A single replacement reaction, also known as a substitution reaction, is a type of chemical reaction where one element replaces another element in a compound. In this type of reaction, one element is released from the compound while the new element is added in its place.
All single displacement reactions begin and end with one compound and one single element. Most of these reactions consist of a solid reacting with a solution, which causes the solid to replace one of the elements in the solution. An example of this reaction is when zinc or iron is added to a solution of hydrochloric acid. When this occurs, the metal reacts with the acid to take the place of the hydrogen, which forms either iron chloride or zinc chloride. Another example is when sodium is added to water, where it again takes the place of the hydrogen.
Single displacement reactions can only occur when a more reactive element is taking the place of a less reactive element in a compound. In the case of the example reactions, the metal is taking the place of hydrogen, as sodium is more reactive than hydrogen. Due to the difference in reactivity, single replacement reactions are not reversible, as it is impossible for the less reactive element to retake its place from the more reactive element.