A double displacement reaction is one in which two chemicals react with each other. Both of the reactants consist of two parts, say AB and CD. The products of the reaction interchange components to produce AD and BC. That is, AB + CD → AD + BC. At the student level, double displacement reactions can be demonstrated in a variety of interesting, instructive ways.
None of the following double displacement laboratory activities require the use of exact quantities or conditions. All that is necessary to achieve success is to first completely dissolve small quantities of each of these chemicals, perhaps a gram or two, in water in separate containers, then combine together the appropriate contents for each reaction.
Some double displacement reactions produce a solid product that is insoluble in water. It settles out as fine particles. One such reaction that can easily be carried out in the school laboratory is
BaCl₂ + Na₂SO₄ → 2 NaCl + BaSO₄↓
This equation says barium chloride reacts with sodium sulfate to give sodium chloride plus barium sulfate. The down-arrow means the barium sulfate settles to the bottom of the reaction vessel. It is a cream-colored precipitate.
Gas Evolving Reactions
Some double displacement reactions produce bubbles of gas. One example is the reaction of sodium bicarbonate -- baking soda, with acetic acid -- vinegar.
Na₂CO₃ + CH₃COOH → CH₃COONa +H₂CO₃
This reaction reads, left-to-right, sodium carbonate plus acetic acid yields sodium acetate and carbonic acid.
This reaction as drawn should not produce bubbles, but the carbonic acid breaks down instantaneously to water and carbon dioxide gas, which escapes to the atmosphere,
H₂CO₃ → H₂O + CO₂↑
Color Change Reactions
Color changes occur less frequently in double displacement reactions than they do in oxidation-reduction reactions. Sometimes they do occur. One such reaction is,
Pb(NO₃)₂ + 2 KI → PbI₂↓ + 2 KNO₃
Lead nitrate combined with potassium iodide gives lead iodide -- a yellow precipitate -- plus potassium nitrate. In carrying out this reaction, it is best to use a closed container that one can shake. Since lead is a component, the reaction ingredients should not be disposed of by pouring them down the drain.
One common variety of double displacement reaction involves the combining of acids and bases to produce a compound called a salt, plus water. Thus, hydrobromic acid combines with lithium hydroxide to produce lithium bromide plus water,
HBr + LiOH → LiBr + H₂O.
This form of reaction is sometimes called an acid-base reaction or a neutralization reaction.
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