08,840
趋向于当个特例

214
00:09:08,860 --> 00:09:11,390
也有别的 但这个是个特例

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00:09:11,410 --> 00:09:15,350
这时氧的氧化态的确不是-2

216
00:09:15,370 --> 00:09:17,090
我们来看这个并算算

217
00:09:17,100 --> 00:09:18,600
过氧化氢的氧的

218
00:09:18,610 --> 00:09:20,080
氧化态是多少

219
00:09:20,090 --> 00:09:23,030
这时 在氢氧键中

220
00:09:23,040 --> 00:09:25,270
氧会吸电子

221
00:09:25,280 --> 00:09:27,510
而氢会失去电子

222
00:09:27,520 --> 00:09:29,000
那么氢是+1

223
00:09:29,020 --> 00:09:30,040
另一边也同理

224
00:09:30,060 --> 00:09:33,160
氧 至少在这个键上 得到+1价

225
00:09:33,180 --> 00:09:34,410
因为它会得到一个电子

226
00:09:34,420 --> 00:09:36,710
那氧之间的键呢？

227
00:09:36,720 --> 00:09:39,660
嗯… 没理由一个氧会

228
00:09:39,670 --> 00:09:41,230
吸走另一个氧的电子

229
00:09:41,250 --> 00:09:43,850
所以这不会对它的氧化态的值

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有任何影响

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因此这情况下 这个氧的氧化态是-1
【译者注：板书更正，氧是-1价而不是+1价】

232
00:09:48,010 --> 00:09:52,770
这个氧也是-1

233
00:09:52,780 --> 00:09:59,400
所以每个氢的氧化态是+1

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00:09:59,420 --> 00:10:03,070
我们说过氧离子中氧的氧化态是-1

235
00:10:03,090 --> 00:10:05,740
所以总的值是0

236
00:10:05,750 --> 00:10:08,790
2乘+1 加上2乘-1 等于0

237
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这是个特例

238
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不过熟悉它很重要

239
00:10:12,980 --> 00:10:14,330
我们再做一个

240
00:10:14,340 --> 00:10:15,840
碳酸铁(Ⅲ)

241
00:10:15,850 --> 00:10:17,690
现在第一次…

242
00:10:17,700 --> 00:10:19,970
我记得我们第一次遇到碳酸铁(Ⅲ)

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你可能会想 呃

244
00:10:21,230 --> 00:10:22,960
为什么这叫碳酸铁(Ⅲ)

