Post on 03-Jan-2016
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SolubilityLesson 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!
Ba2+
1st add Cl-, it reacts with Pb2+ (low) forming a ppt
2nd add SO42-, it reacts with Ba2+ (low) forming a ppt.
Pb2+ Ba2+
NaCl
Na+ Cl-
PbCl2
Filter
Separating Positive Ions
Cl- does not existon its ownAdd NaCl becauseit is soluble- always use Na+ for negative ions
1. Separate Ba2+ & Pb2+
i. Add NaCl Filter out PbCl2(s)
Pb2+ + 2Cl- PbCl2(s)
ii. Add Na2SO4 Filter out BaSO4(s)
Ba2+ + SO42- BaSO4(s)
Ba2+
Na2SO4
Na+ SO4-2
BaSO4
Filter
Cl SO4
Ba ------ ppt
Pb ppt ppt
Separating Positive Ions
2. Separate Cu2+, Mg2+ & Sr2+
Cu2+ Mg2+ Sr2+
Cu2+ Mg2+
Cu2+
Mg2+
Separating Positive Ions
2. Separate Cu2+, Mg2+ & Sr2+
i. Add Na2SO4 Filter out SrSO4(s) Sr2+ + SO4
2- SrSO4(s)
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)
SO42- does not exist
on its ownAdd Na2SO4 becauseit is soluble- always use Na+ for negative ions
SO4 S OH
Cu ---- ppt ppt
Mg ---- ---- ppt
Sr ppt ---- ----
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+
Remove the bottom negative ion first by adding a positive cation.
Look for a cation that is low with OH- and high with Cl-.
Look for a cation that is low with with Cl-
Separating Negative Ions 3. Separate Cl- & OH-
ii. Add Ag+ as AgNO3 and filter out AgCl(s)
Ag+ + Cl- AgCl(s)
Ba2+ does not existon its ownAdd Ba(NO3)2 becauseit is soluble- always use NO3
- to pair with positive ions
i. Ba2+ works so add Ba(NO3)2 and filter out Ba(OH)2(s)
Ba2+ + 2OH- Ba(OH)2(s)
Separating Negative Ions 4. Separate Cl- S2- CO3
2-
Ba2+
Zn2+
Look for a cation that forms a ppt with only CO3
2- (high with Cl- and S2- )
Look for a cation that is low with S2- and high with Cl-
Look for a cation that is low with with Cl-
Separating Negative Ions 4. Separate Cl- S2- CO3
2-
i. Ba2+ works so add Ba(NO3)2 and filter out BaCO3(s)
Ba2+ + CO32- BaCO3(s)
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)
Ba2+ does not existon its ownAdd Ba(NO3)2 becauseit is soluble- always use NO3
- to pair with positive ions
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
You may want to create a table to help you!
Practice:
Page90 #’s 28,29,30,35,36