I was going through my textbook and found some experimental steps which I do not understand why they should be performed.
1. Before reacting some metal strips, you are required to clean it with sandpaper. Why is that so?
2. What is the salt given off when H2SO4 reacts with NH3?
3. In identifying Anions, what is the purpose of adding silver nitrate after adding HNO3? (eg. to test for Cl-)
4. What are the uses of the products of the Haber process? The product here refers to NH3, does it?
Do respond to my queries if you can. Many Many thanks!
please correct me if im wrong! im not very sure too
1. i think becuase metal like aluminium forms a protective layer around itself,making it unreactive. thus use sandpaper to rub away the protective layer in order for it to react.
2.erm. ammonium sulphate. Nh4SO4 and hydrogen gas i think
3.so that the ions that u dun want to have reaction with will no react.
4. NH3 is the only product right? so ya.
Originally posted by bonkysleuth:I was going through my textbook and found some experimental steps which I do not understand why they should be performed.
1. Before reacting some metal strips, you are required to clean it with sandpaper. Why is that so?
2. What is the salt given off when H2SO4 reacts with NH3?
3. In identifying Anions, what is the purpose of adding silver nitrate after adding HNO3? (eg. to test for Cl-)
4. What are the uses of the products of the Haber process? The product here refers to NH3, does it?
Do respond to my queries if you can. Many Many thanks!
Good attempt at helping bonkysleuth, starmoonsun. Though you didn't directly answer his questions, but your comments are fairly correct, if incomplete.
Q1) starmoonsun has given the reason, though he/she didn't explicitly identify the compound, which is the metal oxide (which may or may not be insoluble and corrosion resistant, depending on the metal. Regardless, sandpapering would do the job of removing the oxide layer).
Q2) (Proton transfer) x 2 from the diprotic acid to the base; conjugate base is the deprotonated sulfuric acid aka sulfate(VI) anion, conjugate acid is the protonated ammonia aka ammonium cation; hence salt formed = (NH4)2SO4(aq).
Q3) If a white ppt is observed, the white ppt must be AgCl(s) and hence Cl- ion must have been present. A higher order question here would be, "Give two possible reasons why nitric acid must be added, before adding AgNO3(aq)". For the answer, control-F (ie. "find") "silver nitrate" here :
http://www.sgforums.com/forums/2297/topics/320107
Q4) See
http://www.google.com.sg/search?hl=en&q=uses+of+ammonia&meta=
Actually for Q3, simple reason may be this...
The nitrate in HNO3 will cause the substance you are testing to become soluble aqueous form. i.e. the anion will be in aqueous form.
The Silver nitrate is there to precipitate the aqueous anion so it is visible and identifiable. (nitrate still remains aqueous so it does not precipitate)
E.g. NaCl...
HNO3 + NaCl = Na+(aq) + Cl-(aq) + H+(aq) + NO3-(aq)
Basically you DON'T see anything as all is in clear aqueous form
But if you add silver nitrate, which is actually Ag+(aq) + NO3-(aq)
It becomes
Na+(aq) + H+(aq) + NO3-(aq) + AgCl(s)
The Silver Cloride is insoluble in water (non-aqueous) and thus precipitates as white solid
And for many of you who just completed your 'O' levels, you can look forward to learning (in your JC or Polytechnic course) why some ionic compounds are insoluble in water (and hence precipitate out as a solid) while others are soluble in water.
4. What are the uses of the products of the Haber process? The product here refers to NH3, does it?
React with sulphuric acid to form fertilisers. That's the main reason.
2. What is the salt given off when H2SO4 reacts with NH3?
erm. ammonium sulphate. Nh4SO4 and hydrogen gas i think
Do you see that it's a neutralisation reaction? Hydrogen gas is wrong.
Q2) (Proton transfer) x 2 from the diprotic acid to the base; conjugate base is the deprotonated sulfuric acid aka sulfate(VI) anion, conjugate acid is the protonated ammonia aka ammonium cation; hence salt formed = (NH4)2SO4(aq).
...
You don't change, do you? ;D
3. In identifying Anions, what is the purpose of adding silver nitrate after adding HNO3? (eg. to test for Cl-)
In my opinion, all the above answers have failed to notice something crucial. It is not about the precipitate of AgCl coming out all these stuffs.
Because: If you add silver nitrate after adding nitric acid, THERE WILL BE NO NET REACTION between the two of them.
Let's see. Silver nitrate + nitric acid => ??? NET Change
There is no NET change at all.
This is why you MUST add silver NITRATE with nitric acid. Both of similar anion base.
