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07 – Precipitation Reactions

Chemistry 30 – Unit 2 – Solubility – Ch. 16 in Text. 07 – Precipitation Reactions. Precipitate Reactions. A precipitate is a solid that forms when two aqueous solutions are mixed together. They result because the compound formed is insoluble in water .

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07 – Precipitation Reactions

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  1. Chemistry 30 – Unit 2 – Solubility – Ch. 16 in Text 07 – Precipitation Reactions

  2. Precipitate Reactions • A precipitate is a solid that forms when two aqueous solutions are mixed together. • They result because the compound formed is insoluble in water. • In general, precipitates form during a double replacement reaction. • In a double replacement reaction, the reactants will both be aqueous and one of the products formed will come out of solution in the form of a precipitate, gas or liquid • In precipitate reactions, we are only concerned with the solid precipitate that may form

  3. How do we know when a precipitate reaction actually takes place? • A reaction will only take place if at least one of the products is a solid (a precipitate). • If no precipitate forms, then no reaction actually takes place because all the ions remain in the solution. • A solubility table will be needed to check the solubility of each product.

  4. Representing Precipitation Reactions • Precipitation reactions can be represented in three ways: • 1) Formula Equations • This is the typical chemical reaction that you are used to • Includes the reactant and product substances • 2) Ionic Equations • This is a representation of all of the ions that are present in the solution on the reactants side and the products side • 3) Net Ionic Equations • The chemical equation showing only the ions that make up the precipitate

  5. Representing Precipitation Reactions • i.e.: Lead (II) nitrate reacts with sodium iodide to produce lead (II) iodide and sodium nitrate • 1) Formula equation: • Pb (NO3)2(aq) + 2 NaI(aq) 2 NaNO3(aq) +PbI2(s) • 2) Ionic Equation: • Pb+2(aq)+ 2 NO3 -(aq) + 2 Na+(aq) +2 I-(aq) 2 Na+(aq)) + 2 NO3- (aq) +PbI2(s) • 3) Net Ionic Equation: • Pb+2(aq) +2 I-(aq)PbI2(s)

  6. Net Ionic Equations • These only focus on the ions involved in forming the precipitate compound • The other ions are still present in the solution, however, they do not form a solid compound • Spectator Ions: • Ions that are present in a double replacement reaction, but do not react to form a precipitate • Since their state does not change, they do not actually react. • In our previous example, NO3 -(aq) Na+(aq) would be the spectator ions

  7. Does the Double Replacement Reaction Actually Take Place? • A reaction will only take place if at least one of the products is a solid (a precipitate). • If no precipitate forms, then no reaction actually takes place because all the ions remain in the solution. • In this case, all the ions would be considered spectators.

  8. Examples • Write a balanced equation for the following reactants. Then, write the net ionic equation. • a) copper (II) sulfate and sodium hydroxide CuSO4(aq) + 2NaOH(aq)  Cu(OH)2 (?) + Na2SO4 (?) *Check solubility table to determine the states of products CuSO4(aq) + 2NaOH(aq)  Cu(OH)2(s) + Na2SO4(aq) Net Ionic Equation: Cu2+ (aq) + 2OH-(aq)  Cu(OH)2(s)

  9. Examples: • b) magnesium bromide and sulfuric acid MgBr2(aq) + H2SO4(aq)  MgSO4(?) + 2HBr (?) *Check solubility table to determine the states of products - Remember that acids are ALWAYS aqueous MgBr2(aq) + H2SO4(aq)  MgSO4(aq) + 2HBr (aq) *Since both products are soluble in water, no precipitate forms, and no reaction occurs *You will simply have a homogenous solution of aqueous ions

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