Preparation of Hydrogen

Elemental hydrogen must be prepared from compounds by breaking chemical bonds. The most common methods of preparing hydrogen follow.

From Steam and Carbon or Hydrocarbons

Water is the cheapest and most abundant source of hydrogen. Passing steam over coke (an impure form of elemental carbon) at 1000 °C produces a mixture of carbon monoxide and hydrogen known as water gas:

\(\begin{array}{cc}\text{C}(s)+{\text{H}}_{2}\text{O}(g)\;\stackrel{\phantom{\rule{0.5em}{0ex}}1000\;\text{°C}\phantom{\rule{0.5em}{0ex}}}{\to }& \text{CO}(g)+{\text{H}}_{2}(g)\\ & \hfill \text{water gas}\hfill \end{array}\)

Water gas is as an industrial fuel. It is possible to produce additional hydrogen by mixing the water gas with steam in the presence of a catalyst to convert the CO to CO₂. This reaction is the water gas shift reaction.

It is also possible to prepare a mixture of hydrogen and carbon monoxide by passing hydrocarbons from natural gas or petroleum and steam over a nickel-based catalyst. Propane is an example of a hydrocarbon reactant:

\({\text{C}}_{3}{\text{H}}_{8}(g)+3{\text{H}}_{2}\text{O}(g)\;\underset{\phantom{\rule{0.4em}{0ex}}\text{catalyst}\;}{\overset{900\;\text{°C}}{\to }}\;\text{3CO}(g)+7{\text{H}}_{2}(g)\)

Electrolysis

Hydrogen forms when direct current electricity passes through water containing an electrolyte such as H₂SO₄, as illustrated in the figure below. Bubbles of hydrogen form at the cathode, and oxygen evolves at the anode. The net reaction is:

\({\text{2H}}_{2}\text{O}(l)+\text{electrical energy}\;⟶\;{\text{2H}}_{2}(g)+{\text{O}}_{2}(g)\)

A diagram shows a beaker that contains a liquid, a battery submerged in the liquid, and two test tubes. The battery has the positive and negative terminals labeled. The liquid is connected by a right-facing arrow to an image of two molecules made up of one red atom and two white atoms. It is labeled, “Water,” and, “2 H subscript 2 O ( l ).” The left test tube above the negative sign is connected by a right-facing arrow to an image of two pairs of white atoms. The image is labeled, “Hydrogen,” and, “2 H subscript 2 ( g ).” The right test tube above the positive sign is connected by a right-facing arrow to an image of a pair of red atoms. The image is labeled, “Oxygen,” and, “O subscript 2 ( g ).”

The electrolysis of water produces hydrogen and oxygen. Because there are twice as many hydrogen atoms as oxygen atoms and both elements are diatomic, there is twice the volume of hydrogen produced at the cathode as there is oxygen produced at the anode.

Reaction of Metals with Acids

This is the most convenient laboratory method of producing hydrogen. Metals with lower reduction potentials reduce the hydrogen ion in dilute acids to produce hydrogen gas and metal salts. For example, as shown in the figure below, iron in dilute hydrochloric acid produces hydrogen gas and iron(II) chloride:

\(\text{Fe}(s)+{\text{2H}}_{3}{\text{O}}^{\text{+}}(aq)+{\text{2Cl}}^{\text{−}}(aq)\;⟶\;{\text{Fe}}^{2+}(aq)+{\text{2Cl}}^{\text{−}}(aq)+{\text{H}}_{2}(g)+{\text{2H}}_{2}\text{O}(l)\)

A photo shows a beaker that contains a clear, colorless liquid. It is labeled, “H C l ( a q ).” Beside the beaker is a watch glass with a dropper above it. The dropper is releasing liquid into a fizzing liquid. The fizzing liquid is releasing a white gas.

The reaction of iron with an acid produces hydrogen. Here, iron reacts with hydrochloric acid. (credit: Mark Ott)

Reaction of Ionic Metal Hydrides with Water

It is possible to produce hydrogen from the reaction of hydrides of the active metals, which contain the very strongly basic H⁻ anion, with water:

\({\text{CaH}}_{2}(s)+{\text{2H}}_{2}\text{O}(l)\;⟶\;{\text{Ca}}^{2+}(aq)+{\text{2OH}}^{\text{−}}(aq)+{\text{2H}}_{2}(g)\)

Metal hydrides are expensive but convenient sources of hydrogen, especially where space and weight are important factors. They are important in the inflation of life jackets, life rafts, and military balloons.

Preparation of Hydrogen