Double Replacement Reactions — Swapping Ion Partners
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23 تفاعل
Double replacement (double displacement or metathesis) reactions involve two compounds exchanging ions or groups to form two new compounds. The general form is AB + CD -> AD + CB. These reactions typically occur in aqueous solution when mixing two ionic compounds. The driving force is the formation of a precipitate, a gas, or a weak electrolyte (like water), which removes ions from solution and shifts the equilibrium forward.
Reaction Mechanism
In solution, ionic compounds dissociate into their constituent ions. When two solutions are mixed, the ions can recombine in new ways. A reaction proceeds if one of the products is insoluble (precipitate), volatile (gas), or a molecular compound like water. For example, mixing silver nitrate and sodium chloride solutions produces insoluble silver chloride: Ag+(aq) + Cl-(aq) -> AgCl(s). Solubility rules predict which combinations produce precipitates. Net ionic equations reveal the actual reacting species.
Everyday Examples
Antacid tablets (calcium carbonate) react with stomach acid (HCl) in a double replacement: CaCO3 + 2HCl -> CaCl2 + H2O + CO2. The fizzing is carbon dioxide gas escaping. Hard water forms soap scum through double replacement — calcium ions in water replace sodium in soap to form insoluble calcium stearate. Baking soda and vinegar's famous volcano reaction is also double replacement.
الأهمية الصناعية
Water treatment plants use double replacement to remove toxic metal ions. Adding sodium sulfide to wastewater precipitates heavy metals as insoluble sulfides. The Solvay process for producing sodium carbonate (soda ash) involves a series of double replacement reactions, producing over 55 million tonnes annually for glass, detergent, and paper manufacturing.
Safety Note
Double replacement reactions that produce gases (H2S, CO2, SO2) require fume hoods. Precipitates of heavy metals (lead, mercury, cadmium) are toxic and require proper hazardous waste disposal. Always consult solubility tables before mixing ionic solutions.
Aluminum Hydroxide and Hydrochloric Acid
Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O
Aluminum hydroxide, an amphoteric hydroxide, dissolves in hydrochloric acid to form aluminum chloride and water. This demonstrates the basic character …
Iron(III) Chloride and Sodium Hydroxide
FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl
Iron(III) chloride reacts with sodium hydroxide to form a rust-brown precipitate of iron(III) hydroxide and sodium chloride. This reaction is …
Ammonia and Hydrochloric Acid (White Smoke)
NH₃ + HCl → NH₄Cl
Ammonia gas reacts with hydrogen chloride gas to form white fumes of ammonium chloride solid. This dramatic reaction occurs when …
Calcium Carbonate and Hydrochloric Acid
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Calcium carbonate (limestone or marble) reacts with hydrochloric acid to produce calcium chloride, water, and carbon dioxide. This is the …
Calcium Chloride and Sodium Carbonate
CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl
Calcium chloride reacts with sodium carbonate to form insoluble calcium carbonate precipitate and sodium chloride. This is the basis of …
Barium Chloride and Sodium Sulfate Precipitation
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
Barium chloride reacts with sodium sulfate to form an insoluble white precipitate of barium sulfate and sodium chloride. This is …
Calcium Hydroxide and Carbon Dioxide (Limewater Test)
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Calcium hydroxide solution (limewater) turns milky white when carbon dioxide is bubbled through it, due to the formation of insoluble …
Sodium Hydroxide and Iron(III) Chloride (Rust Color)
3NaOH + FeCl₃ → Fe(OH)₃ + 3NaCl
When sodium hydroxide is added to iron(III) chloride solution, a characteristic rust-brown precipitate of iron(III) hydroxide forms. This gelatinous precipitate …
Sodium Hydroxide and Hydrochloric Acid Neutralization
NaOH + HCl → NaCl + H₂O
Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride and water. This is the archetypal neutralization reaction, producing exactly …
Potassium Hydroxide and Sulfuric Acid
2KOH + H₂SO₄ → K₂SO₄ + 2H₂O
Potassium hydroxide reacts with sulfuric acid in a 2:1 ratio to form potassium sulfate and water. This diprotic acid neutralization …
Sodium Carbonate and Hydrochloric Acid
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
Sodium carbonate reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide gas. The effervescence from CO2 evolution …
Magnesium Hydroxide and Hydrochloric Acid (Antacid)
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Magnesium hydroxide (milk of magnesia) neutralizes hydrochloric acid to form magnesium chloride and water. This is exactly the reaction that …
Sodium Hydroxide and Sulfuric Acid
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
Sodium hydroxide reacts with sulfuric acid to form sodium sulfate and water. This is a standard acid-base neutralization producing a …
Sodium Hydroxide and Phosphoric Acid
3NaOH + H₃PO₄ → Na₃PO₄ + 3H₂O
Sodium hydroxide reacts with phosphoric acid in a 3:1 ratio to completely neutralize the triprotic acid, forming trisodium phosphate and …
Sodium Hydroxide and Nitric Acid
NaOH + HNO₃ → NaNO₃ + H₂O
Sodium hydroxide neutralizes nitric acid to form sodium nitrate and water. This strong acid-strong base reaction produces a neutral solution …
Sodium Sulfide and Copper(II) Sulfate
Na₂S + CuSO₄ → CuS + Na₂SO₄
Sodium sulfide reacts with copper(II) sulfate to form a black precipitate of copper(II) sulfide and sodium sulfate. CuS is extremely …
Sodium Fluoride and Calcium Chloride
2NaF + CaCl₂ → CaF₂ + 2NaCl
Sodium fluoride reacts with calcium chloride to form insoluble calcium fluoride (fluorite) and sodium chloride. Calcium fluoride is a mineral …
Lead Nitrate and Potassium Iodide (Golden Rain)
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
Lead(II) nitrate reacts with potassium iodide to form a bright yellow precipitate of lead(II) iodide and soluble potassium nitrate. When …
Sodium Bicarbonate and Acetic Acid (Vinegar Volcano)
NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂
Baking soda reacts with vinegar to produce sodium acetate, water, and carbon dioxide gas. This endothermic reaction absorbs heat, making …
Silver Nitrate and Sodium Chloride Precipitation
AgNO₃ + NaCl → AgCl + NaNO₃
Silver nitrate reacts with sodium chloride to form an insoluble white precipitate of silver chloride and soluble sodium nitrate. This …
Calcium Hydroxide and Sulfuric Acid
Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O
Calcium hydroxide reacts with sulfuric acid to form calcium sulfate and water. The product calcium sulfate is a sparingly soluble …
Copper(II) Sulfate and Sodium Hydroxide
CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄
Blue copper(II) sulfate solution reacts with sodium hydroxide to form a pale blue gelatinous precipitate of copper(II) hydroxide and sodium …
Sodium Hydroxide and Acetic Acid
NaOH + CH₃COOH → CH₃COONa + H₂O
Sodium hydroxide reacts with acetic acid to form sodium acetate and water. Since acetic acid is a weak acid, this …