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Review of Solubility and Precipitation Reactions

This review discusses the concepts of solubility and precipitation reactions in aqueous solutions. It covers topics such as dissociation, solubility rules, precipitation predictions, chemical equilibrium, solubility product constant (Ksp), and classic Ksp problems. Examples are provided throughout to illustrate the concepts discussed.

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Review of Solubility and Precipitation Reactions

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  1. Review of Solubility and Precipitation Reactions

  2. Aqueous Solutions Compounds dissolved into water. Can contain molecules or ions in a solution. How do you distinguish between ion or molecule?

  3. DISSOCIATION !! • The ability of a compound to breakdown in a solution into individual ions • Ionic Compounds • Break down into cations and anions • Electrical conductors—ions flow through solution • Molecular Compound • Compound remains intact as “molecules,” no breakdown • Generally NOT electrical conductors

  4. Solubility • How much solute dissolves in a solution to produce a saturated solution • Temperature and Pressure dependent • Increase with increasing temperature • Increases with decreasing temperature (ex. Water in lake) • Pressure increases, solubility increases (ex. Soda can)

  5. Which compounds are soluble in water? 1) BaCl2 2) Pb (NO3)2 3) Na2S 4) BaCO3 5) PbS

  6. Precipitation Prediction • Write the reactants in ionic from • breakdown into ionic form if compounds are soluble • leave as molecules if insoluble • Determine the solubility of the products. • Use solubility Rules

  7. Precipitation Predictions (cont.) 3) Check to see if one product is insoluble in water. • Product will fall out of solution, identified as precipitate 4) Write the net ionic equation • Displays which ions are directly involved in the reaction, produce the precipitate • Ions existing on BOTH sides of the equation are “spectator ions” (do NOT participate in precipitate formation) • Spectator ions are eliminated

  8. Example 1: MgSO4 + KOH Write the net ionic equation. Will a precipitate form?

  9. Chemical Equilibrium so far----- • Gases • Acids and Bases • Slightly soluble Salts • Many ionic compounds—only a small fraction dissolves

  10. Ksp/Solubility Equilibria

  11. Solubility Equilibrium • Type of equilibrium between ionic solid and a saturated solution • Equilibrium exists between ions and solid • Saturated vs. concentrated solutions are NOT the same thing • Saturation---at equilibrium

  12. Solubility Product Constant (Ksp)

  13. Example 1: • BaSO4(s)  Ba+2(aq) + SO4-2(aq) • Indicates salt exists in “solubility equilibrium”—some dissolves, some does not

  14. Solubility vs. Solubility Constant • Solubility— • How much compound can be dissolved • Molarity of a saturated solution • How much ionic solid can dissolve in a liter of solution • Solubility Product/constant • Numerical value of equilibrium constant • Specific for an equation representing ionic substance dissolving in water

  15. Solubility Product Constant (Ksp) • Equilibrium constant for slightly soluble salts • Indicates equilibrium between solid salt and the ions found in a solution when it dissociates • Expression represents the product of the concentrations of ions in equilibrium • Temperature dependent • Values found in table along with solubility equation (p. 678, Appendix C—p.A18)

  16. Example 2: Write a Ksp expression for an equilibrium in a saturated aqueous solution of iron (III) phosphate and for an equilibrium in a separate aqueous solution of chromium (III) hydroxide.

  17. Ksp and Solution Molarity • Ksp is an equilibrium constant, NOT concentration • Molarity/concentration separate from Ksp • Ksp values • Considered estimates due to ion attractions to other ions in solution and Ksp values not exact • Used only for slightly soluble salts • Increase concentration of ions, increase Ksp value

  18. 2 Classic Ksp Problems Finding solubility of a chemical substance if given its solubility product (Ksp). Given the molar solubility of a chemical substance, find the solubility product (Ksp).

  19. Example 3: Determine the molar solubility of CaF2 given that Ksp= 3.9x10-11

  20. Example 4: • Based on a Ksp value of 1.4 x 10-5 at 25°C for silver sulfate, calculate this compound’s molar solubility at 25°C. • Ag2SO4(s)2Ag+(aq) + SO4-2(aq)

  21. Example 5: Given that the molarity solubility of MgF2= 2.1x10-3 M, calculate Ksp for MgF2.

  22. Example 6: • Without any calculations, arrange the following in order of INCREASING molar solubility. • MgF2, CaF2, PbCl2, PbI2

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