Whereas, if you added silver SULPHATE (which works, trust me) with nitric acid, what happens is:
Ag2SO4 + 2HNO3 --> 2AgNO3 + H2SO4
A pretty screwed up anion test, huh?
When these two react, well maybe your precipitate wouldn't be so visible. That's the way I'm taught. Btw the maybe part is MY OPINION, don't blame my teacher.
I emphasised the "NET" because there is change. AgNO3 does react with HNO3, but in the end, the overall change is nothing because basically the products=reactants.
And you can use Ag2SO4, but you must use H2SO4 in place of HNO3's function
I hope I haven't lost my touch. :D
>>>
In my opinion, all the above answers have failed to notice something crucial. It is not about the precipitate of AgCl coming out all these stuffs.
Because: If you add silver nitrate after adding nitric acid, THERE WILL BE NO NET REACTION between the two of them.
Let's see. Silver nitrate + nitric acid => ??? NET Change
There is no NET change at all.
This is why you MUST add silver NITRATE with nitric acid. Both of similar anion base.
Whereas, if you added silver SULPHATE (which works, trust me) with nitric acid, what happens is:
Ag2SO4 + 2HNO3 --> 2AgNO3 + H2SO4
A pretty screwed up anion test, huh?
When these two react, well maybe your precipitate wouldn't be so visible. That's the way I'm taught. Btw the maybe part is MY OPINION, don't blame my teacher.
I emphasised the "NET" because there is change. AgNO3 does react with HNO3, but in the end, the overall change is nothing because basically the products=reactants.
And you can use Ag2SO4, but you must use H2SO4 in place of HNO3's function <<<
Good conceptual theorizing attempt, Garrick, and while you're not "wrong", but the point is not so much having 'no net change' in the sense of deliberately adding a 'nitrate' to another 'nitrate' for the sake of keeping the anion as 'nitrate' only, but rather, it is primarily the solubility that we're concerned about. So I'd say you're right, but your phrasing needs elaboratio.
From the way the question is phrased, the best answer still has to do with firstly the Ag+ (forming AgCl(s) with Cl-), and secondly using 'nitrate' (not to prevent 'net change', but rather) to safely avoid any false positive ppts, because nitrates are the safest to use (since ALL nitrates are always soluble).
So again, what you mean to say is, we could use alternative combinations of acid and silver salt, as long as they do not react to cause false positives, by precipitating out the Ag+(aq) ions before they have the chance to precipitate out with the Cl-(aq) ions in the test solution.
>>> Whereas, if you added silver SULPHATE (which works, trust me) with nitric acid, what happens is:
Ag2SO4 + 2HNO3 --> 2AgNO3 + H2SO4
A pretty screwed up anion test, huh? When these two react, well maybe your precipitate wouldn't be so visible. <<<
No, it's not 'screwed up' at all. First of all, your equation isn't correct, for 2 reasons.
Silver sulfate is rather insoluble (ie. minimally soluble), so you can't use it at all. (How can you test for ppt if what you're adding is itself a ppt?)
And even if assuming it was soluble, you would not 'exchange ionic partners' in a clean-cut fashion like your equation implied, because both your RHS products are soluble. (ionic partners only 'exchange' in an ionic precipitation reaction, where one RHS product compound is insoluble).
Rather, it would be an aqueous mixture of all 4 possible aqueous compounds, because all 4 compounds are soluble.
So again, good try, Garrick, but you will want to elaborate further to say the reason why we use silver nitrate acidified with nitric acid, is again :
1) Silver cations to precipitate out any Cl- ions present
2) Nitrate anions because all nitrates are SOLUBLE and hence a) allows use to provide *aqueous* Ag+ ions, and b) won't cause false positive ppts in the test solution (rather than "no net change" as the reason.).
3) Adding protons (H+ ions) for this reason
http://www.sgforums.com/forums/2297/topics/320107 control-F (ie. "find") "silver nitrate"
Garrick's essential point is point#2a, but he/you shouldn't phrase it like "ensuring no net change" and "making the ppt harder to see", but rather emphasize it's about *solubility*, ie. ensuring that Ag+(aq) ions are still available (in aqueous state) AND that no other ppt is formed before adding the mixture to the test solion.
Also, you should note that silver chloride is a white ppt, silver bromide is a cream ppt, and silver iodide is a yellow ppt. So the colour must be stated precisely.
"After adding nitric acid and silver nitrate into the test solution, if a white precipitate is observed, we conclude chloride anions must have been present in the test solution, and that the white ppt must be silver chloride."
Finally, again, I'd recommend everyone read this :
"Give two possible reasons why nitric acid must be added, before adding AgNO3(aq)". For the answer, control-F (ie. "find") "silver nitrate" here :