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Precipitation Reactions and Predicting Solids

Learn about the common results of chemical reactions, specifically the formation of a solid. Understand how to predict the solid that forms in a precipitation reaction and how to write molecular, complete ionic, and net ionic equations.

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Precipitation Reactions and Predicting Solids

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  1. Objectives • To learn more about some of the results of chemical reactions • To learn to predict the solid that forms in a precipitation reaction • To learn to write molecular, complete ionic, and net ionic equations

  2. A. Common Results of a Reaction • Four common eventsaccompanychemical reactions. These often occur when a reaction happens • Formation of a solid • Formation of water • Formation of a gas • Transfer of electrons Can you give examples of each of these?

  3. B. Precipitation Reactions • What Happens When an Ionic Compound Dissolves in Water? H2O NaCl(aq) NaCl(s) But what does NaCl(aq) mean? Na+ and Cl- ions in water ? NaCl molecules in water ? Na and Cl atoms in water?

  4. B. Precipitation Reactions • What Happens When an Ionic Compound Dissolves in • Water? • The ions separate and move around independently – a strong electrolyte

  5. B. A Reaction in Which a Solid Forms • A reaction in which a solid is formed from a solution is called a precipitation reaction • Solid = precipitate = insoluble When positive and negative ions form a precipitate the net charge of the precipitate is zero Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq)

  6. Stalactites in Yosemite Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(aq)

  7. B. Precipitation Reactions • What Happens When Two Ionic Compounds Dissolve in • Water? • K2CrO4(aq) + Ba(NO3)2(aq) Products

  8. B. Precipitation Reactions • How to Decide What Products Form • Determine the possible products from the ions in the reactants. • In our example • K2CrO4(aq) + Ba(NO3)2(aq) Products • The possible ion combinations are

  9. What are the potential products in the reactions between the following aqueous solutions? – Write balanced equations for each • NaOH + CaCl2 • CuBr2 + (NH4)2CO3 • K2SO4 + Fe(NO3)3

  10. B. Precipitation Reactions • How Would We Decide What Product Has Formed? • What is most likely to be the yellow solid formed in the following reaction? • K2CrO4(aq) + Ba(NO3)2(aq) • The possible product combinations are KNO3 and BaCrO4 • KNO3 white solid • BaCrO4 yellow solid

  11. B. Precipitation Reactions – Solubility Rules

  12. B. Precipitation Reactions • Using Solubility Rules • Predicting Precipitates • Soluble solid • Insoluble solid • (Slightly soluble solid) Why are some compounds soluble and others not?

  13. Solubility Rules – Are These Compounds Soluble or Insoluble? Barium Nitrate Potassium Carbonate Sodium Sulfate Copper (II) Hydroxide Mercury (I) Chloride Ammonium Phosphate Chromium (III) Sulfide Lead (II) Sulfate

  14. B. Precipitation Reactions Let’s try some predictions: WOC P271 Q10

  15. C. Describing Precipitation Reactions in Aqueous Solutions • Three types of equations: • Molecular (formula) equation  complete formulas of all reactants and products • Complete ionic equation  all strong electrolytes are shown as ions • Net ionic equation  only those components of the solution that undergo a change • Spectator ion (those that remain unchanged)  not shown in the net ionic equation

  16. C. Describing Precipitation Reactions in Aqueous Solutions • Molecular Equation Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq) • Complete Ionic Equation Pb2+(aq) + 2NO3-(aq) + 2Na+(aq) + 2I-(aq) PbI2(s) + 2Na+(aq) + 2NO3-(aq)

  17. C. Describing Precipitation Reactions in Aqueous Solutions Pb2+(aq) + 2I-(aq) PbI2(s) • Net Ionic Equation • What are the spectator ions in this reaction? A solution of sodium chloride in water reacts with aqueous silver nitrate to give a precipitate of silver chloride – write down the three equations that describe this reaction and identify the spectator ions (P272 Q14 – all three equations and identify spectators)

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