Precipitates - Reactions in solution Precipitation...

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R eactions in solution Precipitation reactions —not all ionic compounds are soluble in water. If PbSO 4 is added to water none of the lead(II)sulfate will dissolve (the interaction between Pb 2+ and SO 4 2- is simply stronger than the attraction of the water to either the Pb 2+ or SO 4 2- (entropy also plays a roll, but we will discuss that next semester).) There is a table on page 150 which describes some simple solubility rules. Simple Rules for the Solubility of Salts in Water 1. Most nitrate (NO 3 - ) salts are soluble. 2. Most salts containing the alkali metal ions (Li + , Na + , K + , Cs + , Rb + ) and the ammonium ion (NH 4 + ) are soluble. 3. Most chloride, bromide and iodide salts are soluble. Notable exceptions are salts containing the ions Ag + , Pb 2+ , and Hg 2 2+ . 4. Most sulfate salts are soluble. Notable exceptions are BaSO 4 , PbSO 4 , HgSO 4 , and CaSO 4 . 5. Most hydroxide salts are only slightly soluble. The important soluble hydroxides are NaOH and KOH. The compounds Ba(OH) 2 , Sr(OH) 2 , and Ca(OH) 2 are marginally soluble. 6. Most sulfide (S -2 ), carbonate (CO 3 2- ), chromate (CrO 4 2- ), and phosphate (PO 4 -3 ) are only slightly soluble. If two solutions are mixed together it is possible that two ions could combine to form an insoluble ionic compound. A solution of silver nitrate is combined with a solution of sodium chloride. The resulting solution contains Na + , Ag + , Cl + , and NO 3 - , but AgCl is not soluble in water. Since Ag + is now in solution with Cl - the two ions will combine to form AgCl, and the AgCl will precipitate from the solution. The reaction can be described in a number of ways (1) A “molecular” equation could be used: AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) 54
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(2) A complete ionic equation could be used: Ag + (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq) AgCl (s) + Na + (aq) + NO 3 - (aq) (3) A net ionic equation could be used. Ag + (aq) + Cl - (aq) AgCl (s) Net ionic equations are found by writing the full equation and then eliminating the spectator ions; spectator ions are ions that do not participate in the reaction. In the example above, Na + , and NO 3 are present as both products and reactants; they do not participate in the reaction.
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Precipitates - Reactions in solution Precipitation...

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