HF (weak or strong acid)

[akcapr]

Isnt dissolving of a solid just hydrolysis?

[budullewraagh]

actually, the strongest base possible would be the best electron donor, ie Li metal.

 

Li3N must be fearsome as a base as well. of course LiNH2 is an immensely strong base as well

[akcapr]

do all of those still form oh- wen added to water?

[jdurg]

I beg to differ just a little bit woelen. Yes KOH and RbOH and CsOH and NaOH all dissociate 100%, but CsOH is more soluble than any of the other hydroxides. As a result, more of the CsOH can dissociate in water and you get a stronger base.

 

For example, NaOH has a solubility of 111 grams in 100 grams of water. That's equal to about 1110 grams per liter of water. With a molar mass of 40, that's equal to a molarity of around 27.75 Molar for a saturated solution. CsOH has a solubility of 8600 grams per liter of water. With a molar mass of 150 g/mole, that equals a molarity of 57.33 Molar. Over twice as strong as sodium hydroxide is. So in aqueous solutions, which is the word I missed earlier, CsOH is unequivically the strongest base that is known.

[budullewraagh]

i hear your last line. could you address my last post (#27)?

[jdurg]

While the lithium ion does have a very negative standard reduction potential, remember that the value is in regards to an aqueous species. The real value that should be looked at is the electronegativity. The lower the electronegativity, the easier it is to pull an electron off. As a result, cesium would be INCREDIBLY willing to donate an electron. So if a base is defined as a substance which is an electron donor, then cesium metal would be the strongest base.

[woelen]

I beg to differ just a little bit woelen. Yes KOH and RbOH and CsOH and NaOH all dissociate 100%' date=' but CsOH is more soluble than any of the other hydroxides. As a result, more of the CsOH can dissociate in water and you get a stronger base.

 

For example, NaOH has a solubility of 111 grams in 100 grams of water. That's equal to about 1110 grams per liter of water. With a molar mass of 40, that's equal to a molarity of around 27.75 Molar for a saturated solution. CsOH has a solubility of 8600 grams per liter of water. With a molar mass of 150 g/mole, that equals a molarity of 57.33 Molar. Over twice as strong as sodium hydroxide is. So in aqueous solutions, which is the word I missed earlier, CsOH is unequivically the strongest base that is known.[/quote']

 

There are two flaws in your reasoning:

1) Solubility is not equivalent to strength. The fact that CsOH dissolves better in water still does not make it a stronger base. You may be able to make a more concentrated solution, but is it stronger in that case? Yes, it is stronger in terms of concentration (e.g. more corrosive), but its intrinsic strength is not higher. With HCl you also do not reason like that. 10% HCl is stronger than 1% HCl, but only in the sense of concentration. The intrinsic strength of the HCl still is the same in both cases.

2) You can dissolve over 57 moles of CsOH in one liter of water, but of course, the volume increases quite a lot if you add almost 9 kilos of solid to 1 liter of water . I'm sure that the solution will take much more than 1 liter, maybe three or even four liters. Even with four liters the density would be almost 2.5 kilo/liter, which is very high already for an aqueous solution. So, the effective molar concentration you can obtain probably will not be that much higher than with NaOH. Of course, with NaOH you also will have an increase of volume, but that will be less, because you only add over 1 kilo of solid to a liter of water. I expect it to become around 1.5 liter.

But again, this is just the concentration, we are talking about, and not the intrinsic strength of an acid or a base.

 

You have to distinguish between the following:

- intrinsic strength of an acid or a base, which is a property of the compound itself. A very nice example of SUPER strong bases are certain organic cage-compounds, which have a small cavity, in which precisely one proton (H+) fits. These compounds are capable of even deprotonating CH4.

- maximum observable alkalinity or acidity, which is determined by the solvent. Bases with a very strong intrinsic strength cannot be distinguised if the intrinsic basicity is stronger than the basisity of the deprotonated species of the solvent. E.g. water cannot distinguish between hydroxides, amides, oxides and nitrides (mentioned in increasing intrinsic strength), they all form hydroxide. E.g. liquid ammonia can distinguish between amides and hydroxides, but not between amides, oxides and nitrides. The latter all form amides and in the case of oxide, the weaker hydroxide is formed as well.

- concentration of an acid or a base. This does determine properties like corrosiveness, but its something different than intrinsic strength of a base or acid.

[jdurg]

I understand what you're saying, but if you look at the hydrogen halides in terms of acid strength, yes HCl, HBr, and HI are all strong acids, but HI is indeed stronger than HCl and HBr are. The same is true of the alkali hydroxides. I don't have a CRC Handbook at the moment, but I'm willing to bet that the Kb of CsOH is not the same as that of NaOH/KOH/LiOH/RbOH.

