Video: Determining the Oxidation State of C in H₂CO₃

What is the oxidation state of C in H₂CO₃? [A] −4 [B] −2 [C] +2 [D] +4 [E] +8

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Video Transcript

What is the oxidation state of C in H2CO3? (A) Minus four, (B) minus two, (C) plus two, (D) plus four, or (E) plus eight.

Oxidation states are generally used as a way to keep track of electrons in reduction–oxidation or redox reactions. In this question, we’ve been tasked with finding the oxidation state of carbon in H2CO3 or carbonic acid. There are a couple of rules about assigning oxidation states to atoms in a compound that we can use to help us solve this question.

The first is that oxidation states of atoms in a compound will sum to be the net charge of the compound. Since carbonic acid is a neutral species, we would expect the oxidation states of all of the atoms in the compound to sum to zero. The next thing we need to know is that the oxidation state of oxygen in a compound is usually minus two, except for when it’s in a peroxide. Since carbonic acid isn’t a peroxide, we’ll assign oxygen an oxidation state of minus two.

The final thing we need to know before we work out the oxidation state of carbon is that the oxidation state of hydrogen is usually plus one, except in metal hydrides. Again, carbonic acid is not a metal hydride, so we’re going to assign hydrogen an oxidation state of plus one.

Now, let’s sum up our total oxidation states so far so we can work out what the oxidation state of carbon must be. We have two hydrogens each with an oxidation state of plus one, which gives us a total of plus two. And we have three oxygens each with an oxidation state of minus two. So, that gives us a total of minus six. Since all these states need to sum to zero, that means carbon must have an oxidation state of plus four.

We were supposed to find the oxidation state of carbon in H2CO3. And we’ve just determined that carbon’s oxidation state in carbonic acid is plus four, which matches answer choice (D).

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