1. Chapter 4: Chemical Quantities & Aqueous Reactions CHE 123: General Chemistry I Dr. Jerome Williams, Ph.D. Saint Leo University

2. Overview Precipitation 101 Expressing Precipitation Reactions

3. 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).

4. Precipitation 101 So far, we can predict the solubility of: (a) CdCO 3 (b) MgO(c) Na 2 S (d) PbSO 4 (e) (NH 4 ) 3 PO 4 (f) HgCl 2

5. Precipitation 101 Now predict whether a precipitate will form for: – (a) NiCl 2 (aq) + (NH 4 ) 2 S(aq)  – (b) Na 2 CrO 4 (aq) + Pb(NO 3 ) 2 (aq)  – (c) AgClO 4 (aq) + CaBr 2 (aq) 

6. 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)

12. 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.

13. Expressing Precipitation Reactions Molecular Equation: All reactants and products are written in molecular (non-dissociated) form along with their phases. – Pb(NO 3 ) 2 (aq) + 2 KCl(aq)  PbCl 2 (s) + 2 KNO 3 (aq) – 2 HCl(aq) + Cu(OH) 2 (s)  CuCl 2 (aq) + 2 HOH(l) – C 2 H 3 O 2 H(aq) + KOH(aq)  KC 2 H 3 O 2 (aq) + HOH(l)

14. Expressing Precipitation Reactions Ionic Equation: All dissolved strong electrolytes in the molecular equation are broken into their ions. – Pb 2+ + 2 NO 3 – + 2 K + + 2 Cl –  PbCl 2 (s) + 2 K + + 2 NO 3 – – 2 H + + 2 Cl – + Cu(OH) 2 (s)  Cu 2+ + 2 Cl – + 2 HOH(l) – C 2 H 3 O 2 H(aq) + K + + OH –  K + + C 2 H 3 O 2 – + HOH(l)

16. Expressing Precipitation Reactions Net Ionic Equation: Spectator ions that occur on both sides are cancelled to give only those species which undergo change. – Pb 2+ + 2 Cl –  PbCl 2 (s) – 2 H + + Cu(OH) 2 (s)  Cu 2+ + 2 HOH(l) – C 2 H 3 O 2 H(aq) + OH –  C 2 H 3 O 2 – + HOH(l)

17. Expressing Precipitation Reactions Write the molecular, complete ionic equation, and net ionic equations for the following reactions: – AgNO 3 (aq) + Na 2 CrO 4 (aq)  – K 2 SO 4 (aq) + MgCO 3 (aq) 