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Author Topic: Why isn't anyone answering me? Please help  (Read 21656 times)

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bethey22111

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Why isn't anyone answering me? Please help
« on: October 04, 2007, 03:39:57 PM »

What are the products for this?
NH4Cl + H2O
I really need some help here.
I was told that it's a synthesis reaction but it's not making any sense to me whatsoever and i'm so confused.
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enahs

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Re: Why isn't anyone answering me? Please help
« Reply #1 on: October 04, 2007, 03:45:39 PM »

Because you have asked the same question 3 times in less then an hour.

A) It might take time.
B) What do you think it might be?

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bethey22111

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Re: Why isn't anyone answering me? Please help
« Reply #2 on: October 04, 2007, 03:51:51 PM »

I've asked it 3 times in an hour because I can't get an answer so I figured maybe no one was looking because my title wasn't catchy enough lol
I'm stumped I have no idea what it might be. If I knew I wouldn't be asking so much.
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agrobert

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Re: Why isn't anyone answering me? Please help
« Reply #3 on: October 04, 2007, 04:36:34 PM »

Do a web search. Ammonium chloride and water
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brentr

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Re: Why isn't anyone answering me? Please help
« Reply #4 on: October 04, 2007, 06:20:28 PM »

What are the products for this?
NH4Cl + H2O
I really need some help here.
I was told that it's a synthesis reaction but it's not making any sense to me whatsoever and i'm so confused.

This is a fairly simple reaction and as many others stated it is present all over google.  I also believe I answered it in your other post...

http://www.chemicalforums.com/index.php?topic=19585.0

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AWK

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Re: Why isn't anyone answering me? Please help
« Reply #5 on: October 04, 2007, 06:53:14 PM »

Its depend on the depth of insight

If you add NH$Cl to water and then evaporate all water you will get NH4Cl unchanged.
But if you check pH of water solution and electrical conductivity you will find pH is acidic and solution conduct electricity. From this point of view NH4Cl should dissociate in solution and lts small part hydrolyse.
Try to write down both reaction
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Valdorod

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No wonder she is confused
« Reply #6 on: October 04, 2007, 07:32:10 PM »

While I understand that posters should give an indication of attempting to answer the question, providng erroneous hints and or answers is inexcusable.  The reaction between ammonium chloride and water is not a simple double replacement (Metatesis) reaction, and that is what is confusing about it.  I am afraid that both AWK and brentr fell into the trap that this question is designed to do.  First AWK if you dissolve ammonium chloride in water and then evaporate all water you will have nothing left.  The two products that are made from this reaction are both gases at room temperature, thus evaporating the water will lead to the escape of the dissolved gases.  and to brentr as mentioned before this is not a straight double replacement reaction.  This reaction has a very distinct odor to it.

NH4Cl(s) + H2O(l) ----> H2O(l) + NH3(g) + HCl(aq)

This reaction is very tipical in general chemistry labs since the evidence of reaction is the smell of ammonia.  This reaction as well as the reaction between a strong acid and an alkaly carbonate always leads to problems for students since they do not follow the rules for double replacement.

The net ionic equation for this reaction is
NH4+  +  OH -  ---->  NH3 + H2O

The other reaction is
2HCl(aq) + Na2CO3(aq) ---->  CO2(g) + H2O(l) + 2NaCl(aq)

The net ionic yields

2H(+)  +  CO3(-2) --->   CO2(g) + H2O(l)

Hope this helps

Valdo
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brentr

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Re: Why isn't anyone answering me? Please help
« Reply #7 on: October 04, 2007, 08:10:54 PM »

Thank you for explaining to better Valdo, I did not research and gave the answer that first came to mind.  It will definitely be noted for next time though :)
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Borek

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Re: No wonder she is confused
« Reply #8 on: October 04, 2007, 09:06:22 PM »

NH4Cl(s) + H2O(l) ----> H2O(l) + NH3(g) + HCl(aq)

Completely off. NH4Cl is an ionic salt and it simply dissolves. While it will be in equilibrium with traces of gaseous ammonia it won't decompose this way in substantial amounts. However, NH4+ is a weak acid and its presence will be obvious due to the pH changes - and that's what will dominate solution behavior.

