How can I identify double displacement reactions?
A double displacement precipitation is a reaction in which two cations switch partners.
Double replacement reactions take the general form:
A⁺B⁻ + C⁺D⁻ → A⁺D⁻ + C⁺B⁻
Double displacement reactions happen because one of the products is a precipitate, a gas, or water.
A. FORMATION OF A PRECIPITATE
The formation of a solid precipitate can pull a reaction forward. For example
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
This is a double displacement reaction because the silver ion and the sodium ion have exchanged partners. It is also a precipitation reaction because one of the products is solid silver chloride.
B. FORMATION OF A GAS
A double displacement reaction will likely occur if one of the products is a gas.
Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂CO₃(aq)
The H₂CO₃(aq) breaks down to H₂O(l) + CO₂(g)
Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l)+ CO₂(g)
The formation of CO₂ pulls the reaction forward.
C. FORMATION OF WATER
These are a special type of double displacement reaction known as neutralization reactions. The reaction occurs between an acid and a base to form a salt and water.
HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l)
The formation of water pulls the reaction forward.
• In a double displacement reaction, there is an exchange of positive ions between two compounds.
• We form two new compounds: AB + CD → AD + BC.
• One of the end products will be:
(a) a precipitate that falls out of solution.
(b) a gas that bubbles out of the mixture.