Acids & Alkali Metals: What's the Difference?

[Microbiologist]

Hi everybody,

Please forgive my question which is probably very obvious, but why are acids based on H ion concentration? Why is HCl a strong acid whereas LiCl is a salt. Both H and Li form + ions in solution. Conversely, why are OH- bases? Why can't NaCl be a strong base?

I'm sure the answer is a simple one, however I have found no where that explains why. Why are hydrogen ions so reactive?

Thanks.

 

[SteamKing]

Hydrogen ions are highly reactive because they have only one electron.

NaCl cannot be a base because it is a salt. If an acid is combined with NaCl, all you get is a salty acid. Ditto LiCl. Hydroxide compounds are bases because they neutralize acids by combining the H+ ions with the OH- ions.

 

[Microbiologist]

Thankyou SteamKing. I understand what you are saying, however I guess my question could be reframed to why is NaCl a salt and HCl is an acid? Why does having only one electron make so much difference? After all, Na and H ions are both 1+ right? So why the massive difference?

 

[SteamKing]

If you want to know why certain compounds are acids, you've got to study the definitions (yes, there are several) of what constitutes an acid:

http://en.wikipedia.org/wiki/Acid

 

[Microbiologist]

Yes. I have read it. I understand the definition. My question is why only hydrogen ions in solution. Why are not sodium ions acidic? Both have a 1+ charge.

 

[SteamKing]

Because the definition of an acid (at least one of them) is a compound which adds H+ ions when in solution. There are many ionic compounds which have +1 charges, but they are not acids. You must remember that the designation of whether a compound is an acid lies in the fact that it contributes H+ ions and not Na+ ions, etc. The terms acid and base have a particular meaning in chemical terms.

 

[Microbiologist]

Thanks SteamKing. I am aware of the definition, however the definition does not answer my question. My question is why do H+ ions in solution cause the solution to behave like an acid, whereas Li+ ions in solution do not. What is the physical/chemical explanation as to why you can't put you hand into a solution with a high H+ ion content but you can put your hand into other solutions with ions of similar charge; for example NaCl. Why does HCl corrode metals rapidly, whereas LiCl does not?

As I have been taught, the 'preference of an atom to gain or lose electrons' determines the majority of it's chemical properties. Na, Li and H all have the same tendency to lose a single electron. Why is Hydrogen so different?

Thank-you again,

I hope this makes more sense.

 

[jim mcnamara]

It looks like confusion with the notion of an electromotive series (how likely metals are to donate electrons) and how many H+ ions are dissociated in a water solution. Acids/bases exist in an aqueous medium. An HCl molecule all by itself in a vacuum is not an acid. When HCl dissociates in water, then it becomes an acid. HCl dissolved other non-ionizing liquids like petroleum ether (if it does in fact dissolve, this is an ad hoc example) does not create an acid medium, just a solution. Water is key.

The electromotive series does not deal with H+ ion concentrations. And does not only apply in aqueous media.

Maybe I have misunderstood this whole thing, but this seems to be the source of confusion.

 

[aclark609]

The thing about acids is that they don't eat through things. The react through things. The H and Cl in HCl, for example, are so reactive it changes the composition of many things it comes in contact with through chemical reactions giving off the effect of "eating away" particular materials when really and truthfully it's just creating new molecules. Na and Li would do the same if they come into contact with something that they react with. To what degree is a matter of proportions (amongst other things) and if the Li or Na will react with the substance in the first place..

Note that HCl is contained in a glass container and doesn't eat away at the container. That's because the glass won't react with the H+ and Cl- dissociated in the container. Glass and plastic make good containers because they are fairly unreactive.

 

[Borek]

H⁺ is a relatively strong oxidizer, able to oxidize many metals. It is also capable of attaching to many organic (and inorganic) molecules with lone pairs (process called protonation), which often induces chemical reactions (like hydrolysis, denaturation of proteins, dehydration of carbohydrates and so on). These properties makes it different from all other cations that you can encounter in the solutions.

 

[snorkack]

The reason why Na+ and K+ are very unreactive is that while they are positively charged, they are well insulated from any chemical reactions by the large and very stable electron clouds of neon and argon octets. While the electrostatic attraction of the positive charge will attract counterions and dipoles in solution, they are unable to get close to sodium nucleus - several of them will form a diffuse surrounding, and none of them can form either a covalent bond or give away an electron to the sodium ion. Even if you do force sodium out of solution, it is still Na+ cation in ionic cristal, without covalent bonds to the counteranions, and if you force electron into sodium atom, it is weakly bonded and readily given away to metallic electron sea, wher sodium is still Na+ ions.

Compared to Na+, H+ is a small and concentrated charge, and H can easily form covalent bonds. This is what makes H+ a strong acid - its tendency to form covalent bonds to electron donors.

There are other things which have the same tendency to form covalent bonds to electron donors. For example, consider Al3+. AlCl3 is even electrically neutral. Yet Al, with its empty p orbital and a partial positive charge, has a strong tendency to form covalent bonds to electron donors. It is actually called acid - with qualification that it is a Lewis acid.

Al3+ can, among other electron donors, react with water. Thus aluminum salts react with water and make the solution acid in containing H+ ions. The reaction is
Al3+ + H2O->AlOH2+ + H+

Note that although Al is a strong electrophile/Lewis acid, it is not an oxidizer. Al has a strong tendency to react with compounds having an available electron pair for a covalent bond - but no tendency to add an electron and not form a bond. AlOH2+ and other such adducts are stable, but Al2+ or Al+ ions are very unstable.

 

[Microbiologist]

The reason why Na+ and K+ are very unreactive is that while they are positively charged, they are well insulated from any chemical reactions by the large and very stable electron clouds of neon and argon octets.

Thankyou snorkack. So to clarify, the reason is that the H atom forms lone + ions (protons) which cannot be accomplished by any other element (within reason) allowing it to get closer to the electrons on other compounds, thus ripping them away? Other atoms electron clouds interfere and repel each other?

 

[snorkack]

Thankyou snorkack. So to clarify, the reason is that the H atom forms lone + ions (protons) which cannot be accomplished by any other element (within reason) allowing it to get closer to the electrons on other compounds, thus ripping them away? Other atoms electron clouds interfere and repel each other?

They interfere, but that is not a complete obstacle. Look at Al3+ again.

Al3+ does have the same 10 electrons as Na+.

But unlike Na+, for one thing, the bigger charge of Al nucleus causes these same 10 electrons to be closer to Al than they are to Na. So Al3+ can get closer to the electrons on other compounds than Na+ can. Not as close as H+; but due to the 3+ charge, the electric field of Al3+ cation is strong enough to form covalent bonds to other atoms.

 

[Microbiologist]

So you are saying that the Al nucleus has a stronger +ve charge, therefore pulls the electrons in closer? So that means that the shape of the election shells is dependent on the charge on the nucleus?

Do you recommend a good source for a more thorough understanding of these processes? Perhaps a textbook or the like?

Thanks again.