Chapter 4:
Chapter 4a:
Reactions in
Aqueous Solutions
Precipitation
Reactions
- Electrolyte Precipitation Reactions
- Acid /Base Reactions
- Oxidation-Reduction Reactions
Chem 111
Dr. Gentry
Chem 111
Dr. Gentry
On
Electrolytes
• Electrolytes:
• Compounds that conduct electricity when dissolved in water
Conduct electricity
• Conductivity due to presence of independently mobile ions
- Soluble ionic compounds
- Acids or bases like HCl or NaOH
‒
+
–
+
+ –
+ –
–
1) Must be soluble AND 2) break into ions
Bulb on
(ions conduct)
Bulb off
(no conductivity)
Off
• Non-Electrolytes:
–
+
–
Pure
Water
NaCl in
Water
+
+
–
–
+
+
+
–
… Except if also contain
… alkali metal cations (Group I)
Off
Do not conduct electricity
- Covalent compounds like sugar
- Or Ionic compounds that remain
solid in water
(insoluble in water)
+–+
–+–
undissolved
solid
–
Soluble Ionic Compounds
Compounds containing
+
Electrolytes: Not a Case of “Either / Or”
• Strong Electrolytes:
–
Highly soluble in water
Strongly conduct electricity
… ammonium cations (NH4+)
neutral
covalent
molecules
–
+
… nitrate anions (NO3–)
NaCl(s)
… chloride (Cl –), bromide (Br –), or
iodide (I –) anions
… Ag+, Hg22+, or Pb2+ cations
… sulfate anions (SO42–)
… Ag+, Hg22+, Sr2+, Pb2+, Ca2+, or Ba2+
Insoluble Ionic Compounds
Compounds containing
… Except if also contain
… carbonate (CO32–), phosphate
(PO43–), chromate(CrO42–) or
sulfide (S 2–) anions
… Li+, Na+, K+, Rb+, Cs+, NH4+ cations
… hydroxide anion (OH-)
… Li+, Na+, K+, Rb+, Cs+, Ba2+, or NH4+
cations
+
+ –
+ –
Na+(aq) + Cl–(aq)
• Weak Electrolytes:
Low, but not zero, dissociation in water
Some of compound dissociates into ions
Most of compound remains as compound
Weakly conduct electricity
bound
+
–
+–
+–
+–
compound
HCH3CO2
H+ (aq) + CH3CO2– (aq)
acetic acid
1
Solubility vs. Temperature
Solubility in Water (g / 100mL)
Glucose
CH3CO2Na
Precipitation
Reactions
NaNO3
• Reactants = all soluble ionic compounds
Products = at least one insoluble ionic compound
KBr
Cu(NO3)2(aq) + NaOH(aq) → Cu(OH)2 (s) + 2 NaNO3(aq)
CuSO4
• Based on swapping cations and anions
NaCl
Ce2(SO4)3
AB + CD →
AD
+ CB
Cu (NO3)2(aq) + Na OH(aq) → Cu (OH)2 (s) +2 Na NO3(aq)
Temperature (°C)
Precipitation Reactions
Precipitation
Reactions
• Reaction of soluble ionic compounds in solution
to form insoluble ionic compound as product
• Soluble reactants combining to form solid product
Cu(NO3)2(aq) + NaOH(aq) → Cu(OH)2 (s) + 2 NaNO3(aq)
Cu2+
NO3–
NO3–
Cu2+ NO3–
NO3–
Na+
+
OH–
OH–
solid
formation
Na+
Na+
NO3–
NO3–
Na+
Cu(OH)2(s)
Cu(NO3)2(aq) + NaOH(aq) → Cu(OH)2 (s) + 2 NaNO3(aq)
• Compare to fully soluble reactants & products
(aq) = notation refers to dissolved (dissociated)
ions in water
Precipitate = solid (s) formed from otherwise
soluble reactants
NaCl (aq)
Na+
Cl–
Net Ionic Equations
Molecular Equation
Cl–
Na+
+
+
KNO3 (aq)
→
K+
NO visible
reaction
NO3–
KCl (aq) + NaNO3 (aq)
K+
NO3–
K+
Na+
Cl–
NO3–
Na+
K+
Cl–
NO3–
Writing Ionic Compounds in Ionic Form
• First decide where to split the compound
Pb(NO3)2 (aq) + 2KI (aq) → 2 KNO3 (aq) + PbI2 (s)
• Afterwards, be careful to include charge on ions
Remember: charge on initial compound had to add to zero
• Watch the number of ions (subscript moves in front)
Ionic Equation
- Show ionic compounds in dissolved aqueous form
Pb2+ + 2 NO3– + 2 K+ + 2 I–
→ 2 K+ + 2 NO3– + PbI2(s)
Mg Cl2 (aq)
Net Ionic Equation
- Ignore the “spectator ions”
Mg Cl2 (aq)
(those that stay unchanged)
Pb2+(aq) + 2 I– (aq) → PbI2(s)
NO3– and K+ found on
both sides of ionic eqn.
Mg2+ (aq) + 2 Cl – (aq)
2
Writing Net Ionic Equations
If Only Given Reactants
Writing Net Ionic Equations
1a) Split the reactants, and recombine to give products
Pb (NO3)2 + K I → K NO3 + Pb I
1) Split the reactants, and recombine to give products
- Assign charges to each ion,
- Balance charges so each ionic compound is zero
1b) Assign ionic charges, balance ionic cmpds
Pb2+ (NO3)‒2 + K+ I‒ → K+ NO3‒
+
Pb2+ I‒2
2) Write Molecular Eqn: assign solubilities, balance rxn
2) Write full Molecular Equation
- Assign solubilities for each compound
- Balance atoms in the reaction
3) Split the aqueous compounds to give Ionic Equation
Pb(NO3)2 (aq) + 2 KI (aq) → 2 KNO3 (aq) + PbI2 (s)
3) Split the aqueous compounds to give Ionic Equation
Pb2+ + 2 NO3– + 2 K+ + 2 I– →
2 K+ + 2 NO3– + PbI2(s)
4) Cancel spectator ions to give Net Ionic Equation
4) Cancel spectator ions to give Net Ionic Equation
Precipitation Reactions - Review
• Which reactions are precipitation reactions?
• What are the net ionic equations?
1) Cu(NO3)2 (aq) + Na2CO3 (aq) → CuCO3 (s) + 2NaNO3 (aq)
Cu 2+ (aq) + CO3 2– (aq)
2) K2SO4 (aq) + CaCl2 (aq)
SO42–
(aq) +
Ca2+
→
→ CuCO3 (s)
2 KCl (aq) + CaSO4 (s)
(aq) → CaSO4 (s)
3) CdCl2 (aq) + (NH4)2S (aq)
Cd 2+ (aq) + S 2– (aq)
→
CdS (s) + 2 NH4Cl (aq)
→ CdS (s)
Pb2+(aq) + 2 I– (aq) → PbI2(s)
NO3– and K+ found on
both sides of ionic eqn.
Precipitation
Reactions
• Reactants = all soluble ionic compounds
Products = at least one insoluble ionic compound
Cu(NO3)2(aq) + NaOH(aq) → Cu(OH)2 (s) + 2 NaNO3(aq)
• Based on swapping cations and anions
Cu (NO3)2(aq) + Na OH(aq) → Cu (OH)2 (s) +2 Na NO3(aq)
3