Experiment 9. Preparation of Hydrogen Gas.
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↑
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 this video
. 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, click here. 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.
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 (watch this video
) is employed in portable devices for generating hydrogen to inflate weather balloons. In contrast, the reaction of sodium hydride (NaH) with water is violent (watch this
) and therefore is not used to make H2
. 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.