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4.2 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

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4.2 Precipitation Reactions

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  1. 4.2 Precipitation Reactions Advanced Chemistry

  2. 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

  3. Precipitation Reactions

  4. 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.

  5. 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.

  6. Solubility Guidelines for Ionic Compounds

  7. Using Solubility Rules • Classify these ionic compounds as soluble or insoluble in water • Sodium carbonate • Na2CO3 • Lead sulfate (PbSO4) • Cobalt(II) hydroxide • Barium nitrate • Ammonium phosphate

  8. 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

  9. Predicting Precipitate Examples • Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed? • Both substances are soluble ionic compounds and strong electrolytes. • Mixing the solution will create the Mg2+, NO3-, Na+, and OH- ions. • The possible combinations of products from the reactants are Mg(OH)2 and NaNO3 • We look at the table and see that Mg(OH)2 is insoluble because hydroxides are generally insoluble.

  10. Now You Try • Predict the identify of the precipitate that forms when aqueous solutions of BaCl2 and K2SO4 are mixed. • What compound precipitates when aqueous solutions of Fe2(SO4)3 and LiOH are mixed? • Will a precipitate form when solutions of Ba(NO3)2 and KOH are mixed?

  11. 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).

  12. Aqueous Solution Equations • Molecular Equations: • AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq) • Ionic Equation: • Ag+(aq) + NO3−(aq) + K+(aq) + Cl−(aq)  AgCl(s) + K+(aq) + NO3−(aq) • Net Ionic Equations • Ag+(aq) + Cl−(aq) AgCl(s)

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

  14. Practice Which ions, if any, are spectator ions in a reaction AgNO3(aq) + NaCl (aq)  AgCl(s) + NaNO3(aq) Write the net ionic equation for the precipitation reaction that occurs when aqueous solutions of calcium chloride and sodium carbonate are mixed.

  15. 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.

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