Solubility Lesson 3 Separating Ions

Basic idea You have an aqueous solution that contains ions. You want to separate the ions. Looking at the Solubility Table, you add an ion to your solution that you KNOW will form a precipitate with only one of your existing ions ( low solubility ) After precipitation, you use filtration to remove the ppt. Repeat until you separate the ions you want.

Separating positive ions

Positive ions react with negative ions to give a precipitate if they have low solubility. A precipitate can be separated from a solution by filtration. The precipitate will be on the filter paper while the soluble ions will go through. To separate ions; one ion must be low solubility and all others high.

Separating Positive Ions   1. Separate Ba2+ & Pb2+ Look at your “Solubility Table" and start with adding Cl- first Remember, if you want to form a ppt, you want to look at the ‘low solubility’ side because it means a ppt will form!

1st add Cl-, it reacts with Pb2+ (low) forming a ppt Ba2+ 1st add Cl-, it reacts with Pb2+ (low) forming a ppt 2nd add SO42-, it reacts with Ba2+ (low) forming a ppt.

NaCl Na+ Cl- Pb2+ Ba2+ PbCl2 Filter

Separating Positive Ions   1. Separate Ba2+ & Pb2+ i. Add NaCl Filter out PbCl2(s) Cl- does not exist on its own Add NaCl because it is soluble- always use Na+ for negative ions Pb2+ + 2Cl-  PbCl2(s) ii. Add Na2SO4 Filter out BaSO4(s)  Ba2+ + SO42-  BaSO4(s)

Na2SO4 Na+ SO4-2 Ba2+ BaSO4 Filter

Cl SO4 Ba ------ ppt Pb

Separating Positive Ions 2. Separate Cu2+, Mg2+ & Sr2+  

Cu2+ Mg2+ Sr2+ Cu2+ Mg2+ Cu2+ Mg2+

Cu2+ + S2-  CuS(s) Separating Positive Ions 2. Separate Cu2+, Mg2+ & Sr2+ i. Add Na2SO4 Filter out SrSO4(s)  Sr2+ + SO42-  SrSO4(s) SO42- does not exist on its own Add Na2SO4 because it is soluble- always use Na+ for negative ions ii. Add Na2S Filter out CuS(s)  Cu2+ + S2-  CuS(s) iii. Add NaOH Filter out Mg(OH)2(s)  Mg2+ + 2OH-  Mg(OH)2(s)

SO4 S OH Cu ---- ppt Mg Sr

Separating Negative Ions

Separating Negative Ions   On “Solubility table” start from bottom working your way up- so start with negative ion on the bottom first. 3. Separate Cl- & OH- Start with OH- because it is on bottom of Solubility Table. Look for a cation/positive ion that is low with OH- (forms ppt) and high with Cl- ( doesn’t form ppt)

Ba2+ Ba2+ Look for a cation that is low with with Cl- Look for a cation that is low with OH- and high with Cl-. Ba2+ Remove the bottom negative ion first by adding a positive cation.

Separating Negative Ions   3. Separate Cl- & OH- i. Ba2+ works so add Ba(NO3)2 and filter out Ba(OH)2(s) Ba2+ + 2OH-  Ba(OH)2(s) Ba2+ does not exist on its own Add Ba(NO3)2 because it is soluble- always use NO3- to pair with positive ions ii. Add Ag+ as AgNO3 and filter out AgCl(s) Ag+ + Cl-  AgCl(s)

Separating Negative Ions    4. Separate Cl- S2- CO32-

Look for a cation that is low with with Cl- Look for a cation that is low with S2- and high with Cl- Zn2+ Look for a cation that forms a ppt with only CO32- (high with Cl- and S2- ) Ba2+

Separating Negative Ions    4. Separate Cl- S2- CO32- i. Ba2+ works so add Ba(NO3)2 and filter out BaCO3(s) Ba2+ + CO32-  BaCO3(s) Ba2+ does not exist on its own Add Ba(NO3)2 because it is soluble- always use NO3- to pair with positive ions ii. Add Zn2+ as Zn(NO3)2 and filter out ZnS(s) Zn2+ + S2-  ZnS(s) iii. Add Ag+ as AgNO3 and filter out AgCl(s) Ag+ + Cl-  AgCl(s)

Summary Separating Positive Ion Look at solubility table starting with Cl, choose a negative ion (Cl or otherwise) that can precipitate only ONE of the ions you have - Filter out the resulting precipitate and repeat!

Summary Separating Negative Ions Look at solubility table starting from bottom, choose a positive ion that can precipitate only ONE of the ions you have. - Filter out the resulting precipitate and repeat!

Summary Practice: Page90 #’s 28,29,30,35,36 You may want to create a table to help you! Practice: Page90 #’s 28,29,30,35,36