Remember this is high school question, in October students still struggle with the new basic concepts which they have yet not understood and with the old basic concepts which they have forgotten during summer break. Your explanation is fine print, yet you put it as if it was dominating thing happening in the solution. It is not and it can be confusing as hell.
« Last Edit: October 04, 2007, 09:35:29 PM by Borek »
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Valdorod

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For every acid there is an ammonia
« Reply #9 on: October 05, 2007, 08:19:03 AM »

Borek,

While you are correct that the equilibrium lies in favor of ammonium and not ammonia, you cannot make the statement that
" NH4+ is a weak acid and its presence will be obvious due to the pH changes - and that's what will dominate solution behavior"
and
"While it will be in equilibrium with traces of gaseous ammonia it won't decompose this way in substantial amounts"
in the same paragraph since
NH4(+) ----> H(+) + NH3
For every mole of acid that is formed there is a mole of ammonia that is also formed, how can H(+) dominate over NH3 if they are formed at the same rate and in the same amount.  If you can see the pH change you can smell the ammonia.

As to this being high school chemistry, My high school students in west Texas just had an exam this week on reactions in aqueous solutions, which covers these types of reactions.  While not all of them understood, these topics and this level of difficulty is part of the curriculum at the high school level.  My students are sophmores at Mission Early College High School.

When covering this topic they are taught two major categories of reactions Metatesis and Redox and the subcategories within each.  For metatesis (double reactions) the student is supposed to be able to always write the net ionic equation.  They are taught that dissolving is a physical change and does not qualify as a chemical change.  Thus when mixing two ionic solutions, they are to be able to predict either the formation of a precipitate, the formation of water in the case of an acid and a base, or no reaction.
For the case of ammoinium chloride with water they are taught that there is a reaction, since as you correclty stated there is a pH change, thus the student has to explain the formation of the H(+) or H3O(+) ion.  In the formation of the ion there is always the formation of ammonia, whithout it the equation is not balanced.

Valdo
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enahs

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Re: Why isn't anyone answering me? Please help
« Reply #10 on: October 05, 2007, 10:37:57 AM »

.


NH4Cl in water gives you mostly
NH4+ + Cl-

NH4+ is a weak acid with a pKa of ~9.2.

Very little goes to NH3.
Which, what little does in water goes to what is represented by NH4OH which is basic.
NH3 in water is a base.
NH3 + H2O -> NH4+ + OH-


If you put NH4Cl in water, sure you get lots of stuff, but you mostly get NH4+ and Cl-. Other wise, the pH would not change as while the NH4+ is acidic the NH4OH is basic and would cancel, they do not; the pH drops due to the different equilibrium.


If you dissolve NH4Cl in water and boil away the water, you get most of it back.

Just because you smell ammonia does not mean there is a substantial amount produced.







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constant thinker

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Re: Why isn't anyone answering me? Please help
« Reply #11 on: October 05, 2007, 12:27:13 PM »

I definitely agree with Borek and enahs. You will get mainly NH4+ and Cl- since this is an ionic salt.

Other things will happen though, but not to a great enough degree for it to be relative to a high school student who is taking what is most likely chemistry I.
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Borek

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Re: For every acid there is an ammonia
« Reply #12 on: October 05, 2007, 12:41:43 PM »

If you can see the pH change you can smell the ammonia.

For 0.1M NH4+ solution pH changes to 5.12 (easily detectable), that means about 7.5x10-6M ammonia in solution. For 1 liter of such solution, assuming all ammonia was liberated from the solution and diluted in 10L of air this gives ammonia concentration in the air below 1 ppm, while individual smell detection limit is listed as 1-20 ppm. Note that assumptions made are in favor of detection, in reality most of the ammonia will be dissolved and will not leave the solution. To have a chance of smelling the ammonia you probably have to use concentrated solution and very small not ventilated air volume.

Quote
For the case of ammoinium chloride with water they are taught that there is a reaction, since as you correclty stated there is a pH change, thus the student has to explain the formation of the H(+) or H3O(+) ion.  In the formation of the ion there is always the formation of ammonia, whithout it the equation is not balanced.

Well, I don't know exact curriculum, so I can be off, but this is rather subtle equilbrium at work. Do they know what a weak acid, weak base, Bronsted-Lowry acid/base and dissociation constants are? Without it they will most likely remember that dissolved NH4Cl decomposes - which is not generally true. At first approximation NH4Cl dissociates during dissolution just as NaCl does.

Besides, there is absolutely no need for ammonia formation, thats what Bronsted-Lowry theory is about. TBH I have no idea what really happens, as different sources give contradicting information on NH4OH existence.

if you dissolve ammonium chloride in water and then evaporate all water you will have nothing left.  The two products that are made from this reaction are both gases at room temperature, thus evaporating the water will lead to the escape of the dissolved gases.

