A double replacement reaction is a type of chemical reaction. The name describes the process very well. Two replacements take place in the reaction. The reaction takes place between two ionic compounds, usually in the form of a solution.
Ionic compounds are neutral, though they are made up of cations (positive ions) and anions (negative ions). A double replacement reaction can be defined as a reaction in which one set of ions of the same charge are exchanged and two new products are formed.
It can be illustrated as follows: a double replacement reaction takes place between two compounds, AB and XY. The double replacement reactions can be shown as
{eq}A^{+}B^{-} + X^{^{+}}Y^{^{-}}\rightarrow A^{+}Y^{-} +X^{+}B^{-} {/eq}
The anion {eq}B^{-} {/eq} goes to {eq}X^{+} {/eq} and the anion {eq}Y^{^{-}} {/eq} goes to {eq}A^{+} {/eq}
Another example showing the charges is:
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Other names for double replacement reactions are:
Double replacement reactions are of three categories. They are:
Precipitation Reaction: Double replacement reactions produce precipitates or insoluble solids as one of the products. These reactions are easy to see and classify. For example, lead(II) chloride dissolves moderately in hot water. Potassium sulfate dissolves readily in water at room temperature. To a solution of lead(II) chloride taken in a test tube, a small quantity of potassium sulfate solution is added and there is an immediate precipitate formation. The reaction can be represented as
{eq}PbCl_{2(aq)}+ K_{2}SO_{4(aq)}\rightarrow 2KCl_{(aq)} + PbSO_{4(s)} {/eq}
It can also be represented as {eq}Pb^{2+}(Cl^{-})_{2} + 2K^{+}(SO_{4}))^{^{2-}} \rightarrow \ 2K^{+}Cl^{-}+ Pb^{2+}(SO_{4})^{2-} {/eq}
Another example of a precipitation reaction is the addition of sodium hydroxide solution to ferrous sulfate solution. Iron(II) hydroxide is produced as a precipitate, with the other product being a sodium chloride solution.
{eq}2NaOH_{(aq)}+ FeCl_{2(aq)} \rightarrow 2NaCl_{(aq)} +Fe(OH)_{2(s)} {/eq}
Formation of a molecular compound:
Another form of double replacement reaction is one that gives rise to a molecular compound. Most often, the molecular compound formed is water. So, neutralization reactions are good examples of this type. Neutralization reactions take place between an acid and a base. The essential reaction is between the hydrogen ion of the acid and the hydroxide ion of the base.
{eq}NaOH_{(aq)} + HCl_{(aq) }\rightarrow NaCl_{(aq)} + H_{2}O_{(l))} {/eq}
The hydroxide ion and the hydrogen ion combine to form water. The other ions, sodium and chloride, remain unchanged. They are spectator ions.
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Gas formation: An example of a double replacement reaction that results in a gas is the reaction between a carbonate or a hydrogen carbonate and dilute acids. In both cases, carbon dioxide is released as a gaseous product.
{eq}K_{2}CO_{3(s)} + H_{2}SO_{4(aq)}\rightarrow K_{2}SO_{4(aq)} +H_{2}O_{(l)} +CO_{2(g)} {/eq}
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{eq}NaHCO_{3(s)} + HCl_{(aq)}\rightarrow NaCl_{(aq)} +H_{2}O_{(l)} +CO_{2(g)} {/eq}
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The following points come into play when doing a double replacement reaction:
The following table lists the solubility of a few compounds. This can aid in determining if the double replacement reaction gives a precipitate.
| No/ Cell | Soluble Compounds | Insoluble Compounds |
|---|---|---|
| 1 | All alkali metal compounds | Insoluble salts |
| 2 | All ammonium compounds | Chlorides of PbII), Hg(II), Ag |
| 3 | All nitrates | Sulfates of Sr, Ba, Pb(II), Hg(II) |
| 4 | All halides | Carbonates except those of alkali metals,and ammonium |
| 5 | All sulfates | Sulfates of Sr, Ba, Hg(II) and Pb(II) |
| 6 | Hydroxides of Sr and Ba | All hydroxides |
If one of the reactants is a compound like carbonates, hydrogen carbonates, or sulfide to name a few, and the other is dilute acid, gas is one of the products.
Here are some examples of each kind of double replacement reaction
Example 1: When solutions of sodium sulfate and silver nitrate are mixed, there is an immediate formation of a precipitate. The precipitate is silver sulfate.
{eq}Na_{2}SO_{4(aq)} + 2AgNO_{3(aq)} \rightarrow 2NaNO_{3(aq)} + Ag_{2}SO_{4(s)} {/eq}
Example 2: Potassium hydroxide solution reacts with dilute nitric acid, undergoing a double replacement reaction. The molecular compound, water, is formed as one of the products.
{eq}KOH_{(aq)}+ HNO_{3(aq)}\rightarrow KNO_{3(aq)}+ H_{2}O_{(l)} {/eq}
Example 3:The reaction between calcium carbonate and dilute hydrochloric acid gives carbon dioxide gas.
{eq}CaCO_{3(s)} +2HCl \rightarrow CaCl_{2(aq)} + H_{2}O_{(l)} + CO_{2(g)} {/eq}
Example 4: The reaction between iron(II) sulfide and dilute hydrochloric acid produces hydrogen sulfide gas. The has a characteristic rotten egg smell. This reaction is also an example of metathesis where a gas is evolved.
{eq}FeS_{(s)} + 2HCl_{(aq)}\rightarrow FeCl_{2(aq)}+ H_{2}S_{(g)} {/eq}
Double replacement reactions are defined as those in which two ionic compounds exchange their ions. Ions of the same kind and two new products are formed. An ionic compound is neutral but has cations that carry positive charges and anions that carry negative charges. Double replacement reactions also go by the names of double displacement reactions, double decomposition reactions, and metathesis. Double replacement reactions fall into three types:
To do a double replacement reaction:
For obtaining information about the physical state of the chemicals, the table given can be used.
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Double displacement reactions occur when two aqueous solutions react to form either water, a solid (insoluble compound) or a gas and an aqueous solution. Let's work on predicting and balancing the reactions.
Write the balanced chemical reaction for each scenario, which includes predicting the products of the reaction.
1) Sodium oxide reacts with silver acetate.
2) Silver nitrate reacts with magnesium chloride.
3) Potassium carbonate reacts with ammonium iodide.
4) Cesium sulfide reacts with sodium hydroxide.
5) Sodium hydroxide reacts with hydrochloric acid.
1) Na2 O + 2AgC2 H3 O2 ⟶ 2NaC2 H3 O2 + Ag2 O
2) 2AgNO3 + MgCl2 ⟶ 2AgCl + Mg(NO3 )2
3) K2 CO3 + 2NH4 I ⟶ 2KI + (NH4)2 CO3
4) Cs2 S + 2NaOH ⟶ 2CsOH + Na2 S
5) NaOH + HCl ⟶ NaCl + H2 O
Notice that in each of these reactions, they're balanced for mass. Reactions 1-4 produce a precipitate, while reaction 5 produces water.
A double displacement reaction takes place between two ionic compounds. Ions are exchanged between them. Only ions of the same charge are exchanged.
An example is HCl + NaOH -> NaCl + H2O
There is a general formula for representing double displacement reactions.
It is
AB + XY -> AY + XB.
A and X have the same number of positive charges and B and Y have the same number of negative charges.
The three types of double replacement reactions are
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