A single replacement reaction represents one of the fundamental processes in chemistry, where a single, uncombined element reacts with a compound, resulting in the exchange of one element for another within that compound. The reaction follows the pattern \(\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}\). This chemical event is a type of oxidation-reduction process, meaning that electrons are transferred between the participating atoms.
Understanding the Components of the Reaction
A single replacement reaction requires two specific reactants: one element standing alone and one compound, typically dissolved in water as an aqueous solution. The lone element, which is always neutral with a zero charge, is the one that initiates the replacement. The compound consists of a positively charged ion (cation) and a negatively charged ion (anion), held together by an ionic bond. The element that does the replacing starts neutral and must lose or gain electrons to change its charge and join the new compound. Conversely, the element being replaced starts as a charged ion and must return to its neutral, uncombined elemental state. The element that replaces the cation is generally a metal, while an element replacing the anion is a nonmetal, such as a halogen.
Predicting the Reaction: The Activity Series
A single replacement reaction will not occur automatically just because the elements are mixed together; it is entirely dependent on the relative reactivity of the involved elements. This conditional rule is governed by the Activity Series, which is a ranked list of elements based on their tendency to lose electrons. For a successful reaction, the single, unbound element must be more reactive than the element it intends to replace in the compound. If the lone element is lower on the series than the element already bonded in the compound, the reaction will not proceed.
For example, if solid zinc metal is placed into a solution of copper(II) sulfate, a reaction occurs because zinc is positioned higher on the Activity Series than copper. The more reactive zinc is able to displace the less reactive copper ion from the compound, forming zinc sulfate and solid copper metal. However, if solid copper metal were placed into a solution of zinc sulfate, no reaction would take place. Copper is less reactive than zinc, meaning it does not have the chemical ability to force the zinc ion out of its compound. This series provides a simple, predictive tool: an element can only replace any element that sits below it on the list.
Common Types of Single Replacement Reactions
Single replacement reactions are primarily categorized based on whether a metal replaces a metal ion, or a nonmetal replaces a nonmetal ion. The first common category involves a metal replacing another metal, or hydrogen, from a compound.
Metals can also replace hydrogen from acids or water, provided they are sufficiently reactive according to the series. For instance, a very reactive metal like sodium will readily replace hydrogen from water, a reaction that can be quite vigorous.
The second main category involves nonmetals, specifically the halogens, replacing a less reactive halogen ion in a compound. For halogens—fluorine, chlorine, bromine, and iodine—reactivity decreases as you move down the group. Chlorine gas, being more reactive than bromine, can replace the bromide ion in potassium bromide, as shown by the reaction \(\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2\). In contrast, iodine cannot replace the chloride ion in potassium chloride because it is the least reactive halogen in this group.