Manganese (Mn) is a transition metal (atomic number 25, Group 7) that appears as a hard, brittle, silvery-gray metal, similar to iron. As is typical of transition metals, manganese is chemically versatile and engages in a variety of reactions. Understanding this diverse chemical behavior is central to knowing what manganese reacts with in different environments.
The Chemical Basis of Manganese Reactivity
Manganese’s wide range of reactivity stems from its electron configuration, allowing it to utilize electrons from both the 4s and 3d orbitals in chemical bonding. The element exhibits a broad spectrum of oxidation states, which represent the degree of oxidation of an atom in a compound. This versatility dictates the compounds manganese forms and their chemical properties.
The most stable and frequently encountered oxidation state is +2, often seen in pale pink aqueous solutions. Other common states include +3, +4, and +7, each resulting in compounds with distinct characteristics and colors. For example, the +4 state is found in black-brown manganese dioxide, while the +7 state is present in the intensely purple permanganate ion. The ability to shift between these states allows manganese to act as both a reducing agent (losing electrons) and an oxidizing agent (gaining electrons), depending on the reaction conditions.
Reactions with Atmospheric and Solid Nonmetals
Manganese readily engages in direct combination reactions with various nonmetals, often requiring heat to initiate the process. Its reactivity with atmospheric components like oxygen and nitrogen is a primary example of this behavior.
Reaction with Oxygen (Oxidation)
When exposed to air, the surface of manganese metal slowly tarnishes. When heated, however, manganese reacts vigorously with oxygen gas (\(\text{O}_2\)) to form manganese oxides. A common product is manganese(II, III) oxide (\(\text{Mn}_3\text{O}_4\)), which contains manganese in two different oxidation states. Finely divided manganese powder is particularly flammable and burns readily in air due to its increased surface area.
Reaction with Halogens
Manganese reacts vigorously with halogens (highly reactive nonmetals in Group 17), such as chlorine (\(\text{Cl}_2\)) and fluorine (\(\text{F}_2\)). These exothermic reactions form manganese halides. For example, manganese burns in chlorine gas to produce manganese(II) chloride (\(\text{MnCl}_2\)). With fluorine, the reaction can yield both manganese(II) fluoride (\(\text{MnF}_2\)) and manganese(III) fluoride (\(\text{MnF}_3\)), demonstrating its ability to adopt multiple oxidation states.
Reaction with Sulfur and Nitrogen
Manganese combines with other nonmetals when sufficient energy is supplied. Upon heating, manganese reacts with elemental sulfur (\(\text{S}\)) to form manganese(II) sulfide (\(\text{MnS}\)). This reaction is important in the steel industry, where manganese desulfurizes steel by forming high-melting manganese sulfide, preventing the embrittlement caused by liquid iron sulfide. Manganese also reacts with nitrogen gas (\(\text{N}_2\)) at high temperatures to form manganese nitride (\(\text{Mn}_3\text{N}_2\)).
Behavior in Aqueous Environments (Water and Acids)
The reactivity of manganese changes significantly when the metal is placed in liquid solutions like water and acids. In these aqueous environments, the reaction is often characterized by the formation of ions and the liberation of hydrogen gas.
Reaction with Water
Manganese metal is relatively unreactive with cold water. However, the reaction proceeds slowly with hot water or steam. This slow reaction produces manganese hydroxide and releases hydrogen gas (\(\text{H}_2\)). This moderate reactivity is characteristic of its position in the electrochemical series, indicating it is not among the most reactive metals.
Reaction with Acids
Manganese dissolves readily in dilute, non-oxidizing acids, such as sulfuric acid (\(\text{H}_2\text{SO}_4\)) or hydrochloric acid (\(\text{HCl}\)). This classic metal displacement reaction oxidizes the manganese metal to the manganese(II) ion (\(\text{Mn}^{2+}\)) and displaces hydrogen from the acid. The reaction forms an aqueous manganese(II) salt and liberates hydrogen gas. This spontaneous reaction confirms that manganese is more reactive than hydrogen, placing it above hydrogen in the electrochemical series.
Strong oxidizing acids, like concentrated nitric acid, react differently and drive manganese toward higher oxidation states. While dilute acids form the +2 ion, strong oxidizing agents can push manganese to the +4 oxidation state, forming compounds such as manganese dioxide. The nature of the acid controls the final oxidation state of the manganese product. The reaction with concentrated nitric acid produces a manganese nitrate salt and various nitrogen oxides.