245
00:10:22,970 --> 00:10:25,250
而这儿只有两个铁分子

246
00:10:25,260 --> 00:10:26,430
两个铁原子？

247
00:10:26,440 --> 00:10:27,820
你马上就会学到了

248
00:10:27,830 --> 00:10:29,410
我们来看看氧化态

249
00:10:29,430 --> 00:10:31,410
氧的

250
00:10:31,420 --> 00:10:35,870
氧的氧化态 一般是-2

251
00:10:35,890 --> 00:10:38,210
是-2

252
00:10:38,220 --> 00:10:40,650
现在 如果碳(C)连在氧(O)上…

253
00:10:40,670 --> 00:10:42,540
我们看元素周期表吧

254
00:10:42,550 --> 00:10:45,680
碳和氧成键

255
00:10:45,700 --> 00:10:48,510
碳怎么变都行

256
00:10:48,520 --> 00:10:51,270
碳 有时它喜欢贡献电子

257
00:10:51,300 --> 00:10:52,870
有时它希望得到电子

258
00:10:52,890 --> 00:10:55,100
而碳和氧成键是

259
00:10:55,110 --> 00:10:58,110
这里可是个电子狂

260
00:10:58,130 --> 00:11:01,140
如果一定要说谁夺走了电子

261
00:11:01,150 --> 00:11:02,670
答案会是氧

262
00:11:02,680 --> 00:11:03,890
对吧？

263
00:11:03,900 --> 00:11:06,860
所以碳会贡献它的电子

264
00:11:06,870 --> 00:11:09,760
不过碳能拿出几个电子呢？

265
00:11:09,780 --> 00:11:11,470
嗯 看看

266
00:11:11,480 --> 00:11:14,990
它有1 2 3 4个价电子

267
00:11:15,010 --> 00:11:16,070
所以它真正最多能

268
00:11:16,080 --> 00:11:18,560
贡献4个价电子

269
00:11:18,570 --> 00:11:21,450
回到碳酸根

270
00:11:21,460 --> 00:11:26,110
所以碳最多能

271
00:11:26,120 --> 00:11:29,370
贡献4个价电子

272
00:11:29,390 --> 00:11:32,400
那么碳酸根的总的氧化态

273
00:11:32,420 --> 00:11:33,870
是多少？

274
00:11:33,880 --> 00:11:36,390
CO3的

275
00:11:36,400 --> 00:11:38,770
所以这是+4价

276
00:11:38,790 --> 00:11:40,290
因为它只有4个了

277
00:11:40,300 --> 00:11:42,680
如果它和氧成键 它就会贡献全部

278
00:11:42,690 --> 00:11:43,880
氧是个超级电子狂

279
00:11:43,890 --> 00:11:46,270
每个氧是-2

280
00:11:46,280 --> 00:11:48,790
我们来想想

281
00:11:48,800 --> 00:11:54,130
我有+4 加【译者注：口误】 3乘-2

282
00:11:54,150 --> 00:11:55,210
对吧？

283
00:11:55,260 --> 00:11:57,220
我有3个氧原子

284
00:11:57,240 --> 00:12:00,350
所以是4-6等于-2

285
00:12:00,370 --> 00:12:02,850
所以我们可以把整个碳酸根

286
00:12:02,860 --> 00:12:05,790
看作是-2价的

287
00:12:05,810 --> 00:12:10,640
那么 如果整个碳酸根是-2

288
00:12:10,660 --> 00:12:14,370
它对整个分子的氧化态的

289
00:12:14,390 --> 00:12:16,940
贡献是…

290
00:12:16,950 --> 00:12:18,730
这个碳酸根

291
00:12:18,740 --> 00:12:20,630
我们有3个碳酸根

292
00:12:20,640 --> 00:12:22,630
每个的贡献是-2

293
00:12:22,640 --> 00:12:24,680
所以这有-6的贡献值

294
00:12:24,700 --> 00:12:28,360
如果这是-6 并且这是个中性分子

295
00:12:28,380 --> 00:12:32,210
那么我们两个铁就必须要

296
00:12:32,220 --> 00:12:35,370
贡献+6价

297
00:12:35,380 --> 00:12:36,820
因为全部加起来是0

298
00:12:36,830 --> 00:12:40,160
如果两个铁加起来

299
00:12:40,180 --> 00:12:43,380
贡献了+6价

300
00:12:43,400 --> 00:12:46,410
那么每个铁就是+3价

301
00:12:46,420 --> 00:12:49,600
或者 在我们的推理里 如果这成立

302
00:12:49,620 --> 00:12:52,650
至少每个铁会有3个电子

303
00:12:52,670 --> 00:12:54,800
乐意和碳酸根待一起

304
00:12:54,840 --> 00:12:58,120
那为什么这叫碳酸铁(Ⅲ)咧？

305
00:12:58,120 --> 00:13:00,400
我想现在你就懂了

306
00:13:00,420 --> 00:13:04,870
因为这是铁价态为3的情况

307
00:13:04,880 --> 00:13:06,070
铁… 很多金属

308
00:13:06,090 --> 00:13:07,550
特别是过渡金属

309
00:13:07,560 --> 00:13:09,560
能有几种价态

310
00:13:09,570 --> 00:13:11,290
如果你有碳酸铁(Ⅲ)

311
00:13:11,300 --> 00:13:12,680
实际上就表示

312
00:13:12,690 --> 00:13:14,330
它的价态是3

313
00:13:14,340 --> 00:13:17,810
或者说分子里铁的氧化态

314
00:13:17,820 --> 00:13:19,600
会是+3

315
00:13:19,610 --> 00:13:21,940
现在 我们再做一个

316
00:13:21,960 --> 00:13:23,030
这很有趣

317
00:13:23,050 --> 00:13:24,200
醋酸(乙酸)

