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Does corrosion of a sample of iron increases or decreases its mass? I think that the mass will increase because of additional oxygen atoms.

Rody Oldenhuis
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Yoda
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  • Well, look at it this way: are any particles (atomic or subatomic, or photons) given off as the iron oxidises? Do those particles take away more energy (aka mass, thanks to Einstein) than the added mass of the oxygen atoms? – Carl Witthoft Jan 07 '14 at 15:59
  • possibly very related :) http://www.youtube.com/watch?v=zLm4sgnNsfw – Nephente Jan 07 '14 at 16:06

2 Answers2

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The following reactions take place :

$O_2 + 4 e^− + 2 H_2O → 4 OH^−$
$Fe → Fe^{2+} + 2 e^-$
$4 Fe^{2+} + O_2 → 4 Fe^{3+} + 2 O^{2−}$
$Fe^{2+} + 2 H_2O ⇌ Fe(OH)_2 + 2 H^+$
$Fe^{3+} + 3 H_2O ⇌ Fe(OH)_3 + 3 H^+$
$Fe(OH)_2 ⇌ FeO + H_2O$
$Fe(OH)_3 ⇌ FeO(OH) + H_2O$
$2 FeO(OH) ⇌ Fe_2O_3 + H_2O$
(source : Wikipedia )

Now since initially only $Fe$ was present and finally its oxides are present in the sample, there is definitely an increase in the mass of the sample.

Rijul Gupta
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Technically, rust expands, so it increases in mass during the rusting process (in a closed system), but it also has lower density AFAIK.

Stilez
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