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Section 4.2

Section 4.2. Precipitation Reactions. Precipitation Reactions. Mixing ions that form insoluble compounds The insoluble solid formed is a precipitate. Solubility. The amount of substance that can dissolve in a given quantity of solvent at a given temperature. Solubility Guidelines.

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Section 4.2

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  1. Section 4.2 Precipitation Reactions

  2. Precipitation Reactions Mixing ions that form insoluble compounds The insoluble solid formed is a precipitate

  3. Solubility • The amount of substance that can dissolve in a given quantity of solvent at a given temperature

  4. Solubility Guidelines

  5. Steps for Predicting Precipitation • Note ions present in reaction • Consider possible combinations of cations and anions • Use Table 4.1 to determine if any of those combinations are insoluble

  6. Example • Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed?

  7. Step 1: Ions present • Mg 2+ • NO3– • Na + • OH -

  8. Step 2: Possible combinations • Mg 2+ withOH – • Na + with NO3–

  9. Step 3: Table 4.1 • Hydroxides generally insoluble, and Mg is not an exception • NaNO3 is soluble Mg(NO3)2 (aq) + 2NaOH (aq)  Mg(OH)2 (s) + 2NaNO3 (aq)

  10. Metathesis- Greek, “to transpose” Pattern of precipitation formation AX + BY  AY + BX AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Metathesis (Exchange) Reactions

  11. AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Metathesis (Exchange) Reactions

  12. Balancing Metathesis Reactions • Determine ions present • Write chemical formulas of products by combining cation from one reactant with anion of the other • Use charges of ions to determine subscripts • Balance the equation

  13. Sample Problem • Predict the identity of the precipitate that forms when solutions of BaCl2 and K2SO4 are mixed.

  14. Step 1: Determine ions • BaCl2 and K2SO4 are mixed: • Ba 2+ • Cl - • K + • SO4 2-

  15. Step 2: Combine • Reactants: BaCl2 and K2SO4 BaCl2 + K2SO4 BaSO4 + K2Cl2

  16. Step 3: Subscripts Ba 2+ Cl - K +SO4 2- BaCl2 + K2SO4 BaSO4 + KCl

  17. Step 4: Balance BaCl2 + K2SO4 BaSO4 + 2KCl Did a precipitate form?

  18. Solution BaCl2 (aq) + K2SO4(aq)  BaSO4(s) + 2KCl (aq)

  19. AgNO3 (aq) + KCl(aq) AgCl(s) + KNO3 (aq) Molecular Equation • Reactants and products in their molecular form • NOT ionic character

  20. All soluble strong electrolytes shown as ions Spectator ions- appear as reactants and products in identical form Ag+(aq) + NO3-(aq) + K+ (aq) + Cl- (aq)  AgCl(s) + K+(aq) + NO3-(aq) Complete Ionic Equation

  21. Net Ionic Equation • Omit spectator ions Ag+ (aq) + Cl- (aq)  AgCl(s)

  22. Writing Net Ionic Equations • Write a balanced molecular equation. • Dissociate all strong electrolytes. • Identify and cancel spectator ions

  23. Sample Problem • Write the net ionic equation for mixing calcium chloride and sodium carbonate.

  24. Step 1: Molecular equation • Calcium chloride and sodium carbonate CaCl2 + Na2CO3  CaCO3 + 2NaCl

  25. Step 2: Dissociate strong electrolytes Ca 2+ + 2Cl- + 2Na++CO3 2- CaCO3 + 2Na+ + 2Cl- All are strong electrolytes, but CaCO3 is insoluble in water

  26. Step 3: Omit spectators Ca 2+ (aq) +CO3 2-(aq) CaCO3 (s)

  27. Homework • 4.19-4.24 on page 158

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