A chemical reaction is generally understood as a process where two or more distinct substances interact to form new products. However, a reaction can indeed occur with only one starting substance, or reactant, provided that substance is a compound. In these specific cases, the single compound undergoes a transformation, resulting in the creation of entirely new chemical entities with different properties.
Defining Single-Reactant Change
A single-reactant change fundamentally differs from a combination reaction, which involves two or more substances joining together. Initiating this change requires overcoming the inherent stability of the starting compound. This demands a significant input of energy to disrupt the existing chemical bonds and facilitate a rearrangement of atoms.
This necessary energy is typically supplied in the form of heat, light, or electricity, acting as the driving force for the transformation. The energy disrupts the stable configuration of the reactant molecule, causing it to break apart or allowing its atoms to shift into a new structural arrangement. This mechanism of bond breaking and subsequent product formation defines the various types of single-substance reactions.
Decomposition Reactions
Decomposition reactions are the most common examples of chemical change involving only one reactant. In this process, a single, more complex compound breaks down into two or more simpler substances, which can be elements or smaller compounds. The general equation for this type of reaction is \(\text{AB} \rightarrow \text{A} + \text{B}\).
The energy source used to initiate the breakdown classifies the decomposition reaction. In thermal decomposition, heat energy is applied to break the bonds, such as when calcium carbonate is heated to produce calcium oxide and carbon dioxide. Electrolytic decomposition, also known as electrolysis, uses electrical energy to drive the reaction. A prominent example is the electrolysis of water, where electric current splits the \(\text{H}_2\text{O}\) molecule into hydrogen gas (\(\text{H}_2\)) and oxygen gas (\(\text{O}_2\)).
Photochemical decomposition, or photolysis, is driven by light energy, specifically photons. For instance, hydrogen peroxide (\(\text{H}_2\text{O}_2\)) slowly breaks down into water and oxygen when exposed to light, which is why it is stored in opaque containers. The energy supplied by the light disrupts the bonds within the molecule, leading to the formation of multiple, simpler products.
Other Single-Substance Transformations
Beyond decomposition, other distinct types of single-substance reactions exist where the atoms are not necessarily separated into simpler substances but are instead rearranged or joined together. Isomerization is one such transformation, where a compound converts into an isomer, represented as \(\text{A} \rightarrow \text{A’}\). An isomer is a molecule that has the exact same chemical formula as the starting material but a different structural arrangement of atoms, resulting in different chemical and physical properties.
Isomerization
A simple industrial example is the conversion of normal butane, a straight-chain alkane, into its branched-chain isomer, isobutane, by heating it in the presence of a catalyst. In biological systems, the light-driven conversion of cis-retinal to trans-retinal in the eye is a fundamental isomerization reaction that initiates the process of vision. The atoms are merely rearranged within the same molecular shell.
Polymerization
Polymerization involves the joining of many identical smaller molecules, called monomers, into a single, much larger molecule, known as a polymer, written as \(\text{n(A)} \rightarrow (\text{A})_n\). In this addition process, the chemical bonds within the monomer are broken and reformed to link the units together in a long chain. For example, the monomer ethylene (\(\text{C}_2\text{H}_4\)) can be induced to link repeatedly to form the polymer polyethylene, a common plastic. Although a catalyst is often required to start the process, the single monomer substance is the sole chemical reactant contributing mass to the final product.