Chapter 4: Chemical Quantities & Aqueous Reactions CHE 123: General Chemistry I Dr. Jerome Williams, Ph.D. Saint Leo University
Overview • Precipitation 101 • Expressing Precipitation Reactions
Precipitation 101 • A precipitate is an insoluble solid; the supernatant liquid is the portion found above the settled precipitate. • We use solubility rules as a separation tool (remove a component from a mixture).
Precipitation 101 • So far, we can predict the solubility of: (a) CdCO3(b) MgO (c) Na2S (d) PbSO4(e) (NH4)3PO4(f) HgCl2
Precipitation 101 • Now predict whether a precipitate will form for: • (a) NiCl2(aq) + (NH4)2S(aq) • (b) Na2CrO4(aq) + Pb(NO3)2(aq) • (c) AgClO4(aq) + CaBr2(aq)
Precipitation 101 • How do we predict products? • The Beaker Analogy • 1. Draw beaker • 2. Place ions into beaker • 3. Identify candidates • 4. Write out products (identify precipitate)
Expressing Precipitation Reactions • Three basic equations seen for precipitation reactions: • Molecular Equation: All reactants and products are written in molecular form. • Ionic Equation: All dissolved strong electrolytes are written as the dissociated ions. • Net Ionic Equation: All ions that are identical on both sides are deleted.
Expressing Precipitation Reactions • Molecular Equation: All reactants and products are written in molecular (non-dissociated) form along with their phases. • Pb(NO3)2(aq) + 2 KCl(aq) PbCl2(s) + 2 KNO3(aq) • 2 HCl(aq) + Cu(OH)2(s) CuCl2(aq) + 2 HOH(l) • C2H3O2H(aq) + KOH(aq) KC2H3O2(aq) + HOH(l)
Expressing Precipitation Reactions • Ionic Equation: All dissolved strong electrolytes in the molecular equation are broken into their ions. • Pb2+ + 2 NO3– + 2 K+ + 2 Cl– PbCl2(s) + 2 K+ + 2 NO3– • 2 H+ + 2 Cl– + Cu(OH)2(s) Cu2+ + 2 Cl– + 2 HOH(l) • C2H3O2H(aq) + K+ + OH– K+ + C2H3O2– + HOH(l)
Expressing Precipitation Reactions • Net Ionic Equation:Spectator ions that occur on both sides are cancelled to give only those species which undergo change. • Pb2+ + 2 Cl– PbCl2(s) • 2 H+ + Cu(OH)2(s) Cu2+ + 2 HOH(l) • C2H3O2H(aq) + OH– C2H3O2– + HOH(l)
Expressing Precipitation Reactions • Write the molecular, complete ionic equation, and net ionic equations for the following reactions: • AgNO3(aq) + Na2CrO4(aq) • K2SO4(aq) + MgCO3(aq)