Hydrogen’s characteristics, properties, and interactions with oxygen

Hy­dro­gen is a chem­i­cal el­e­ment lo­cat­ed si­mul­ta­ne­ous­ly in two groups of the Pe­ri­od­ic Ta­ble of el­e­ments, in­dica­tive of op­po­site prop­er­ties. It can form a sim­ple sub­stance, and can ad­di­tion­al­ly be a com­po­nent of many com­pounds.

Hy­dro­gen’s chem­i­cal char­ac­ter­is­tics

Hy­dro­gen is a chem­i­cal el­e­ment lo­cat­ed in sub­group A of the first group, and in sub­group A of the sev­enth group in the first pe­ri­od.

Hy­dro­gen’s main prop­er­ties, as can be de­ter­mined from its po­si­tion as the first el­e­ment in the pe­ri­od­ic ta­ble, are as fol­lows:

• hy­dro­gen has an atom­ic num­ber of 1, and has 1 elec­tron and 1 pro­ton;
• hy­dro­gen’s atom­ic mass is 1.008. Hy­dro­gen has 3 iso­topes with mass­es of 1, 2, and 3. Their prop­er­ties are very dif­fer­ent – for hy­dro­gen, even adding just one AMU (atom­ic mass unit) means dou­bling its mass;
• as its out­er lev­el only con­tains 1 elec­tron, hy­dro­gen has both ox­ida­tive and re­duc­tive prop­er­ties. If it sur­ren­ders its elec­tron, hy­dro­gen is left with a free or­bital, which can form chem­i­cal bonds ac­cord­ing to a donor-ac­cep­tor mech­a­nism;
• hy­dro­gen is a good re­duc­er, and so is in sub­group A of the first group;
• when in­ter­act­ing with strong re­duc­ers (such as al­ka­li met­als), hy­dro­gen be­comes an ox­i­diz­er, ac­cept­ing an elec­tron. These com­pounds are called hy­drides. Due to this prop­er­ty, hy­dro­gen con­ven­tion­al­ly be­longs to the halo­gen group;
• hy­dro­gen’s low atom­ic mass makes it the light­est el­e­ment. As such, hy­dro­gen is rec­og­nized as a stan­dard of light­ness.

Hy­dro­gen is a chem­i­cal el­e­ment un­like any oth­er, with spe­cial prop­er­ties of enor­mous sig­nif­i­cance.

Phys­i­cal prop­er­ties of hy­dro­gen

Hy­dro­gen as a pure sub­stance has the fol­low­ing phys­i­cal pa­ram­e­ters:

• a melt­ing point of -259.2°C;
• a boil­ing point of -252.8°C;
• from (-259.2°C) to (-252.76°C), hy­dro­gen is a col­or­less, odor­less liq­uid;
• liq­uid hy­dro­gen can se­vere­ly freeze the skin upon con­tact;
• in nor­mal con­di­tions, hy­dro­gen is a col­or­less, odor­less, taste­less gas that is both com­bustible and ex­plo­sive;
• at high pres­sures, hy­dro­gen forms sol­id, snow-like crys­tals;
• in spe­cial con­di­tions, hy­dro­gen is ca­pa­ble of as­sum­ing a metal­lic state;
• hy­dro­gen is in­sol­u­ble in wa­ter, so it can be ob­tained in a lab­o­ra­to­ry set­ting via the elec­trol­y­sis of wa­ter (the de­com­po­si­tion of wa­ter into oxy­gen and hy­dro­gen);
• it dis­solves well in some met­als be­cause of its abil­i­ty to dif­fuse through them;
• hy­dro­gen is 14.5 times lighter than air;
• its crys­tal struc­ture is molec­u­lar and breaks down eas­i­ly due to weak bonds be­tween the lat­tice nodes.