318
00:13:24,210 --> 00:13:25,560
我想这是第一次

319
00:13:25,570 --> 00:13:29,990
我秀出醋酸的结构式

320
00:13:30,010 --> 00:13:33,160
我不会讲太多有机化学的内容

321
00:13:33,170 --> 00:13:37,620
不过我们会看到不同的价态

322
00:13:37,640 --> 00:13:39,200
或者说氧化态

323
00:13:39,210 --> 00:13:41,560
[少一句]

324
00:13:41,570 --> 00:13:43,280
有时候你会看到这个形式

325
00:13:43,290 --> 00:13:44,600
你会觉得OK

326
00:13:44,610 --> 00:13:48,050
氧 每个氧会是-2

327
00:13:48,070 --> 00:13:50,890
[少一句]

328
00:13:50,920 --> 00:13:53,270
每个氢会是+1

329
00:13:53,280 --> 00:13:56,560
[少一句]

330
00:13:56,570 --> 00:13:57,600
目前的情况如何？

331
00:13:57,630 --> 00:14:01,520
这些氧一共就是-4

332
00:14:01,540 --> 00:14:03,290
至于氢

333
00:14:03,300 --> 00:14:08,930
这里是+3 然后这还有+1

334
00:14:08,950 --> 00:14:12,070
你把它们都加起来就是0 你会想 噢

335
00:14:12,090 --> 00:14:14,890
所以碳就没氧化态咯

336
00:14:14,900 --> 00:14:16,780
它们的氧化态一定是0

337
00:14:16,790 --> 00:14:20,480
因为只看氢和氧

338
00:14:20,490 --> 00:14:23,040
我们已经是0了

339
00:14:23,050 --> 00:14:25,670
那么我们来看是否确实如此

340
00:14:25,680 --> 00:14:28,770
所以如果碳和氢成键

341
00:14:28,780 --> 00:14:30,230
谁更吸电子？

342
00:14:30,240 --> 00:14:34,670
碳氢键

343
00:14:34,680 --> 00:14:37,160
越往右边 电负性越强

344
00:14:37,170 --> 00:14:39,330
碳的电负性更强

345
00:14:39,350 --> 00:14:42,690
比起氢 碳更喜欢保留电子

346
00:14:42,700 --> 00:14:43,800
或者说贪图电子

347
00:14:43,810 --> 00:14:45,470
所以在氧化还原角度看

348
00:14:45,480 --> 00:14:46,980
氢失电子

349
00:14:46,990 --> 00:14:50,600
它是个共价键 不过当然

350
00:14:50,620 --> 00:14:51,870
我们知道讨论氧化态的时候

351
00:14:51,880 --> 00:14:53,260
就假装是离子键

352
00:14:53,280 --> 00:14:56,620
所以这时 你的氢将会

353
00:14:56,630 --> 00:14:58,140
失电子

354
00:14:58,160 --> 00:15:00,170
所以它们的氧化态分别是+1

355
00:15:00,190 --> 00:15:02,940
这和我们目前知道的一致

356
00:15:02,950 --> 00:15:04,740
不过其实这是另一回事

357
00:15:04,760 --> 00:15:06,680
我做这里这练习的时候

358
00:15:06,710 --> 00:15:07,750
我第一时间假设氢

359
00:15:07,760 --> 00:15:10,810
氧化态为+1

360
00:15:10,820 --> 00:15:11,850
我这样是因为 噢

361
00:15:11,860 --> 00:15:13,620
分子里的剩下的只有碳和氧

362
00:15:13,630 --> 00:15:15,800
它们的电负性都更大

363
00:15:15,810 --> 00:15:17,950
所以氢会变成它的+1价

364
00:15:17,960 --> 00:15:19,670
如果这里是

365
00:15:19,680 --> 00:15:21,500
碱金属 或者碱土金属

366
00:15:21,520 --> 00:15:22,910
我就不会那么肯定了

367
