precipitates (1 Viewer)

[za]

wat does Pb react with, coz i thought it was only with Cl- (conquering textbook) but apparently it also reacts with sulfate and carboante( q 15 2001 HSC paper).

 

[Tommy_Lamp]

if your talking about identifying Pb2+, then you add Cl- (as HCl)
if your talking about using Pb2+ to identify an anion, then you add it last.

 

[za]

im saying that pb2+ not only forms a precipitate with Cl but also with sulfate and carbonate.

 

[Tommy_Lamp]

When identifying Pb2+, then you add Cl- (as HCl), this the first test when determining cations.
When using Pb2+ to determine anions, you have to put it last it reacts with everything, so you add Pb(NO3)2, if a white precipitate forms Cl- is present

 

[za]

http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2001exams/pdf_doc/chemistr_01.pdf


can u look at the multiple choice q15 and tell me wat u think the answer is.

 

[shazzam]

The answer is C.
If you refer to your solubility table (eg in Conquering chemistry prelim) you'd see that lead forms a precipitate with almost every anion, including all three that the student in this question used.
I think that all these precipitates are white.
NB lead also forms a distinct yellow precipitate with iodide ions, and this is a better way (well if this reaction occurs then you KNOW it's lead immediately) of testing for lead ions.

 

[tina_goes_doo]

Ok, some nice little info on your solubility rules:

1. All carbonates are insoluble except group 1 elements and ammonium. This rules out B and D.

2. Generally sulfates are soluble EXCEPT lead and barium. This rules out A.

3. Generally chlorides are soluble except silver and mercury. But lead chloride is only SLIGHTLY soluble. It dissolves again in hot water.

However as we have already ruled out A, B and D, the answer is C.

 

[za]

oh k then

thnx

 

[cko]

doesn't Pb form a ppte with I? like u add KI to unknown solution containing Pb and u PbI and its yellow i think...

 

[Skywalker]

NB lead also forms a distinct yellow precipitate with iodide ions, and this is a better way (well if this reaction occurs then you KNOW it's lead immediately) of testing for lead ions.

That's due to the iodide ion, not the lead cation.

Silver iodide is also yellow. Similarly, lead/silver bromide are both pale yellow.

 

[ashtor]

how the hell are we suposed to have known all this in the test?...i did that question as well Mr. Za and put in B. Plz tell me whatever's not in conquering chem is outta the syllabus...

 

[za]

are ther any other rules that arnt in conquering chem tha we should be awre of?

 

[mystify]

This might help...

Solubility rules

SOLUBLE SUBSTANCES INCLUDE: all salts of group 1 elements (in periodic table) metals, all salts formed with the ammonium ion (NH4+), all nitrates and acetates, all chlorides, bromides and iodides except those of silver, lead and mercury, all sulfates except lead, barium, mercury and strontium. Calcium and silver sulfates are only slightly soluble.

INSOLUBLE SUBSTANCES: all carbonates, hydroxides and phosphates except those of group1 elements and ammonium.


TESTS FOR ANIONS:

1) To test for chloride you add silver ions and you get a white ppt. of silver chloride. When you add dilute nitric acid or barium nitrate there is no reaction.
2) To test for carbonate you can:
i. add dilute nitric acid and bubbles of gas evolve (test with limewater to confirm carbon dioxide gas production)
ii. add silver nitrate solution and get a yellow ppt. of silver carbonate.
iii. add barium nitrate and get a white ppt. ob barium carbonate.
3) To test for phosphate you can:
i. add silver nitrate soln and get a yellow ppt. of silver phosphate.
ii. add barium nitrate and get a white ppt of barium phosphate.
iii. if you add nitric acid there is no reaction.
4) To test for sulfate you can:
i. add silver nitrate and there'll be no ppt OR a faint white ppt of sparingly soluble silver sulfate.
ii. add barium nitrate and get a dense white ppt. of barium sulfate.
iii. if you add nitric acid no reaction takes place.

TESTING FOR CATIONS

1) To test for lead you can:
i. add dilute HCl and get white ppt of lead chloride (dissolves on heating)
ii. add dilute sulfuric acid and get a white ppt. of lead sulfate
iii. add dilute sodium hydroxide and get a white ppt. of lead hydroxide.
2) To test for copper you add dilute NaOH and get a blue ppt. of copper hydroxide. No reaction occurs if you add HCl or sulfuric acid.
3) To test for ferrous ion (Fe2+) add dilute NaOH and you get a pale green ppt. of iron (II) hyrdoxide which quickly turns brown. No reaction occurs if you add HCl or sulfuric acid.
4) To test for ferric ions (Fe3+) you add dilute NaOH and get a reddish brown ppt. of iron (III) hydroxide. No reaction occurs if you add HCl or sulfuric acid.
5) To test for calcium ions you can:
i. add dilute sulfuric acid and get a white ppt. of calcium sulfate.
ii. faint white ppt. of sparingly soluble calcium hydroxide.
iii. if you add dilute HCl no reaction occurs.
6) To test for barium ions add dilute sulfuric acid and you get a white ppt. of barium sulfate. No reaction occurs with NaOH or HCl.

Hope that helps! Btw it is just as important to know if a reaction does not occur to when it does because it helps you eliminate options in exams to get to a final answer.

 

[Xayma]

You also have to know the flame tests.

I find it easier to remember it like this:

Cations.

Add NaCl ---> Lead forms white ppt of partially soluble PbCl₂
If no reaction
Add Na₂SO₄ ---> If white ppt, flame test:
Na₂SO₄ ---> If white ppt Ca²⁺ apple green
Na₂SO₄ ---> If white ppt Ba²⁺ brick red
If no reaction:
Add NaOH ---> Cu²⁺ forms blue ppt
Fe²⁺ pale green that turns brown
Fe³⁺ brown ppt.

Similar to the above for anions but adding NO₃ to get salts out.

 

[tina_goes_doo]

Isn't your flame colours the other way around?

Calcium = red
Barium = green?

 

[mr EaZy]

yeah thats right
ca= brick red
BA = apple green

 

[mr EaZy]

by the way, 2001 q14, m/c the membrane filter question, is the answer B?

Also, can i say that membrane filters work on reverse osmosis? coz there's a few ways in which they work.