[edawards]

do alloys have any chemical bond? if alloys are mixture do they have possible to make a chemical bond? do mixtures have any chemical bond? i heard that alloys change the chemical properties of
made of their metals but we know alloys are mixtures and mixtures keep their chemical properties

[lavoisier]

The question is very interesting, almost philosophical in that it's aimed to solving an apparent contradiction.
I guess the answer lies in the fact that metallic bonds are quite different from covalent bonds. When you melt the two metals together, they actually lose their identity. In old theories they were described as 'nuclei held together by a sea of electrons'.
I'm not a physical chemist, but I would say alloys ARE mixtures. Your definition says it clearly: in a mixture the components keep their properties. If they don't, it's not a mixture.

A mechanical mixture of Fe and S can be separated by physical methods which don't break any chemical bond (e.g. you can attract Fe with a magnet), but what if you wanted to get back the pure metals from an alloy? Shouldn't you give it enough energy to break this more or less defined 'metallic bond'?

PS Physical chemists out there please correct me if I'm wrong.

[Borek]

what if you wanted to get back the pure metals from an alloy?

Melt, boil and distill. Apart from technical difficulties it should work.

Most alloys are mixtures, but AFAIR sometimes they contain intermetallic compounds.

[Borek]

Your definition says it clearly: in a mixture the components keep their properties. If they don't, it's not a mixture.

One thing more: while components of the mixture retain their properties, properties of mixture can be different.

Physical properties - water boils at 100 deg C, ethanol at about 78, 50/50 mixture slightly over 80.

Chemical properties - one of the components can dominate mixture behaviour. IIRC stainless steel gets covered with chromium oxides, thus iron is not rusting so easily. That's without creating any intermetallic compounds, that's just the way it is

[tamim83]

I want to say that alloys are compounds, that is the metals in an alloy are bonded together via metallic bonds.  I actually did an xray diffraction experiment on some copper-nickel alloys I made that suggests that alloys are compounds.  We made alloys with different mole fractions of copper in them.  The x-ray diffraction pattern showed that the alloy spectrum was different from that of a physical mixture of copper and nickel.  The physical mixture just shows an addition of the spectra of both metals while the alloy spectrum is its own distinct spectrum (i.e.- you cannot see any peaks from the two metals separately if you really made an alloy). 

But, I am not so sure about separating an alloy into two separate metals, if that would be a physical process or not.  Metallic bonds are just so different from making ionic and covalent bonds, but electrons are involved in all three bonds.  It's really inky I suppose... 

[Borek]

The x-ray diffraction pattern showed that the alloy spectrum was different from that of a physical mixture of copper and nickel.

IIRC x-ray diffraction is just about atoms localisation, not about bonds. And as such it must give different results depending on the proportions of components, as the structure will be different.

Crystallography is AWK's domain, his comments will be most interesting probably.

[lavoisier]

I'm not a physical chemist, but I would say alloys ARE mixtures. Your definition says it clearly: in a mixture the components keep their properties. If they don't, it's not a mixture.

I quote myself because I wrote it wrong: I meant 'I would say alloys AREN'T mixtures'. Sorry. Besides, the rest of my reasoning was conflicting with the statement that alloys were mixtures.
However, I see this has raised quite a lot of comments and different opinions, and I still wouldn't be sure of the answer.

The two metals in an alloy DO lose their identity in some cases. Brass (Zn+Cu) is not attacked by dilute mineral acids, whereas pure Zn is. If it was a mechanical mixture, as finely divided as you want, the acid would still be able to go and dissolve the Zn crystals.

Yet in other cases it doesn't happen. For instance, if I remember well, the preparation of Raney Nickel involves treating a Ni-Al alloy with aqueous NaOH. Al is dissolved by NaOH, leaving a microporous Ni.

I'm not entirely convinced by Borek's reply, i.e. that you can separate the metals by distilling the alloy, hence it's a mixture. As it was said before, the metallic bond is different from covalent bonds, so I wouldn't be sure that the energy you supply for the distillation isn't actually breaking the bonds instead of just breaking the interactions between individual, discrete metal particles.