This is simply not true. After water has evaporated you will be left with crystallic ammonium chloride. If you will heat it up to 335 deg C it will sublime.
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FireBall

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Re: Why isn't anyone answering me? Please help
« Reply #13 on: October 05, 2007, 01:26:32 PM »

Valdorod, why not do the actual experiment? Dump some NH4Cl into water, smell it, evaporate the water, and get back to us.

I'm a chemistry teacher. I use the dissolution of NH4Cl in water as an example of an endothermic process every year, and I've never detected the odor of ammonia.
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Valdorod

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Question in context
« Reply #14 on: October 05, 2007, 03:56:23 PM »

I am not disagreeing with any of you on where the equilibrium lies, not on how much ammonia is produced, but the question has to be taken in context.  Now the high school class that I have are taking college level chemistry in a special program, thus they are supposed to be ahead of the curve, (their last exam scores might not agree, but that is another issue).

Of course I am making assumptions, however, after reading the thread on high school chemistry books; several of those that were mentioned such as those by Raymond Chang and Zumdahl, are books that we have used or are using.  Currently we are using Raymond Chang's 9th edition.

This type of question is typical during the chapter on aqueous solutions, usually chapter 4 or 5 on most current General Chemistry books.  The curriculum is divided into double replacement reactions which include precipitation and acid base reactions, and redox reactions, which include the rest including, displacement, combustion, disproportionation, combintaion and decomposition.

For double replacement reactions the students are asked to produce a molecular formula, then separate the molecules into ions to produce the ionic equation, last they are asked to form the net ionic equation by removing ions that did not react, ie are on both sides of the equation.  Thus the reaction is condensed into only those ions that produce either a solid (precipitation), water (acid/base) or gases (reactions involving the ammonia/ammonium ions and carbon dioxide/carbonate/bicarbonate ions).  If no solids, liquids or gases are formed, then the student is to assume that no chemical reaction took place.  Keeping in mind that early on they are told that dissolving salts is a physical and not a chemical process.  Thus merely dissolving a salt is not considered a chemical reaction.

Now the question asked was this

"What are the products for this?
NH4Cl + H2O
I really need some help here.
I was told that it's a synthesis reaction but it's not making any sense to me whatsoever and i'm so confused."

As the student prepares the equation the student typically swaps ions
NH4Cl + H2O <----> HCl + NH4OH

Then separating to produce the ionic equation, we have what all of us agree, ammonium and chlorine.  However in this form the student assumes that no reaction occurs since only ionic species were produced.

Nonetheless, this reaction, as all of you have mentioned, causes a change in pH in the solution, removes heat (endothermic), and in a few instances, a few students will detect the smell of ammonia.

The student is asked that if the raction did not produce anything other than ions and while the H+ explains the change in pH, the teacher points out that OH is created in equal ammounts thus negating the pH change.  Then what caused the observed changes.

If we answer her question in terms of only ammonium and chloride ions, then there are no new products, water is not needed in the equation, then why have it in the first place?  And what is the synthesis product, if you already had all the components before the reaction, you did not synthesize anything new.

This reaction, or any example involving the ammonium ion is most often used to explain to students that not all double replacement reactions produce a precipitate or water, but that in a few cases gases are also produced.  The same reasoning is used to explain the reaction between CaCO3 and HCl.  also a double replacement reaction that produces CO2, granted a more obious one than the one we are discussing.  When asked to categorize these types of reactions, they do not fit any of the other models that they are taught.

Thus once again I am not arguing that ammonium and chloride are not produced, nor that they are not produced in large quantities, when compared to the amount of ammonia.  And yes you need large molarities to even detect a hint of ammonia, thus the pH change is typically used more often as a means to detect evidence of reaction.  And yes evaporating the solution will produce ammonium chloride crystals.  Nonetheless, this reaction is a special one in the chapter on aqueous reactions. 

One last point, in laboratories, when a lab asks for the students to use ammonia in a reaction, the students are always confused, because they do not have any bottles labeled ammonia, and they always tell me that we forgot to provide it to them.  At which point I remind them that we can only use ammonia in the laboratory in the form of ammonium hydroxide, and to use the bottle labeled ammonium hydroxide.

In terms of carrying out the experiment, a few General Chemistry Laboratories, use the preparation of an ammonium chloride solution and subsequent evaporation to demonstrate the law of conservation of mass.  However, these labs usually require scales with  sensitivity of 0.001, which I am sad to say we do not have in our chemistry lab for students to use.

Valdo
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