Displacement reactions involve the exchange of elements or groups in chemical compounds. Single shift reactions involve the replacement of a less reactive element with a more reactive one, while double shift reactions occur between ionic compounds. Acid-base reactions are a common form of double shift. Examples include metal plating, the production of hydrogen gas, and the thermite reaction.
A displacement reaction is one in which an element or group is displaced from one chemical compound by another element or group. In single move reactions, the more responsive element moves the less responsive one. In double shift reactions, which most commonly occur between aqueous solutions of ionic compounds, the positively charged cations and negatively charged anions exchange. The reaction generally proceeds because one of the products is insoluble and either precipitated as a solid or evolved as a gas, or because the reactants are an acid and a base, which will always produce a salt and water.
Single shift reactions include the replacement of one metal in a salt with a more reactive metal. For example, iron will replace copper from a copper sulphate solution because it is more reactive: Fe+ CuSO4 -> Cu + FeSO4. This type of reaction can be used for metal plating.
Metals that are more reactive than hydrogen will displace it from acids. For example, if zinc is added to hydrochloric acid, the reaction will produce zinc chloride and hydrogen, because zinc is more reactive than hydrogen: Zn + 2HCl -> ZnCl2 + H2. Metals that are less reactive than hydrogen, such as copper, lead, silver, and gold, will not displace it from an acid.
The well-known thermite reaction, in which powdered aluminum is used to displace a metal from a compound, usually an oxide, is an example of a displacement reaction between solids. In its most common form, iron is produced by the reaction of aluminum with iron III oxide: Fe2O3 + 2Al -> 2Fe + Al2O3. So much heat is developed that the iron is released as a red-hot liquid; it is sometimes used as a method of welding broken rails together.
Shift reactions between nonmetals are also common. An example is the displacement of iodine from an iodide salt by chlorine. Chlorine is more reactive than iodine, so if chlorine gas is bubbled into a sodium iodide solution, it will replace the iodine: 2NaI + Cl2 -> 2NaCl + I2. This method is used for the industrial production of iodine.
An example of a double shift is the reaction of solutions of barium chloride and sodium sulfate. When these are mixed, they react to produce sodium chloride and barium sulfate. The latter is insoluble and forms a precipitate: BaCl2 + Na2SO4 -> BaSO4 + 2NaCl. The barium chloride solution can then be used to test for sulfate ions.
Acid-base reactions are another form of double shift. These are also known as neutralization reactions. For example, hydrochloric acid and sodium hydroxide will react to produce sodium chloride and water: HCl + NaOH -> NaCl + H2O.
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