Double Replacement Reaction
A double replacement reaction (also called a double displacement or metathesis reaction) is a type of chemical reaction in which the cations and anions of two ionic compounds exchange partners, following the general pattern AB + CD → AD + CB. These reactions typically occur in aqueous solution and are driven by the formation of a precipitate, a gas, or a weakly ionised product (such as water). Double replacement reactions underlie many important processes, including precipitation reactions used in water treatment, neutralisation reactions in acid-base chemistry, and qualitative analysis in analytical chemistry.
Key Formula
AB + CD → AD + CB (cations and anions of two compounds exchange partners)
LaTeX: AB + CD \rightarrow AD + CB
| Symbol | Meaning | Unit |
|---|---|---|
| A | Cation of first compound | dimensionless |
| B | Anion of first compound | dimensionless |
| C | Cation of second compound | dimensionless |
| D | Anion of second compound | dimensionless |
Worked Example
Problem
Write the balanced equation for the reaction between silver nitrate (AgNO₃) and sodium chloride (NaCl) solutions.
Solution
Step 1: Identify the reactants — AgNO₃ and NaCl (both aqueous ionic compounds). Step 2: Exchange cations and anions: Ag⁺ pairs with Cl⁻, Na⁺ pairs with NO₃⁻. Step 3: Write the products: AgCl (silver chloride) and NaNO₃ (sodium nitrate). Step 4: Unbalanced equation: AgNO₃ + NaCl → AgCl + NaNO₃ Step 5: Count atoms — Ag: 1=1 ✓; N: 1=1 ✓; O: 3=3 ✓; Na: 1=1 ✓; Cl: 1=1 ✓ Step 6: Equation is already balanced. Step 7: Identify driving force — AgCl is insoluble (white precipitate forms).
Answer
AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)
Driving Forces for Double Replacement Reactions
| Driving Force | Example | Evidence | Product Formed |
|---|---|---|---|
| Precipitate formation | AgNO₃ + NaCl | White solid appears | AgCl(s) |
| Gas formation | Na₂CO₃ + HCl | Bubbling/fizzing | CO₂(g) |
| Water formation | HCl + NaOH | Temperature rises | H₂O(l) |
| Weak acid formation | CH₃COONa + HCl | Vinegar smell | CH₃COOH(aq) |
| Complex ion formation | CuSO₄ + NH₃(aq) | Deep blue colour | [Cu(NH₃)₄]²⁺ |
Interactive Tools
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Related Terms
Single Replacement Reaction
A single replacement reaction (also called a single displacement reaction) is a type of chemical reaction in which one element displaces another element from a compound, following the general pattern A + BC → AC + B. The reaction occurs when the displacing element is more reactive (higher on the activity series) than the element it replaces; if the displacing element is less reactive, no reaction occurs. Single replacement reactions are widely used in metallurgy and electrochemistry — for example, extracting metals from their ores or in galvanic cells.
Chemical Reaction
A chemical reaction is a process in which one or more substances (reactants) are transformed into one or more different substances (products) through the breaking and forming of chemical bonds. Chemical reactions involve changes in the arrangement of atoms, resulting in new materials with different properties from the original substances. They are fundamental to all biological, industrial, and environmental processes, from cellular respiration to the manufacture of medicines.
Balancing Chemical Equations
Balancing chemical equations is the process of adjusting the stoichiometric coefficients in front of each formula in a chemical equation so that the number of atoms of each element is equal on both the reactant and product sides, satisfying the Law of Conservation of Mass. Only coefficients (not subscripts) may be changed during balancing, because altering subscripts would change the identity of the compounds. Balanced equations are essential for calculating the exact quantities of reactants and products in stoichiometric calculations used in laboratories and industry.
The term "double displacement" or "metathesis" comes from Greek "metathesis" meaning "transposition", from "meta-" (change) and "tithenai" (to place), reflecting the exchange of partners between two compounds.