[woelen]

I understand what you're saying, but if you look at the hydrogen halides in terms of acid strength, yes HCl, HBr, and HI are all strong acids, but HI is indeed stronger than HCl and HBr are. The same is true of the alkali hydroxides. I don't have a CRC Handbook at the moment, but I'm willing to bet that the Kb of CsOH is not the same as that of NaOH/KOH/LiOH/RbOH.

Have a look at page 8 of this document:

 

http://www.cgcc.cc.or.us/Academics/dept/chemistry/RKovacich/notes/Notes%20Acids%20and%20Bases.doc

 

The strong acids are mentioned separately all with their own Ka, which indeed differ by quite large factors. The strong bases, however, all are mentioned in a single group, with just one (uncertain) Kb value specified. So, there might be differences, but they are marginal at best. I still think that this is due to the fact that these strong bases are totally ionic and then the dissociation is approximately the same for all.

 

When opening the file, please be prepared that it is quite large (>5 Mbyte). Of course, this single reference is not a proof of my theory, but it is at least some evidence supporting it .

[jdurg]

Another reason why I still believe that CsOH is a stronger base than NaOH, and that KOH is a stronger base than NaOH is in regards to making soaps. If you make a saponify an oil using NaOH, the resulting product is generally a solid. If you do the same with KOH, however, the resulting product winds up being a liquid and you get a liquid soap. (I believe this is caused by the KOH breaking apart the fat to a greater extent. Something that a stronger base would do). Also, look at the ionic nature of each of the alkali hydroxides. Cesium salts are VERY ionic due to the incredibly low electronegativity of cesium. NaOH and KOH are less ionic.

 

The problem with trying to find the proper answer to this is that CsOH and RbOH aren't really used all that much due to their cost.

[woelen]

Cesium salts are VERY ionic due to the incredibly low electronegativity of cesium. NaOH and KOH are less ionic.

As far as I know, all Na-salts and K-salts are 100% ionic. No covalent bonding at all. So, the corresponding Cs-salt cannot be more ionic.

 

The problem with trying to find the proper answer to this is that CsOH and RbOH aren't really used all that much due to their cost.

Just an experiment of though. Suppose I have 1 mol of CsI and 1 mol of KOH. I also have 1 mol of CsOH and 1 mol of KI.

 

I take 250 ml of water and I dissolve the CsI and KOH in it. I take another 250 ml of water and I dissolve the KI and CsOH in it. Which solution would be "stronger", i.e. more corrosive?

[jdurg]

I haven't gotten a chance to read the document yet, but another experimental observation I can make is the base's ability to attack glass. All the alkali hydroxides will attack glass, but the rate at which they do it is not the same. LiOH takes forever to make any change to the glass, while NaOH and KOH don't take nearly as long. CsOH, however, will visibly attack the glass. I.E. you can see it happening in front of your eyes. No I know that a lot of strong bases attack glass, so obviously the OH- ion is able to, in some way, attack the glass molecules. Therefore, if the heaver alkali hydroxides attack glass at a much quicker rate, wouldn't lead you to believe that they are stronger bases?

[budullewraagh]

woelen, Cs actually does dissociate more than the other alkali metals. consider CsOH vs NaOH. the Cs S orbital is huge, so it doesn't overlap well

[woelen]

Therefore, if the heaver alkali hydroxides attack glass at a much quicker rate, wouldn't lead you to believe that they are stronger bases?

No, not really. HF also attacks glass quickly, yet it is a weak acid.

 

In fact, I intend to buy some CsI. It is affordable at a price of $16 per 10 grams. If I have that stuff, then I'll try whether a solution of KOH and CsI is more corrosive than a solution of KOH only of the same concentration.

 

BTW, the CsI is going to serve as a place-holder for Cs in my collection of elements, until I have more $$$ for a decent Cs-sample

 

But coming back on the experiment of thought, what do you think is more corrosive: CsI/KOH or CsOH/KI? Could you also give an explanation to this. I'm curious about your answer and maybe we can settle the issue on strength of alkalies.

[woelen]

woelen, Cs actually does dissociate more than the other alkali metals. consider CsOH vs NaOH. the Cs S orbital is huge, so it doesn't overlap well

Can you mention any solvent, which shows the difference? As far as I know, NaOH already is 100% ionized, so how could CsOH be more ionized than NaOH?

[budullewraagh]

hey, i'm only quoting professor garry procter. he explained that this is why CsOH is used in solid phase peptide synthesis. if you have any questions, you can email him. pm me for his email address