4 2 Precipitation Reactions Advanced Chemistry Precipitation Reactions

4. 2 Precipitation Reactions Advanced Chemistry

Precipitation Reactions Precipitation Reaction: a reaction between two solutes that produces a precipitate. Precipitate: an insoluble solid formed by a reaction in solution Precipitation Reactions are generally double replacement reactions Also known as exchange reactions or metathesis

Precipitation Reactions

Precipitation Reaction Precipitation reactions occur when pairs of oppositely charged ions attract each other so strongly they form an insoluble solid. To predict whether certain combinations of ions form insoluble compounds, we must consider guidelines concerning the solubilities of common ionic compounds.

Solubility Guidelines for Ionic Compounds Solubility: amount of the substance that can be dissolved in a given quantity of solvent at a given temperature. Solubility is determined experimentally If no compounds are insoluble in a reaction, THE REACTION DOES NOT OCCUR Any substance with a solubility less than 0. 01 mol/L will be referred to as insoluble. Attraction between the oppositely charged ions in the solid are too great for the water molecules to separate the ions.

Solubility Guidelines for Ionic Compounds

Using Solubility Rules Classify these ionic compounds as soluble or insoluble in water 1. Sodium carbonate 2. Na 2 CO 3 3. Lead sulfate (Pb. SO 4) 4. Cobalt(II) 5. Barium hydroxide nitrate 6. Ammonium phosphate

Predicting Precipitates To predict whether precipitate forms when we mix aqueous solutions of two electrolytes: 1) Note the ions present in the reactants 2) Consider the possible cation-anion combinations 3) Use the solubility table to determine if any of these combinations are insoluble

Predicting Precipitate Examples Will a precipitate form when solutions of Mg(NO 3)2 and Na. OH are mixed? 1. Both substances are soluble ionic compounds and strong electrolytes. 2. Mixing the solution will create the Mg 2+, NO 3 -, Na+, and OH- ions. 3. The possible combinations of products from the reactants are Mg(OH)2 and Na. NO 3 4. We look at the table and see that Mg(OH)2 is insoluble because hydroxides are generally insoluble.

Now You Try Predict the identify of the precipitate that forms when aqueous solutions of Ba. Cl 2 and K 2 SO 4 are mixed. What compound precipitates when aqueous solutions of Fe 2(SO 4)3 and Li. OH are mixed? Will a precipitate form when solutions of Ba(NO 3)2 and KOH are mixed?

Aqueous Solution Equations In writing equations for reactions in aqueous solutions, it is often useful to indicate whether the dissolved substances are present predominately as ions or as molecules. Molecular Equation: shows the complete chemical formulas of reactants and products (reactants and products in molecular form Ionic Equation (complete ionic equation): all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. Net Ionic Equation: includes only the ions and molecules directly involved in the reaction To form a net ionic equation, cross out anything that does not change from the left side of the equation to the right in a ionic equation. The things that didn’t change (and were deleted from the net ionic equation) are called spectator ions).

Aqueous Solution Equations Molecular Equations: Ag. NO 3(aq) Ionic + KCl(aq) Ag. Cl(s) + KNO 3(aq) Equation: Ag+(aq) + NO 3−(aq) + K+(aq) + Cl−(aq) Ag. Cl(s) + K+(aq) + NO 3−(aq) Net Ionic Equations Ag+(aq) + Cl−(aq) Ag. Cl(s)

Writing Net Ionic Equations 1. Write a balanced molecular equation. 2. Dissociate all strong electrolytes. 3. Cross out anything that remains unchanged from the left side to the right side of the equation. 4. Write the net ionic equation with the species that remain. *If every ion in a complete ionic equation is a spectator, no reaction occurs.

Practice Which ions, if any, are spectator ions in a reaction Ag. NO 3(aq) + Na. Cl (aq) Ag. Cl(s) + Na. NO 3(aq) Write the net ionic equation for the precipitation reaction that occurs when aqueous solutions of calcium chloride and sodium carbonate are mixed.

Now You Try Write the net ionic equation for the precipitation reaction that occurs when aqueous solutions of silver nitrate and potassium phosphate are mixed.