Hy­dro­gen’s re­ac­tions with sim­ple sub­stances

When hy­dro­gen is heat­ed, a com­bi­na­tion re­ac­tion takes place be­tween the el­e­ment and sim­ple sub­stances such as chlo­rine, sul­fur, and ni­tro­gen. The fol­low­ing is a com­mon demon­stra­tion of the re­ac­tion be­tween hy­dro­gen and oxy­gen:

A stream of pure hy­dro­gen re­leased from a gas tube is ig­nit­ed in air, and the gas burns with an even, al­most un­no­tice­able flame. The stream of burn­ing hy­dro­gen is then di­rect­ed into a flask con­tain­ing oxy­gen. The hy­dro­gen con­tin­ues to burn, and the walls of the flask are cov­ered with drops of wa­ter that form as a re­sult of the re­ac­tion be­tween hy­dro­gen and oxy­gen. The com­bus­tion of hy­dro­gen is ac­com­pa­nied by the re­lease of a large amount of heat. The tem­per­a­ture of the hy­dro­gen-oxy­gen flame reach­es over 2,000 °C.

The chem­i­cal re­ac­tion of hy­dro­gen with oxy­gen is a com­bi­na­tion re­ac­tion that pro­duces wa­ter and more­over is an ox­i­da­tion-re­duc­tion re­ac­tion: oxy­gen ox­i­dizes hy­dro­gen.

If you col­lect hy­dro­gen in an up­side-down test tube, hav­ing re­moved the air, and bring a burn­ing match to the test tube neck, the mix­ture of hy­dro­gen and air will ex­plode with a loud re­port. This is called a “det­o­nat­ing” mix­ture.

As mix­ing hy­dro­gen and oxy­gen re­sults in oxy­hy­dro­gen, there have been ac­ci­dents in­volv­ing air­ships in­flat­ed with hy­dro­gen. When such a di­ri­gi­ble's her­met­ic seal is bro­ken, a sin­gle spark can cause an ex­plo­sion. For this rea­son, mod­ern air­ships are filled with he­li­um or con­stant­ly-heat­ed air.

Hy­dro­gen — re­ac­tion with oxy­gen

As we know, a mix­ture of hy­dro­gen and oxy­gen forms oxy­hy­dro­gen — an ex­plo­sive mix­ture. Oxy­hy­dro­gen is most ex­plo­sive at a hy­dro­gen:oxy­gen ra­tio of 2:1, also known as Brown's Gas, or 2:5 for hy­dro­gen and air.

Hy­dro­gen gas is high­ly flammable and will burn in air at an ex­treme­ly wide range of con­cen­tra­tions (be­tween 4% and 75% by vol­ume). Con­se­quent stud­ies on the re­ac­tion be­tween oxy­gen and hy­dro­gen have as­cer­tained that large vol­umes of hy­dro­gen can be ex­plo­sive even at small con­cen­tra­tions. In oth­er words, the greater the vol­ume of hy­dro­gen, the low­er the con­cen­tra­tion re­quired for an ex­plo­sion.

Ini­tial­ly, the ex­plo­sive na­ture of hy­dro­gen was stud­ied in lab­o­ra­to­ries with small amounts of the sub­stance. As the re­ac­tion of hy­dro­gen with oxy­gen is a chem­i­cal chain re­ac­tion that takes place ac­cord­ing to the free rad­i­cal mech­a­nism, the ter­mi­na­tion of free rad­i­cals on the walls of chem­i­cal ves­sels is crit­i­cal for con­tin­u­ing the chain. If there is a chance of for­ma­tion of a lim­it­ing con­cen­tra­tions of hy­dro­gen in large vol­umes (for ex­am­ple in sheds, ware­hous­es or fac­to­ry fa­cil­i­ties), we should note that a tru­ly ex­plo­sive con­cen­tra­tion of oxy­gen with hy­dro­gen may be ei­ther slight­ly more or less than 4%. At present, sci­en­tists are re­search­ing hy­dro­gen’s rel­a­tive­ly-un­ex­plored propen­si­ty to spon­ta­neous­ly com­bust from a dras­tic drop in pres­sure.