Chapter 4: Chemical Quantities

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Chapter 4 Chemical Quantities Aqueous Reactions

• CHE 123 General Chemistry I
• Dr. Jerome Williams, Ph.D.
• Saint Leo University

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Overview

• Precipitation 101
• Expressing Precipitation Reactions

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Precipitation 101

• A precipitate is an insoluble solid while a
  supernatant is the liquid portion found above a
  settled precipitate.
• We use solubility rules as a separation tool
  (remove a component from a mixture).

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Precipitation 101

• So far, we can predict the solubility of
• (a) CdCO3 (b) MgO (c) Na2S (d) PbSO4 (e)
  (NH4)3PO4 (f) HgCl2

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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) ?

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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)

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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.

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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)

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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)

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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)

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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) ?