In fact there are countless organic compounds that decompose if distilled at atmospheric pressure, because they have a covalent bond which is weak enough to be broken by the available thermal energy.
Finding two different substances in the distillate fractions wouldn't make you conclude that the starting material is a mixture, would it?

But still, I may be wrong...

[Borek]

I'm not entirely convinced by Borek's reply, i.e. that you can separate the metals by distilling the alloy, hence it's a mixture. As it was said before, the metallic bond is different from covalent bonds, so I wouldn't be sure that the energy you supply for the distillation isn't actually breaking the bonds instead of just breaking the interactions between individual, discrete metal particles.

Good point. Another good point is, that alloys have their properties (different from the properties of their components) to some extent regardless of their composition, which is clearly a point against compound existence; compound in the classical meaning (like water, with precisely given ratio of atoms).

I think it is just a matter of convention. Atoms in metal are - as tamim already wrote - connected by metallic bonds. Metallic bond is different from classic bond (ionic, covalent) and it may exist between atoms of the same metal as well as between atoms of different metals. Whether we will call such atoms connected with metallic bond a compound or mixture is a matter of convention.

Just like it is a matter of convention whether Pluto is a planet

For me alloy is not a compound, even if it may have some characteristics of such. There are intermetallic compounds, but they have well defined stoichiometry.

[tamim83]

Yeah, I am not that certain about it myself, but I am leaning towards compound. 

As for Pluto, heck yeah it's a planet, my favorite as a mater of fact  . 

[Will]

Another good point is, that alloys have their properties (different from the properties of their components) to some extent regardless of their composition,

Just to back up what Borek said I heard that Zn/Zr alloys are ferromagnetic below 35K (28.5K for ZnZr₂ alloy according to wikipedia), and neither Zn and Zr are ferromagnetic. There are quite a few other examples of a similar nature.

[kayamusty]

I want to say that alloys are compounds, that is the metals in an alloy are bonded together via metallic bonds.  I actually did an xray diffraction experiment on some copper-nickel alloys I made that suggests that alloys are compounds.  We made alloys with different mole fractions of copper in them.  The x-ray diffraction pattern showed that the alloy spectrum was different from that of a physical mixture of copper and nickel.  The physical mixture just shows an addition of the spectra of both metals while the alloy spectrum is its own distinct spectrum (i.e.- you cannot see any peaks from the two metals separately if you really made an alloy). 

But, I am not so sure about separating an alloy into two separate metals, if that would be a physical process or not.  Metallic bonds are just so different from making ionic and covalent bonds, but electrons are involved in all three bonds.  It's really inky I suppose... 

Alloys are not compound. they are solution. so there is no chemical bonds between them.

[lavoisier]

Alloys are not compound. they are solution. so there is no chemical bonds between them.

Kayamusty, I am with you in that I don't think alloys are 'compounds' in the most general sense (as in organic compounds, for instance).

But if they are 'solutions' (or 'mixtures', as thermodynamic more generally defines solutions), it's not clear why the components sometimes lose their individual chemical properties. See my previous post, I think I pointed it out quite clearly with the examples of brass and Raney nickel.

Let's imagine that I mix methanol and toluene. This is a mixture, or a solution if you like. And in fact, if I add sodium it will still react with methanol; if I add water, most methanol will go into the aqueous layer and most toluene will stay in the organic layer.

So why the Zn which is contained in an alloy such as brass (Zn+Cu) doesn't react with mineral acids, leaving pure Cu?

Don't you think that some interactions stronger than simple VDV forces are playing a role here, if they are able to change the electrochemical potential of an element?

[Will]

I think it depends on the alloy you are talking about. Some people are talking about alloys like they are all the same. Alloys can be compounds, solid solutions or mixtures AFAIK.

Don't you think that some interactions stronger than simple VDV forces are playing a role here

Yes

It might be worth someone creating a poll on what an alloy is if there are so many strong opinions on it.