The Effect of pH on Dissolution

[paved88]

Basically, my friend and I conducted an experiemtn for 10th grade chem in which we used five liquids with different pH levels and measured the time it took for different liquids to dissolve a Jolly Rancher. I'm working on the final paper, and most of it is straight forward, but I can't find any information on the relationship between the pH of solvents and the time it takes to dissolve solutes. Does anyone know of any similar experiments or about the relationship (if there is one)? We found that the more alkaline solvents dissolve the candy faster, which wasn't what we expected. Does it depend on the solute? Can anyone help?

[TWJian]

What exactly(chemically) is a Jolly Rancher?

 

If the solute in question does not react(chemically) with H3O+ or OH-, then the pH levels should not matter. If it does, and the solubility of the product is greater then the original solute (if the product is less soluble then the following arguments would also be reversed)

 

1) If the solute reacts with H3O+, then increasing the concentration of H3O+ (solution is more acidic) would increase the rate of the forward reaction, yielding more of the more soluble product, which corresponds to an increase in solubility. Increasing the concentration of OH- would decrease the concentration of H3O+ (solution becomes more alkaline) due to the autoionization potential of water (or whatever polar solvent you are using)

 

2)If the solute reacts with OH-, then increasing the concentration of OH-(solution becomes more alkaline) would increase the solubility, and vice versa. (et cetera, et cetera as in (1))

 

This holds true even if the original reactant is not soluble (and hence, not a solute) since a soluble product would certainly be more soluble then a non-solute.

 

Keep in mind, however, that the solute could also react with other ions present in the solution due to the acids or bases you use, as well as molecules and ions present if you are not using water as your solvent. I am assuming it doesn't due to the information you provided. I can't presume that the solute reacts with anything else since the 'anything else' is unknown, and hence I ignored it. I could be wrong.

 

A list of of all major reactants would be useful.

[hermanntrude]

jolly ranchers can almost certainly be considered to be essentially 100% sugar, probably glucose and sucrose.

 

since this is a 10th grade experiment we can also probably guess that the acid solutions are HCl and the alkaline solutions are NaOH.

 

is this correct paved?

[thedarkshade]

jolly ranchers can almost certainly be considered to be essentially 100% sugar, probably glucose and sucrose.

It depends of what Jolly Rancher product are you talking about. And I don't think it can be considered 100% sugar though!

 

There are sugar free jolly ranchers too!

[paved88]

jolly ranchers can almost certainly be considered to be essentially 100% sugar, probably glucose and sucrose.

 

since this is a 10th grade experiment we can also probably guess that the acid solutions are HCl and the alkaline solutions are NaOH.

 

is this correct paved?

 

Ah, I should've thought to include a list of all the solutions when I posted this originally. The acidic solutions were sprite and vinegar, and the acidic solutions were bleach and ammonia, using water as a control.

 

TWJian, so basically the rate of the chemical reaction depends on certain ions in the solvents that the solute may be reacting with? It looks like the jolly ranchers were reacting with OH- (assuming they are present) in the bleach and ammonia respectively? Isn't NH3 the chemical formula for ammonia?

 

Thanks, by the way.

[TWJian]

paved88,

 

Let me guess.. you wanted to study the effects of acidic and alkaline solutions upon the solubility of a Jolly Rancher right? That means it is about the effects of different concentrations of H3O+ and OH-, therefore you would try to avoid other ions which might act with the Jolly Rancher...

 

Avoid oxidizers at all cost. Bleach is sodium hypochlorite, which, although alkaline, could oxidize sugars into ketones, aldehydes, and carboxyllic acids (which are mostly soluble but a pain in the neck to isolate in this experiment), especially if the solution is acidic. (bleach actually has added HCl to become neutral, and the resulting oxidation would yield more HCl, increasing acidity and.. well, you get the point)

 

If you are not sure what chemicals are oxidizers, avoid any acid with an O in there, including nitric acid. Sulfuric acid, incidentally, would dehydrate sugar instead, yielding insoluble carbpn if sufficient in high enough concentrations.

 

Avoid chemicals with -ate or -ite unless you know what you are doing. They contain oxygen.

 

Sprite contains carbonic acid. Shouldn't matter much.. but I'm going to drop carbon in case an organic reaction occurs.

