Volume 1

General Information • Applications of Hydrogen • Physical Properties of H2 • Preparation of Hydrogen. Experiment • Chemical Properties of H2 • Exercises

1.11.1. General Information. As a chemical element, hydrogen has the symbol H and atomic weight of 1 a.m.u. Hydrogen is the first element in the periodic table and is the lightest chemical element known. The valence of hydrogen in its compounds is 1. As a simple substance, hydrogen is a gas that consists of diatomic molecules H2.

While oxygen is the most abundant element on Earth, hydrogen is the most abundant element in the universe, over 70% by mass. However, there is almost no hydrogen gas in the atmosphere, just about 0.0001% by volume.

1.11.2. Applications of Hydrogen. Hydrogen has many important applications. The largest consumers of pure H2 are the petrochemical and chemical industries. While huge amounts of hydrogen are utilized in the processing of fossil fuels, even larger quantities of H2 are used to make ammonia (NH3) for the manufacturing of fertilizers. To get an idea of the tremendous scale and importance of the ammonia production, the industrial synthesis of ammonia currently consumes more than 1% of all energy produced in the world and provides food to up to one-half of the entire population of our planet. An important application of hydrogen in the food industry is in the manufacturing of margarine from vegetable oils.

1.11.3. Physical Properties of H2. Hydrogen is a colorless and odorless gas. Being the lightest gas known, H2 is nearly 15 times lighter (less dense) than air. At normal pressure, the boiling and melting points of hydrogen are -253 oC and -259 oC, respectively.

1.11.4. Preparation of Hydrogen. Experiment. As an element, hydrogen is ubiquitous on Earth, being present in water, natural gas, oil, and all living organisms. However, unlike oxygen (O2) that is readily available (in air), hydrogen as a simple substance (H2) naturally occurs on our planet only in trace quantities and therefore has to be made for its numerous applications. More than 90% of all hydrogen currently produced comes from the steam reforming of natural gas, methane (Figure 1-35). This reaction is conducted at high temperatures of 700 – 1,100 oC in the presence of a nickel catalyst.
Figure 1-35. Steam reforming of methane.

In the laboratory, hydrogen is conventionally obtained by the reaction of some acids with certain metals. Most commonly zinc (Zn) or iron (Fe) and aqueous sulfuric acid (H2SO4) are used (Figure 1-36).
Figure 1-36. Laboratory methods to make hydrogen.

Note that although Fe can be divalent or trivalent, it is always a divalent Fe compound that is formed in reactions of Fe with H2SO4 and most other acids.
Digression. Some people think that most of the hydrogen gas (H2) is effortlessly and cleanly produced from water, which is abundant and free to everyone. This is not the case. To make H2 from H2O, one needs to use electrolysis. Electrolysis, as we will learn in Volume 2, is a method to perform chemical reactions using a direct electric current. For making large industrial quantities of hydrogen from water, this process is not economical because electricity is expensive. (Just recall that electric heating is considerably more expensive than oil or gas heating.) Furthermore, the production of electricity by conventional industrially feasible means is not environmentally friendly. As a matter of fact, making cheap electricity by burning coal is an environmentally malevolent process. Although some modern means to generate electricity, such as wind mills and solar cells, are environmentally benign, their capacity currently remains just a small fraction of the demand.
Experiment 9. Preparation of Hydrogen Gas. Please carefully read and understand the following:

DISCLAIMER: Although most of the experiments described in this subsection and elsewhere in this website are regarded as low hazard, I expressly disclaim all liability for any occurrence, including, but not limited to, damage, injury or death which might arise as consequences of the use of any experiment(s) discussed, listed, described, or otherwise mentioned in the free online course Chemistry from Scratch. Therefore, you assume all the liability and use these experiments at your own risk (see Terms of Use).

If you decide not to do the experiment, still read this subsection.

Clean up an iron nail with sandpaper and place it in a glass containing white vinegar. Observe the formation of small bubbles of H2 on the surface of the nail. Depending on the strength of the vinegar and purity of the iron, the evolution of H2 may be noticeable right away or you may need to wait for a few minutes before the bubbles become visible on the nail surface. If necessary, use a magnifying glass to see the bubbles. The chemical equation for the reaction is as follows.

Fe + 2 C2H4O2 = Fe(C2H3O)2 + H2 (C2H4O2 is the formula of acetic acid)

You are expected to remember that vinegar is a solution of acetic acid. The concentration of acetic acid in white vinegar can vary from 4% to 8%. The higher the concentration of acetic acid in vinegar, the faster it reacts with iron. If your vinegar happens to be rather weak, you can speed up the reaction by warming up the vinegar before dropping the nail in it. With some very rare and interesting exceptions, chemical reactions proceed faster at higher temperatures. The effect of temperature on the reaction of vinegar with iron can be seen in Video 1-29.
Video 1-29. The effect of temperature on the reaction of vinegar with iron. The higher the temperature, the faster the reaction (source).

Warning: NEVER attempt to microwave vinegar with a nail or any other metal object in it. Warm up vinegar in a microwave, turn off the oven, and take the glass out. Place the nail in the warm vinegar. To see what happens to iron nails on microwaving, watch Video 1-30. Now imagine what might happen if hydrogen, a flammable gas that forms explosive mixtures with air (or oxygen), was produced in the microwave chamber containing red-hot iron nails giving off large sparks.
Video 1-30. Iron nails in a microwave (source).

