# What is a double-replacement reaction in chemistry?

Apr 25, 2017

See explanation

#### Explanation:

Double replacement (sometimes referred to as double displacement) reactions are when parts of ionic compounds are switched to form two new ionic compounds.

Heres an image:

The way I think of it, since we're dealing with ionic compounds, is that when I write out a reaction I begin like this.

color(green) (A^+)color(red)(B^-) + color(purple)(C^+)color(red)(D^-)->?+?

We swap the anions so that $A$ is now with $D$ and $C$ is now with $B$ Visually,

$\textcolor{g r e e n}{{A}^{+}} \textcolor{red}{{B}^{-}} + \textcolor{p u r p \le}{{C}^{+}} \textcolor{red}{{D}^{-}} \to \textcolor{g r e e n}{{A}^{+}} \textcolor{red}{{D}^{-}} + \textcolor{p u r p \le}{{C}^{+}} \textcolor{red}{{B}^{-}}$

Heres an real example:

Say you have Sodium Cyanide and you react it with Hydrogen Bromide:

color(green)(Na)color(red)(CN)_(aq) + color(purple)(H)color(red)(Br)_(aq)->?+?

Well we have to remember what are the charges of cation and anion. Since this is an easy example, they all have a charge of +1 for the cations and -1 for the anions but we can just write $+ \mathmr{and} -$

color(green)(Na^+)color(red)(CN^-)_(aq) + color(purple)(H^+)color(red)(Br^-)_(aq)->?+?

Next we can just switch the anions to get

$\textcolor{g r e e n}{N {a}^{+}} {\textcolor{red}{C {N}^{-}}}_{a q} + \textcolor{p u r p \le}{{H}^{+}} {\textcolor{red}{B {r}^{-}}}_{a q} \to \textcolor{g r e e n}{N {a}^{+}} {\textcolor{red}{B {r}^{-}}}_{a q} + \textcolor{p u r p \le}{{H}^{+}} {\textcolor{red}{C {N}^{-}}}_{g}$

(This forms Hydrogen cyanide gas by the way)

I'm not the best at explaining chemistry so if this was confusing I apologize and recommend checking out Tyler DeWitt's video on "Types of Chemical Reactions" as well as the his videos.