00:15:22,930 --> 00:15:24,540
我会说 氢可能会夺走它们的电子

368
00:15:24,550 --> 00:15:26,470
好啦

369
00:15:26,490 --> 00:15:30,270
所以这些都会给碳一个电子

370
00:15:30,290 --> 00:15:32,270
因此光从氢身上

371
00:15:32,290 --> 00:15:39,200
碳就得到-3价了

372
00:15:39,220 --> 00:15:41,450
这些失电子

373
00:15:41,460 --> 00:15:43,170
而这家伙得到了3电子

374
00:15:43,190 --> 00:15:44,620
所以它的价态减少3

375
00:15:44,630 --> 00:15:47,450
碳碳键 嗯 没理由

376
00:15:47,470 --> 00:15:49,620
一个碳可以从另一个碳上夺走电子

377
00:15:49,630 --> 00:15:51,400
碳碳生而平等

378
00:15:51,410 --> 00:15:53,770
所以没有电子转移

379
00:15:53,800 --> 00:15:56,120
所以这碳被氧化了3价

380
00:15:56,140 --> 00:15:57,490
现在这边又怎么样呢？

381
00:15:57,500 --> 00:16:00,130
已知这氢的氧化态

382
00:16:00,140 --> 00:16:02,750
会是+1

383
00:16:02,770 --> 00:16:04,830
它会把电子给这个氧

384
00:16:04,850 --> 00:16:07,880
而这个氧 就跟大部分氧一样

385
00:16:07,890 --> 00:16:09,680
需要吸两个电子

386
00:16:09,690 --> 00:16:12,960
一个来自碳 一个来自氢

387
00:16:12,970 --> 00:16:15,800
所以氧的氧化态是-2

388
00:16:15,810 --> 00:16:19,100
这个氧也需要吸走两个电子

389
00:16:19,110 --> 00:16:20,700
这情况下 它们两个都会

390
00:16:20,720 --> 00:16:22,100
来自这个橙色的碳

391
00:16:22,120 --> 00:16:24,490
那么它的氧化态就是-2

392
00:16:24,510 --> 00:16:26,720
所以这个碳的氧化态是多少？

393
00:16:26,730 --> 00:16:30,190
它给了两个电子这儿家伙

394
00:16:30,200 --> 00:16:35,350
然后有给了1个电子下面这个氧

395
00:16:35,360 --> 00:16:37,700
提醒一下 这家伙得到一个碳的电子

396
00:16:37,710 --> 00:16:38,830
和一个氢的

397
00:16:38,840 --> 00:16:41,600
这里没了1个 那里没了两个

398
00:16:41,610 --> 00:16:43,390
它失去了3个电子

399
00:16:43,410 --> 00:16:47,570
所以事实上 它是+3价的

400
00:16:47,590 --> 00:16:52,010
因此结果是醋酸的碳的

401
00:16:52,020 --> 00:16:54,990
平均氧化态是0

402
00:16:55,010 --> 00:16:56,990
因为如果你取-3和+3的平均值

403
00:16:57,000 --> 00:16:58,300
会得到0

404
00:16:58,320 --> 00:17:00,520
这就是为什么我说 噢可能它们是0

405
00:17:00,530 --> 00:17:03,180
但如果你真的写出它们的氧化态

406
00:17:03,200 --> 00:17:07,300
这个绿色的碳 氧化态-3

407
00:17:07,310 --> 00:17:10,020
而这个橙色的C 橙色碳

408
00:17:10,030 --> 00:17:12,640
氧化态是+3

409
00:17:12,660 --> 00:17:14,050
如果你懂了这个例子

410
00:17:14,070 --> 00:17:16,060
我并不觉得这太复杂

411
00:17:16,080 --> 00:17:21,880
你会变成氧化态迷的

412
00:17:21,890 --> 00:17:23,750
那么我想你都准备好了

413
00:17:23,760 --> 00:17:25,150
下集 我们会开始研究

414
00:17:25,160 --> 00:17:28,150
氧化还原反应