 

Vinegar. An organic compound. Which means you need to know organic chemistry due to the ensuing reaction between ketones, carboxyllic acids, etc. Drop this.

 

Ammonia is NH3, but if it is dissolved in water, it becomes NH4OH. This is important because of the ammonium ion, NH4+.

 

Don't use ammonia since it tends to form complexes (complex coordination ions) with transition metals and undergoes complex reactions with organic compounds, forming amines and god knows what.

 

You already have some organic compounds in the Jolly rancher. Try not to introduce more organic compounds unless you want to do organic chemistry.

 

There are however, some safe reagents which you could use which other ions will probably not react with sugars...

 

As hermanntrude said, the standard chemicals would be HCl and NaOH.

(Hydrochloric acid/Some drain cleaners/murcuric acid and Sodium Hydroxide/Lye)

You could also use KOH, potassium hydroxide but it's more expensive and harder to find.

 

This is because Cl- won't react with the sugars (unless you have oxidizers present) and Na+ compounds are all soluble. (so it will still remain as Na+ as long as you have water)

 

Are you performing an experiment by yourself using household chemicals, and not using laboratory chemicals? If possible, ask for some lab-grade chemicals to help your research.

[John Cuthber]

"Ammonia is NH3, but if it is dissolved in water, it becomes NH4OH."

Oh No it doesn't.

Anyway, none of the acids or bases used is strong enough to affect sugar to any great extent.

All of them are overwhelmingly water so there probably won't be much effect. I guess the bubbles in the sprite might make some difference.

[TWJian]

Fine.. maybe I should have said when ammonia dissolves in water, it is practically identical to NH4OH, but technically, it is not. Then I would have to explain what I meant...

 

Ammonia deprotonates a small amount of water to form NH4+ and OH-. This reaction is reversible, and NH3, H20, NH4+, OH- exists in equilibrium (although there's no actual equilibrium, but a shorthand way of describing how they behave, which otherwise would have taken a wall of text to describe.

 

The chemical behaviour of NH3 and H2O together resembles that of NaOH, KOH and other soluble bases, and hence most scientists and engineers would also refer to a solution of ammonia as NH4OH, ammonium hydroxide. This is technically incorrect, but is done for convenience. (For the same reasons, most people write H+ instead, and benzene and acetic acid are drawn with double bonds, VSEPR theory is used to predict molecule shapes, etc. etc.)

 

Chemically, though, ammonia in solution is treated as NH4OH, even though it is technically incorrect as it is impossible to isolate NH4OH, and hence, is assumed to not exist.

 

This is reflected in the calculations for the disassociation of ammonia in solution, which is treated as NH4OH.

 

Kb (ammonia)=1.79 x 10^-5, meaning that the amount of moles of OH- is squareroot(1.79*10^-5) per mole of NH4OH, and hence per mole of NH3 present in the solution. (even this is technically incorrect, since I am ignoring the change of the amount of NH4OH, which is rather negligible anyway. A more accurate solution would be arrange it as x^2+1.79(10^-5)x-1..79*10^-5 and solve for x using [-b+squareroot(b^2-4ac)]/2a (Note + instead of plus minus since the value must be positive). Even then, it is still technically incorrect since I'm ignoring a whole bunch of other factors, such as shielding, change of temperature, and so on.)

 

Most of the ammonia will exist as NH3 within the solution (Which is written as NH4OH rather then NH3+H20). Since the concentration of OH- is low, ammonia is a weak base. However, both ammonia and the NH4+ ion are quite reactive, (delta G for most reactions tend to be really negative, and it also forms a lot of organic compounds and inorganic complexes) so it tends to react with other chemicals even if it is a weak base. Should NH4+ be responsible for the reaction, it will shift the disassociation of ammonium hydroxide (note that saying disassociation of ammonia is even more incorrect) towards the product side, yielding a constant supply of NH4 to take part in the reaction. This also has a side effect of increasing the pH of the solution.

 

In fact, the strength of an acid or base does not affect its reactivity(rate of reaction), unless it involves acid-base reactions which generally yield a salt and water. Hydrofluoric acid for example, has a disassociation constant, k(a) of 7.2*10-4. Quite a weak acid, but you wouldn't want to go about dripping it on your skin unless you have a (quite painful) death wish (several drops could kill if left untreated). Same goes for formic acid, though it's significantly less dangerous.