In subsection 1.8.4 on substitution reactions (see also Figure 1-25), we learned that sodium reacts with water to give hydrogen gas and NaOH.

2 Na + 2 H2O = 2 NaOH + H2

However, due to its violent nature (as well as high cost of sodium), this reaction is not used to make hydrogen.

There is an interesting class of compounds called metal hydrides. A metal hydride is a hydrogen derivative of a metal, such as NaH (sodium hydride) and CaH2 (calcium hydride). Metal hydrides react with water to give hydrogen gas, as exemplified by the following equation.

CaH2 + 2 H2O = Ca(OH)2 + 2 H2

This rather smooth reaction of calcium hydride with water (Video 1-31) is employed in portable devices for generating hydrogen to inflate weather balloons. In contrast, the reaction of sodium hydride (NaH) with water is violent (Video 1-32) and therefore is not used to make H2.
Video 1-31. Calcium hydride (CaH2) smoothly reacts with water (H2O) to produce hydrogen gas (H2) (source).
Video 1-32. The reaction of sodium hydride (NaH) with water (H2O) is violent enough to result in self-ignition of the hydrogen (H2) produced (source).

1.11.5. Chemical Properties of H2. As we already know, hydrogen burns in oxygen or air to produce water:

2 H2 + O2 = 2 H2O

This clean reaction is highly exothermic (a large amount of heat is released) and environmentally benign as it produces water as the only chemical product.
Digression. Upon initial consideration, one would have thought that hydrogen as a fuel is an ideal source of energy. Unfortunately, the hydrogen economy does not seem to be viable at the moment for a number of reasons. First, H2 is not as cheap as many people think. Second, H2 is too light to be transported economically. Third, due to the extremely small size of the H2 molecule, hydrogen gas leaks exceptionally easily through all sorts of materials, including those that gas pipelines and hosepipes are made of (steel, rubber, and various plastics). Last but not least, mixtures of H2 with oxygen or air are flammable and can even be explosive. (As we discussed earlier, it is nonflammable helium rather than hydrogen that is used to inflate balloons and airships, despite the fact that H2 is not only lighter but also vastly cheaper than He.) Note however, that a stream of pure H2 burns smoothly in air; it is mixtures of hydrogen with air or oxygen in certain proportions that are explosive.
Hydrogen reacts not only with oxygen, but also with some other simple substances (Figure 1-37).
Figure 1-37. Reactions of hydrogen with selected simple substances.

An important chemical property of H2 is its ability to pluck oxygen from some (but not all) metal oxides. For example, heating black copper (II) oxide (CuO) in an atmosphere of H2 gas results in the formation of red copper metal (Cu) and water (Figure 1-38 and Video 1-33).
Figure 1-38. Reaction of copper (II) oxide with hydrogen.
Video 1-33. Reaction of black copper oxide (CuO) with hydrogen (H2) to give red copper metal (source).

Reactions in which a metal oxide is converted to the metal by H2 are called reduction reactions. An important reduction reaction is the formation of tungsten metal (W) from its oxide WO3 (Figure 1-39).
Figure 1-39. Reduction of tungsten oxide (WO3) with hydrogen.
Digression. The reaction of WO3 with H2 (Figure 1-39) is used to make high purity tungsten for light bulb filaments. After an incandescent lamp is turned on, the filament should quickly reach a very high temperature of 2,500 - 3,000 °C to emit light while staying that hot but not melting. Tungsten is a metal with the highest melting point (3,422 °C) and is the best material for making filaments for light bulbs as well as for vacuum tubes and X-ray tubes. The modern tungsten filament is one of many things that we take for granted; we use incandescent light bulbs on a daily basis, while not realizing that they are, in fact, a miraculous scientific and engineering achievement. Watch this excellent short video on the big challenges that had to be overcome to make the production of tungsten filament light bulbs possible.
1.11.6. Exercises.

1. At elevated temperatures and high pressures, aluminum (Al) reacts with H2 to give aluminum hydride. Knowing the valence of Al (Section 1.9, Table 1), write the chemical formula of aluminum hydride and a balanced chemical equation for its formation from Al and H2. Answer

2. Balance the following chemical equations.

(a) H2 + F2 = HF

(b) H2 + Li = LiH

(c) H2 + Ca = CaH2

(d) H2 + N2 = NH3

(e) Fe2O3 + H2 = Fe + H2O

3. Copper-tungsten (Cu-W) composites are valuable materials that have a very high thermal resistance and excellent heat and electrical conductivity, while expanding only slightly at high temperatures. Since molten W and Cu metals are not miscible and cannot form an alloy, Cu-W composites are prepared by sintering fine powders of the two metals. You have copper (II) oxide, tungsten (III) oxide, sulfuric acid, and iron nails. Suggest a method to make copper and tungsten metals using these materials. Answer

4. As described in subsection 1.11.4, both iron (Fe) and zinc (Zn) react with sulfuric acid (H2SO4) to give H2 (Figure 1-36). You have 1 g of Zn, 1 g of Fe, and plenty of H2SO4. You are asked to make as much H2 as possible using these reagents, but you are allowed to use only either the zinc or the iron. Which of the two metals should you use? Try to solve this problem mentally, just by looking up the chemical equations above and the atomic masses of Zn and Fe in the periodic table. Answer