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My understanding was that since the acceptance of Lavoisier's revised Easter Memoir of 1778, all acids were understood to be oxidising agents.*

However, formic acid is often listed as a reducing agent (i.e. the opposite of an oxidising agent), despite being an acid.

Please can you reconcile this apparent contradiction for me?

* What is meant by "oxidising" or "oxidising agent" has, of course, changed since then, now referring to electron transfer rather than necessarily involving the element oxygen.

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  • $\begingroup$ might be easier to describe iron as both reducing agent in Fenton reaction and oxidizer in H-W reaction, just depends on who wants the electrons more. $\endgroup$ – user27634 Mar 7 '16 at 16:35
  • $\begingroup$ I realize this is an old post, but since it's risen to the top again, I figured I would mention that the issue isn't with formic acid but, rather, with (modern interpretation of) Lavoisier's memoir. Simply put, formic acid is an acid because of its pKa. Additionally, it is a reducing agent because it oxidizes to $\ce{CO2}$ in the course of reducing other compounds (through hydrogenation). $\endgroup$ – SendersReagent Mar 7 '16 at 17:53
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Formic acid is a carboxylic acid with the formula of $\ce{CH_2O_2}$ and is easily oxidized to carbon dioxide. Other carboxylic acids are not as easy to oxidized.

Added:
If a compound gets oxidized (as formic acid can), then another compound has to be reduced. The compound that gets oxidized is called a reducing agent. The compound that gets reduced is called an oxidizing agent. Most carboxylic acids cannot oxidize other compounds (the acid is not strong enough).

Lavoisier was referring to mineral acids ($\ce{HNO_3, H_2SO_4, }$...) in the text. These acids can oxidized other compounds (such as changing iron into an iron ion).

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  • $\begingroup$ Thanks, but please could you expand on your answer to address the more fundamental question of how an acid can be a reducing agent (or, if you prefer: why this substance, that is a reducing agent, is still called an acid)? $\endgroup$ – sampablokuper Aug 23 '14 at 14:00
  • $\begingroup$ OK, I'm learning! From Wikipedia: "In organic chemistry, reduction more specifically refers to the addition of hydrogen to a molecule... In organic chemistry, good reducing agents are reagents that deliver H2." This is deeply weird to someone like me who's fairly ignorant of modern organic chemistry, but it explains the first part of your answer; and that in turn makes the next part of it comprehensible. It still, however, does not explain why formic acid is considered an acid. $\endgroup$ – sampablokuper Aug 25 '14 at 3:37
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    $\begingroup$ Formic acid is classified as a carboxylic acid. A carboxylic acid is an organic compound that can release a proton when mixed with water. They are different from minearal acids since not all the protons are released when the compound is mixed in water (only a fraction of the compound is disassociated). Formic acid is easily oxidized to carbon dioxide, whereas most other carboxylic acids require stronger oxidyzing agents. en.wikipedia.org/wiki/Carboxylic_acid $\endgroup$ – LDC3 Aug 25 '14 at 3:56
  • $\begingroup$ Thanks for the explanation of why carboxylic acids are acids and why, therefore, formic acid is an acid. I've accepted your answer :) $\endgroup$ – sampablokuper Aug 25 '14 at 12:19
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A molecule might be an oxidising agent under the electron transfer definition, and simultaneously be a reducing agent under the alternative definition: i.e. that reduction is a loss of oxygen or a gain in hydrogen.

For your specific question, formic acid can be deemed to be an oxidisation agent since it loses a hydrogen during protonation of another molecule. The really silly way to think about this is following deprotonation, formic acid has a higher percentage of oxygen than it did before, i.e. it possesses more oxygen than before.

The case of formic acid being a reducing agent is due to the more modern definition of electron transfer. To see the mechanism of this for clarity, see here. It's clear that the formic acid is donating an electron pair (i.e. itself is being oxidised) and is making the other molecule accept those electrons (i.e. that molecule is reduced). The formic acid is therefore a reducing agent.

So your original issue is due to two different definitions of oxidisation and reduction which still exist to this day, and which are by no means mutually exclusive!

EDIT: Of course, the case of formic acid is more simple because it isn't the acid doing the reducing, it's its conjugate base i.e. formate. So formic acid is an oxidising agent under the proton definition, yet formate is an reducing agent under the electron transfer definition.

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    $\begingroup$ Thanks. I followed your link, but it didn't clarify anything for me, not least because it seems to be talking about deprotonated formic acid rather than simply formic acid. I'm still finding this all a bit baffling, so further clarity would be welcomed! $\endgroup$ – sampablokuper Aug 25 '14 at 3:25
  • $\begingroup$ Yes of course! You've got it right (I mentioned it in my edit). I meant in general that it's possible for a molecule to be an oxidising agent and a reducing agent under the two different definitions that exist, but formic acid is more simple: formic acid is the oxidising agent however it's formate that is the reducing agent! $\endgroup$ – user7232 Aug 25 '14 at 12:04

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