Tabla de Contenidos
A double displacement reaction, also known as a salt metathesis reaction, exchange, and double decomposition reaction, is a type of reaction in which two ionic compounds exchange ions to form two new compounds, typically resulting in the formation of a product that is a precipitate. The general pattern of double displacement reactions looks like this:
AB + CD → AD + CB
The reaction occurs most often between ionic compounds, although technically the bonds formed between chemical species can be ionic or covalent in nature. Acids or bases also participate in double displacement reactions, the bonds formed in the product compounds are the same type of bonds that are seen in reactant molecules, normally the solvent for this type of reaction is water.
The easiest way to identify a double displacement reaction is to check if the cations exchanged anions with each other, another way to know is that if the states of matter are cited, it is to look for aqueous reagents and the formation of a solid product (already the reaction usually produces a precipitate).
Types of Double Displacement Reactions
Double displacement reactions can be classified into several categories, including counterion exchange, alkylation, neutralization, acid-carbonate reactions, aqueous metathesis with precipitation, and aqueous metathesis with double decomposition. The two types most commonly found in chemistry classes are precipitation reactions and neutralization reactions.
Next we will explain in more detail the precipitation and neutralization reactions:
precipitation reactions
They are reactions in which an insoluble product (not dissolved) is formed in the medium, usually water. When an ionic substance is put into a solvent, its ions can separate (dissolve) or stay together (do not dissolve). This depends on the nature of the reagents, temperature, degree of division of the solute, among many other factors. A precipitation reaction occurs when when mixing ionic substances, the ions existing in the medium tend to be united, forming a precipitate. It is important to mention that in this type of reaction an insoluble product is formed.
Here is an example of reaction between lead(II) nitrate and potassium iodide to form potassium nitrate (soluble) and lead iodide (insoluble).
Pb (NO 3 ) 2 (aq) + 2 KI (aq) → 2 KNO 3 (aq) + PbI 2 (s)
Lead iodide forms the precipitate, while the solvent (water) and soluble reagents and products are called the supernatant. The formation of a precipitate drives the reaction forward as the product comes out of solution.
neutralization reactions
Neutralization reactions are the reactions between an acid and a base, in order to determine the concentration of the different substances in the solution. They occur when an acid fully reacts with a base, producing salt and water. There is only one case where water is not formed in the reaction, it is the combination of a non-metal oxide with a metal oxide.
The reaction between vinegar and baking soda in the baking soda volcano is an example of a neutralization reaction, this particular reaction proceeds to release a gas (carbon dioxide), which is responsible for the resulting fizz. The initial neutralization reaction is:
NaHCO 3 + CH 3 COOH (aq) → H 2 CO 3 + NaCH 3 COO
You will be able to tell that the cations exchanged anions, but because of the way the compounds are written, it is a little more difficult to notice the anion exchange. The key to identifying the reaction as a double displacement is to look at the atoms of the anions and compare them on both sides of the reaction.
examples
The reaction between silver nitrate and sodium chloride is a double displacement reaction, with silver exchanging its nitrite ion for sodium chloride ion, causing the sodium to pick up the nitrate ion.
AgNO 3 + NaCl → AgCl + NaNO 3
Here’s another example:
BaCl 2 (aq) + Na 2 SO 4 (aq) → BaSO 4 (s) + 2 NaCl (aq)
In this example the cations are BA2 and NA+ and the anions are Cl- and SO24, if we exchange the anions or the cations we obtain as products: BaSO4 and NaCl.
Conclusions:
- A double displacement reaction is a type of chemical reaction in which reactant ions exchange places to form new products.
- Normally, a double displacement reaction results in the formation of a precipitate.
- Chemical bonds between reactants can be covalent or ionic.
- A double displacement reaction is also called a double replacement reaction, salt metathesis reaction, or double decomposition.