 

Yours verbosely,

TWJian

 

(See the wall of text now? Granted, most of my posts are long, but explaining every single thing like this would have raised the verbosity level by an order of magnitude or more)

[paved88]

Lol. I think I get the point about the ammonia dissolving. TWJian, thanks for all the advice in your second post. I turned in the paper earlier today, but it will help me know what to research if I do a follow up experiment next year. I was able to incorporate a lot of the information in your first post, so thanks a lot. I was afraid I wouldn't have anything substantial to include in my conclusion, since my library access was taken for the year and I couldn't look anything up in there. I'll probably follow this up next year in AP Chemistry. Thanks a lot everyone.

[John Cuthber]

I think the essense of the problem is that this "Most of the ammonia will exist as NH3 within the solution (Which is written as NH4OH rather then NH3+H20). Since the concentration of OH- is low" is a contradiction.

There's a whole lot more ammonia there than OH so practically none of the NH3 is, in fact, present as NH4OH.

Since practically no NH4OH is present, even as a dissociated species and even less is present as as such it's plain wrong to say that it is formed.

This being a scientific forum I think many people would agree with me that putting stuff that's plain wrong is unhelpful.

 

As for " most scientists and engineers would also refer to a solution of ammonia as NH4OH, ammonium hydroxide."

The scientists I know would generally prefer to call a spade a spade accordingly they would call aqueous ammonia aqueous ammonia.

Ammonia, per se is vastly more likely to react with a carbohydrate than the NH4 ion so the fact that some tiny fraction of the stuff gets protonated isn't important to the question in hand. I doubt that many sweets have significant concentrations of transition metals that would be complexed by ammonia.

 

If you hadn't bothered to mention the (substantially irrelevant as well as non existent) NH4OH, not only would you have avoided your second verbose post, but your first one would have been shorter. If you want, I will debate the fact that the reaction NH3 + H+ can be studied in the gas phase; In that instance you can decide whether or not NH3 is a strong base and you don't need to involve NH4OH at all so the assertion "Since the concentration of OH- is low, ammonia is a weak base. " isn't generally true either but I doubt anyone is listening.

In short please don't say stuff that's just plain wrong, even if, like references to NH4OH, it's a common mistake.

[TWJian]

paved88: You are welcome. Pleased to be of help.

 

Cuthber:

 

Ammonia deprotonates water to form NH4+ and OH-. Therefore:

 

If

"Most of the ammonia will exist as NH3 within the solution (Which is written as NH4OH rather then NH3+H20)

 

Then

"Since the concentration of OH- is low, ammonia is a weak base"

 

Anything wrong with that?

Unless you are quoting me out of context as:

Since the concentration of OH- is low...<missing text>, most of the ammonia will exist as NH3 within the solution (Which is written as NH4OH rather then NH3+H20).

 

Please don't reverse my premises to get a (substantially irrelevant as well as non-existent) contradiction.

 

As for you:

 

You seemed to have accepted my explanation that NH4OH is a shorthand way of writing NH3 + H2O (in order to show that as a base, it reacts similiarly to most metallic hydroxide). Then you claimed:

 

If "There's a whole lot more ammonia there than OH so practically none of the NH3 is, in fact, present as NH4OH.)

 

Then "Since practically no NH4OH is present, even as a dissociated species and even less is present as as such it's plain wrong to say that it is formed".

 

You seemed to have missed the essence of my argument since NH4OH represents (NH3+H2O). So if there are a lot of NH3 present in water, the concentration of NH3 + H2O would be high, and therefore the same would go for NH4OH.

 

Removing the word 'practically' would also help. Unless you want to suggest that theoretically, NH4OH (as the molecule, not the representation of NH3+H2O), would be formed, but the amount is so small as to be negligible.

 

In that case you are shooting yourself in your foot since that would allude that NH4OH (as the molecule) actually would be formed, according to theory(which means it is consistent with true descriptions of reality, as of that moment, or else it would be a hypothesis instead). Care to explain what you really meant?

 

Regardless, this is irrelevant either way as I have already conceded that NH4OH (as the molecule) does not form at all. I was merely using NH4OH as a shorthand way to allude that NH3+H2O behaves like a metallic hydroxide.

