Over the summer I used some very powerful neodymium magnets to obtain some magnetite, with the hopes of being able to purify it to thermite-grade (I failed). I know that HCl reacts with Fe2O3, so I decided to use some of my magnetite to observe a Fe3O4 reaction. When I put the reactants in the test tube, I noticed that only a few bits of sand mixed in with the magnetite seemed to be reacting. A few days later, I ran a magnet over the solid left at the bottom of the test tube, and it's still magnetite. Why didn't it react? I suppose it's possible that it reacted, and the Fe3O4 was in excess, but the test tube doesn't seem to have lost any magnetite at all.

umm generally you wouldnt see a reaction... when you add hcl to rust nothing happens, at least visibly

After a few days, you would notice that the rust was gone...

umm generally you wouldnt see a reaction... when you add hcl to rust nothing happens, at least visibly

 

Are you kidding? Maybe you should try it for yourself. You should actually observe a steady reaction between rust and HCL. The rust will dissolve and the solution should turn green. The overall reaction is as follows:

 

Fe2O3 + 6HCL --> 2FeCl3 + 3H2O

But what about Fe3O4?

HCl (or any other other acid) only reacts with oxides at an appreciable rate if they are not calcined or crystalline. HCl indeed reacts (slowly) with fresh rust (hydrous Fe2O3) or freshly precipitated Fe(OH)3. The solutions formed this way are not green, but bright yellow, due to formation of the complex ion FeCl4(-).

 

Reaction with Fe3O4 probably is nill, because that also is a very compact, waterfree oxide.

 

This effect is quite common. Many freshly precipitated metal hydroxides or oxides easily dissolve in HCl or dilute H2SO4, but aged and dried oxides, or calcined oxides, usually do not dissolve anymore and are remarkably inert. Examples of these are Cr2O3, Al2O3, TiO2, Fe2O3, Nb2O5, Co3O4.

So then treating my sand-Fe3O4 mixture with HCl will work well to purify it (at least removing some impurities). Thats good.

HCl (or any other other acid) only reacts with oxides at an appreciable rate if they are not calcined or crystalline. HCl indeed reacts (slowly) with fresh rust (hydrous Fe2O3) or freshly precipitated Fe(OH)3. The solutions formed this way are not green' date=' but bright yellow, due to formation of the complex ion FeCl4(-).

 

Reaction with Fe3O4 probably is nill, because that also is a very compact, waterfree oxide.

 

This effect is quite common. Many freshly precipitated metal hydroxides or oxides easily dissolve in HCl or dilute H2SO4, but aged and dried oxides, or calcined oxides, usually do not dissolve anymore and are remarkably inert. Examples of these are Cr2O3, Al2O3, TiO2, Fe2O3, Nb2O5, Co3O4.[/quote']

 

The rust/HCl reaction is slow like I was trying to say, and it would make sense that the color of the solution would be yellow. Mine happened to turn green because my HCl came from blue toilet bowl cleaner. My bad....