Iron(III) chloride and sodium hydroxide: FeCl3 + NaOH → Fe(OH)3 + NaCl Balanced Equation: FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl Total Ionic Equation: Fe3+ + 3Cl- + 3Na+ + 3OH- → Fe(OH)3 + 3Na+ + 3Cl- Net Ionic Equation: Fe3+ + 3OH- → Fe(OH)3 Spectator Ions: Na+ and Cl- Precipitate: Fe(OH)3 Magnesium sulfate and potassium phosphate (potassium produces a soluble compound): MgSO4 + K3PO4 → Mg3(PO4)2 + K2SO4 Balanced Equation: 3MgSO4 + 2K3PO4 → Mg3(PO4)2 + 3K2SO4 Total Ionic Equation: 3Mg2+ + 3SO4^2- + 2K+ + 3PO4^3- → Mg3(PO4)2 + 2K+ + 3SO4^2- Net Ionic Equation: 3Mg2+ + 3SO4^2- + 3PO4^3- → Mg3(PO4)2 Spectator Ions: K+ and SO4^2- Precipitate: Mg3(PO4)2
The Correct Answer and Explanation is:
Double Replacement Reactions: Precipitation and Ionic Equations
In aqueous chemistry, double replacement reactions often lead to the formation of an insoluble compound, or precipitate, which separates out of solution. Let’s analyze two such reactions step-by-step to determine the balanced equations, total and net ionic equations, and identify the spectator ions and the precipitate.
Reaction 1: Iron(III) chloride and sodium hydroxide
Molecular Equation:
FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq)
Balanced Equation:
This equation is already balanced. One Fe³⁺ ion reacts with three OH⁻ ions to form iron(III) hydroxide, a brown precipitate. Three Na⁺ ions balance the three Cl⁻ ions to form sodium chloride, which remains dissolved.
Total Ionic Equation:
Fe³⁺(aq) + 3Cl⁻(aq) + 3Na⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) + 3Na⁺(aq) + 3Cl⁻(aq)
Net Ionic Equation:
Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
Spectator Ions:
Na⁺ and Cl⁻ are unchanged on both sides of the equation and do not participate in the reaction.
Precipitate:
Fe(OH)₃ – an insoluble brown solid.
Reaction 2: Magnesium sulfate and potassium phosphate
Molecular Equation:
3MgSO₄(aq) + 2K₃PO₄(aq) → Mg₃(PO₄)₂(s) + 3K₂SO₄(aq)
Balanced Equation:
Three magnesium sulfate units provide 3 Mg²⁺ ions, and two potassium phosphate units provide 2 PO₄³⁻ ions. These combine to form magnesium phosphate, Mg₃(PO₄)₂, an insoluble solid. Potassium sulfate remains in solution.
Total Ionic Equation:
3Mg²⁺(aq) + 3SO₄²⁻(aq) + 6K⁺(aq) + 2PO₄³⁻(aq) → Mg₃(PO₄)₂(s) + 6K⁺(aq) + 3SO₄²⁻(aq)
Net Ionic Equation:
3Mg²⁺(aq) + 2PO₄³⁻(aq) → Mg₃(PO₄)₂(s)
Spectator Ions:
K⁺ and SO₄²⁻ are not involved in the actual precipitation.
Precipitate:
Mg₃(PO₄)₂ – a white solid precipitate.
Conclusion:
In both reactions, an insoluble hydroxide or phosphate compound forms as a precipitate. The net ionic equations clearly show the actual chemical change, while spectator ions remain unchanged. Recognizing these patterns is fundamental in predicting reaction outcomes in aqueous solutions.
