In chemistry, there are multiple terms used to describe a reaction based on the nature of its reactants. Double displacement reactions also called double replacement reactions or metathesis reactions, occur when parts of two ionic compounds are exchanged, making two new compounds. These compounds can be thought of as “switching partners”; that is, the two reactants, each “lose” their partner, and forms a bond with a different partner.
The overall pattern looks like this:
Types of Double Displacement Reactions
There are two main types of double-displacement reactions; namely precipitation reactions, as in the separation of chemicals on the basis of solubility (Ksp), and neutralization reactions, as in acid-base reactions.
In precipitation or ionic reactions, the reactants are usually in aqueous states and one of the products formed is necessarily insoluble in that solvent. The precipitate species is obtained after cations (positively charged ions) from one of the reactants combine with the anions (negatively charged ions) from the other reactant.
Examples of Precipitation reactions
CuSO4 + 2NaOH → Na2SO4 + Cu(OH)2 (↓)
Na2SO4 + SrCl2 → 2NaCl + SrSO4 (↓)
AgNO3 + NaCl → NaNO3 + AgCl (↓)
CdSO4 + K2S → K2SO4 + CdS (↓)
The insoluble product is called precipitate, and the (solvent + soluble components) of the reaction are called the supernatant or supernate. The driving force that makes the reaction proceed in the forward direction is the formation of the solid (insoluble) product.
A chemical reaction in which an acid and base quantitatively react together to form salt and water as products. The neutralization of a strong acid and strong base is a complete reaction and has a resultant pH of 7.
The reason neutralization reactions are called double displacement reactions is that both partner molecules interchange their species and get new identities.
Examples of Neutralization reactions
HCl(aq) + KOH(aq) → H2O(ℓ) + KCl(aq)
HCl(aq) + NaOH(aq) → H2O(ℓ) + NaCl(aq)
However, neutralization with weak acid or weak base does not normally result in a pH of 7, although water and salt are still produced. If the reaction involves a weak acid and a strong base, then the resulting pH will be slightly basic (pH>7). Similarly, when the neutralization reaction involves a strong acid and weak base, the resulting pH will be slightly acidic (pH<7).
If both the acid and base are weak then the final pH will be dependent on the pKa value of the acid and base. It can result in acidic, basic, or neutral pH.
These reactions also often do not proceed all the way to completion due to the limited dissociation of the weak acid and weak base.
How can I identify double displacement reactions?
A double displacement reaction can be easily identified by checking that the parts of two ionic compounds are exchanged, making two new compounds.
The form of the reaction must be like:
AB + CD → AD + BC
Are acid-base reactions double replacement?
Acid base reactions are double displacement because acid and base quantitatively react together to form different products like salt and water.
What is a double displacement reaction in chemistry?
A double displacement reaction is a type of reaction where two ionic compounds react by exchanging ions to produce two new compounds.
Will there be a double displacement reaction between Na2CO3 and C3H5O(COOH)3?
There is an acid-base neutralization reaction between sodium carbonate and citric acid but it cannot be called double displacement reaction.
2C3H5O(COOH)3 + 3 Na2CO3 → C3H5O(COONa)3 + 3H2O + 3CO2
Since, we have three products instead of two, at the end.