Chemical Reactions and Equations · Lesson 6 of 9
Double Displacement Reaction
“Two compounds swap ions, and sometimes one product drops out as a solid.”
• Distinguish double displacement from single displacement. • Explain the reaction between sodium sulphate and barium chloride. • Define precipitate and precipitation reaction. • Explain ion exchange in a double displacement reaction. • Use state symbols to identify dissolved substances and an insoluble product.
In a displacement reaction, a free element replaces another element in a compound. Double displacement is different: two compounds exchange ions. The sodium sulphate and barium chloride reaction makes this exchange visible because one new product is an insoluble white solid.
Double Displacement Reaction
A Double Displacement Reaction is a reaction in which ions are exchanged between two reactants to form new products.
Sodium Sulphate And Barium Chloride
Sodium sulphate solution and barium chloride solution are both aqueous. When they are mixed, a white insoluble substance appears. The solid is barium sulphate. Sodium chloride remains dissolved in the solution.
Precipitate And Precipitation Reaction
A Precipitate is an insoluble solid formed during a reaction in solution.
A Precipitation Reaction is a reaction in which an insoluble solid product is formed from substances in solution.
The state symbols make the key observation visible in the equation. Sodium sulphate and barium chloride are marked (aq), indicating that they are dissolved in water. Barium sulphate is marked (s), showing that it leaves the solution as a solid precipitate.
Understanding The Ion Exchange
In solution, sodium sulphate supplies sodium ions and sulphate ions, while barium chloride supplies barium ions and chloride ions. Barium ions combine with sulphate ions to form insoluble barium sulphate. Sodium and chloride ions remain in solution as sodium chloride.
Lead Nitrate And Potassium Iodide Revisited
The earlier reaction between lead nitrate and potassium iodide also fits the double displacement pattern. The ions exchange partners and lead iodide forms as an insoluble product.
Displacement And Double Displacement Compared
| Feature | Displacement | Double Displacement |
|---|---|---|
| Key change | One element replaces another | Ions of two compounds exchange |
| Typical reactants | Element + compound | Compound + compound |
| Example | Fe + CuSO₄ → FeSO₄ + Cu | Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl |
Quiz
What is formed when sodium sulphate solution and barium chloride solution are mixed?
What does the symbol (s) show for BaSO₄ in the reaction?
Which description best defines double displacement?
Which product remains in solution in Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl?
Why is the barium sulphate reaction also called a precipitation reaction?
Practice Problems
- Write the balanced equation with state symbols for sodium sulphate reacting with barium chloride.
- Explain how the state symbols in that equation help identify the precipitate.
- Define precipitate and precipitation reaction and connect both definitions to barium sulphate.
- Explain the ion exchange that occurs when sodium sulphate and barium chloride solutions are mixed.
- Compare displacement and double displacement reactions using one equation for each.
- Write the balanced equation for lead nitrate reacting with potassium iodide and identify the insoluble product.
Key Takeaways
• Double displacement reactions involve exchange of ions between two compounds. • Mixing sodium sulphate and barium chloride forms insoluble barium sulphate and aqueous sodium chloride. • A precipitate is an insoluble solid formed from a reaction in solution. • State symbols help distinguish dissolved reactants from a solid precipitate. • Lead nitrate and potassium iodide provide another example of double displacement and precipitation.