 

"This being a scientific forum I think many people would agree with me that putting stuff that's plain wrong is unhelpful."

 

True, but just because something is false, it doesn't mean that it is just plain wrong, especially if it is more easily understood and offers an excuse for what is happening, and even more so if it provides a gateway to the truer explanation (which is probably still false anyway but is better)

 

Furthermore, scientific textbooks and journals should have higher standards of differentiating what is 'plain wrong' and 'wrong, but convenient explanation that is accepted by most of the community' I've seen aqueous ammonia being referred to as ammonia hydroxide in biology and even chemistry textbooks (university ones too), and even in some research papers, scientific indices, and journals. (http://www.cipav.org.co/lrrd/lrrd5/3/syria3.htm , http://potency.berkeley.edu/chempages/AMMONIUM%20HYDROXIDE.html) They usually explain briefly that it is just a representation of NH3 and H2O (or allude to such an explanation), which I also did.

 

If I should not post such a 'common mistake' here, wouldn't those scientists be of greater error, as they are subject to a more stringent and heavier burden of truth than a "science forum"?

 

Of course, if you want everybody to use the truest explanation all the time, regardless of its convenience, start by petitioning universities, publishers, etc. to stop putting in words like "ammonium hydroxide", "H+", stop using diagrams which picture delocalized electrons with double and single bonds (You can't even find such bonds since the electrons are all over the place, and hence delocalized)", and so on.

 

And,

"If you hadn't bothered to mention the (substantially irrelevant as well as non existent) NH4OH, not only would you have avoided your second verbose post, but your first one would have been shorter."

Yep. Shouldn't have done that due to the can of worms opened up. Hopefully, they are stuffed into the can by now.

 

"If you want, I will debate the fact that the reaction NH3 + H+ can be studied in the gas phase..."

No thanks, this thread is getting off-topic enough at this rate.

 

"In that instance you can decide whether or not NH3 is a strong base and you don't need to involve NH4OH at all.."

 

NH3 is a weak base. Period.

 

"so the assertion "Since the concentration of OH- is low, ammonia is a weak base. " isn't generally true either but I doubt anyone is listening."

 

If it is such a big mistake that it is just "plain wrong", then why isn't anybody listening?

 

Of course, if this is on the debate forums or the questioner is a college/university student, then I would use the longer and 'truer' explanation.

 

As things are, NH4OH is suitable enough for a 10th grade chemistry class. Calling a spade a spade means I'm going to use the most convenient and suitable explanation for a particular condition. I am not going to call it a "yellow spade", "plastic spade", or a "digging implement with a handle made out of a matrix of sclerenchyma cells (read: wood) and a curved trapezoidal wedge made out of a composite of austenite, bainite, martensite, cementite, ledeburite, pearlite, and spheroidite (read:steel) despite them being 'better' and 'truer' explanations unless the situation calls for it.

 

End argument.

 

(If possible, I would rather not respond to your subsequent post (if you reply), as this is getting off-topic and paved88 has already mentioned that his research is already finished.)

 

Thank you.

[John Cuthber]

At the start of this thread NH4OH didn't exist, and it still doesn't now.

As you have pointed out, the original project is done. That's why I think nobody's listening.

"You seemed to have missed the essence of my argument since NH4OH represents (NH3+H2O). So if there are a lot of NH3 present in water, the concentration of NH3 + H2O would be high, and therefore the same would go for NH4OH"

That simply isn't true; there's lots of water, lots of ammonia, not a lot of ammonium or hydroxide ions and even less undissociated ammonium hydroxide. Why call the stuff after the least common species present?

In principle there must be some tiny concentration of H3O+ and NH2- present too. Is it sensible to refer to ammonia solution as hydroxonium amide?

If not then calling it ammonium hydroxide doesn't make sense either.

I'm not planning to tell all the books' publishers to change nor do I intend to lobby the universities. I will just wait for people to look on the wiki page which says "Thus, the term ammonium hydroxide is a misnomer".

I think the pH of household ammonia is about 12 so the OH- concentration is about M/100 or a dozen or so miligrams per hundred ml, the NH4+ concentration will be about the same. Compare that to the 10 g or so of ammonia there. I don't think it makes sense to call the stuff ammonium hydroxide when that is only a small fraction of the material.