The element hydrogen is the lightest and most abundant element in the universe, consisting of a single proton and a single electron in its neutral form. This simple \(1s^1\) electron configuration is the source of its high chemical reactivity. Hydrogen seeks stability by either losing its lone electron to form a positively charged ion (\(\text{H}^+\)), gaining an electron to form a negatively charged hydride ion (\(\text{H}^-\)), or sharing its electron to form a covalent bond. The specific type of reaction hydrogen undergoes depends on the element it reacts with and surrounding conditions, such as temperature and pressure.
Synthesis of Water and Halogen Acids
Hydrogen’s most well-known reactions are those it undergoes with highly electronegative nonmetals, particularly oxygen and the halogens, where it forms stable covalent compounds. The reaction of hydrogen gas (\(\text{H}_2\)) with oxygen gas (\(\text{O}_2\)) produces water (\(\text{H}_2\text{O}\)). This combustion reaction is exothermic and is the basis for its potential use as a clean fuel.
Although the reaction is highly exothermic, a mixture of hydrogen and oxygen at room temperature is stable and requires an initial input of energy, like a spark, to begin the reaction. Once initiated, the heat released from the first molecules reacting provides the energy to trigger surrounding molecules, leading to an explosive, rapid chain reaction. For every mole of hydrogen reacted, a significant amount of heat is liberated.
Hydrogen also reacts directly with the halogens, such as fluorine, chlorine, and bromine. These reactions form hydrogen halides (e.g., \(\text{HCl}\)), which are colorless gases at room temperature. The reaction with fluorine is the most violent due to fluorine’s high electronegativity, while reactions with chlorine and bromine are also exothermic. When dissolved in water, these hydrogen halides form strong acids, such as hydrochloric acid, which have many industrial applications.
Formation of Ionic and Covalent Hydrides
Hydrogen forms two main categories of compounds: ionic and covalent hydrides. When hydrogen reacts with highly electropositive metals, specifically the alkali metals (Group 1) and alkaline earth metals (Group 2), it gains an electron to form the negatively charged hydride ion (\(\text{H}^-\)). This results in the formation of ionic or saline hydrides, such as lithium hydride (\(\text{LiH}\)) or calcium hydride (\(\text{CaH}_2\)).
These ionic hydrides are salt-like, crystalline solids formed by reacting the metal with hydrogen gas at elevated temperatures. The hydride ion is a strong base, which makes these compounds highly reactive with water, producing hydrogen gas and a metal hydroxide. This vigorous reaction is why ionic hydrides must be handled carefully, but it also allows them to be used as portable sources of hydrogen.
In contrast, hydrogen forms covalent hydrides when it reacts with most nonmetals and metalloids, where the bond involves the sharing of electrons. Examples include the reaction of hydrogen with elements like carbon, silicon, and boron to form compounds like methane (\(\text{CH}_4\)) and silane (\(\text{SiH}_4\)). The character of the bond in these compounds varies, becoming more polar as the electronegativity difference between hydrogen and the other element increases. While most covalent hydrides are gases or liquids, some, like the complex hydrides of boron and aluminum, are solids that are extensively used as sources of the hydride ion in chemical synthesis.
Function as a Chemical Reducing Agent
Hydrogen is a powerful chemical reducing agent, meaning it facilitates the reduction of other substances. In this functional role, hydrogen donates electrons or effectively adds hydrogen atoms to a molecule, causing a decrease in the oxidation state of the element it reacts with. This property is widely utilized in industrial chemistry, particularly in the production of pure metals from their ores.
Hydrogen reduction of metal oxides is a high-temperature process used to extract pure metals like tungsten, molybdenum, and iron. The general reaction involves hydrogen gas reacting with the metal oxide, removing the oxygen to form water vapor and leaving behind the elemental metal. This is a cleaner process than methods that produce carbon dioxide.
Another significant application of hydrogen’s reducing power is in the process known as hydrogenation. Hydrogenation involves the addition of hydrogen atoms across double or triple bonds in unsaturated organic molecules, such as vegetable oils, making them saturated. This reaction typically requires a catalyst, such as finely divided nickel, palladium, or platinum, to proceed efficiently. This process is used to convert liquid oils into solid fats for products like margarine and to produce various industrial chemicals.