Sciencemadness Discussion Board

Why does vinegar only react with certain metal oxides?

SnailsAttack - 11-7-2022 at 02:27

Is there an explanation as to why 5% vinegar reacts with some metal oxides but not others? I've found that it dissolves zinc oxide readily, but not iron(III) oxide or manganese(II,III) oxide. Why? Copper(II) oxide also reacts with vinegar but very very slowly.

macckone - 11-7-2022 at 08:40

It depends on the enthalpy of formation and solvation.
Even chromium (III) oxide will eventually dissolve in 5% acetic acid.
You may not live long enough to observe it, but it happens.
All acetates are soluble but a lot of oxides are not which slows things down.

Texium - 11-7-2022 at 13:28

A more practical consideration is the surface area and physical structure of the metal oxide, which is largely determined by how it was prepared. Many metal oxides, especially those used in pottery, are calcined. This means they are heated to extremely high temperatures, which improves their stability in ceramics, but ruins their reactivity towards acids. Essentially, calcination increases particle size and drastically decreases the surface area of the particles. This can often make them quite inert to anything but fusion with molten alkali.

See here for a paper that discusses calcination temperature vs surface area and particle size for various metal oxides:

[Edited on 7-11-2022 by Texium]

CharlieA - 11-7-2022 at 13:47

Absent physical changes to the surface, say though calcining, doesn't the activity series of metals play a part in this?

Texium - 11-7-2022 at 15:21

Nope. If you have copper(II) oxide and dissolve it in HCl to make copper(II) chloride, there’s no redox reaction happening, so activity series is irrelevant. Reducing oxides to their metals is a different story.

teodor - 12-7-2022 at 01:03

At room temperature acetic acid exists mainly as a dimer, two carboxyl groups are connected via hydrogen bonding. Some solutes can break this dimer others are incapable of doing so.
Also among acetates probably only some alkali and alkali earths are ionic. Other salts are more or less covalent (Co, Li), has polymeric structure (some forms of Cu acetate), or chelate complexes (Cu, Cr, Zn, UO2 ...). This makes this family rather complex and does not always behave as the acid + oxide = normal salt.