SS2 Chemistry Lesson Note on Hydrogen

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TOPIC: HYDROGEN

CONTENT:

1. Configuration, possible oxidation, Isotopes of hydrogen and unique Position of hydrogen of hydrogen in periodic table,
2. Laboratory and Industrial preparation of hydrogen
3. Physical and chemical properties,
4. Compounds of hydrogen, Uses and test of hydrogen.

PERIOD 1: CONFIGURATION, POSSIBLE OXIDATION AND ISOTOPES OF HYDROGEN AND UNIQUE POSITION OF HYDROGEN OF HYDROGEN IN PERIODIC TABLE.

Hydrogen is the first element in the periodic table with nucleus and electron. It is a gas; diatomic molecule in which two atoms of hydrogen are covalently bonded. The atomic number of hydrogen is one. The electronic configuration of hydrogen is 1s¹. It has oxidation number of +1 and -1

ISOTOPES OF HYDROGEN

Hydrogen exhibits Isotope with Hydrogen or protium [₁¹H], Deuterium or heavy hydrogen [²₁H or D] and tritium [³₁H or T] as the Isotopes. These Isotopes have relative atomic masses of 1, 2 and 3 respectively.

HYDROGEN ISOTOPES

The Isotopes have similar chemical properties but different physical properties although tritium is radioactive.

UNIQUE POSITION OF HYDROGEN IN THE PERIODIC TABLE

It can lose this single electron to form positive ions[H⁺] just like the elements in groups [Li, Na, K, Rb, Cs and Fr] and based on the argument it can be considered to be placed in group 1 along with group 1 elements. However, the H⁺ ion is not stable like ions formed by group 1 elements. Most of the properties of H⁺ are not similar to the properties of group I elements.

 Hydrogen can also gain one electron to form hydride ion (H^–) similar to halogens (group 7 elements) forming halides ion. Thus, hydrogen is unique and anomalous in being similar to two different groups of elements. With electro positive alkali and alkali-earth metals, hydrogen forms compounds like LiH, NaH, and CaH₂ etc. Similar to halogens form LiCl, Na, CaCl₂ etc. On being electrolysed these hydrides yield hydrogen or halogen at the anode showing the similarities between the two. The ionisation energy of hydrogen and halogen are comparable. While those of alkali- metal are very low. It is similar to halogen in been a non-metal, a bad conductor of heat and electricity and diatomic. Hydrogen forms covalent compound like those of halogens.

EVALUATION

1. To which group does hydrogen belong?
2. What is the position of hydrogen in the periodic table?
3. Mention the existing Isotopes of hydrogen
4. Discuss the uniqueness of hydrogen

PERIOD 2: LABORATORY AND INDUSTRIAL PREPARATION OF HYDROGEN.

Laboratory preparation of hydrogen:

Hydrogen is liberated when active metals react with dilute mineral acid, water or steam.e.g action of zinc on H₂SO₄

Zn_(s) + H₂SO_{4(aq)   →}    ZnSO_{4 (aq)} +H_{2 (g)}

The hydrogen gas is dried by passing it through Calcium  chloride. The apparatus is set up as shown below.

Dilute hydrochloric acid can also be used in place of H₂SO_4. Magnesium and iron can be used in the place of zinc.

Zn_(s) + 2HCl _(aq)  →     ZnCl_{2 (aq)} + H_{2 (g)}

Mg_(s) + H₂SO_{4 (aq)}    →        MgSO_4(aq) + H_{2 (g)}

Dilute trioxonitrates (v) acid is never used in the preparation of hydrogen because of its strong oxidizing property and it produces water instead of hydrogen on reaction with metals. However, very dilute trioxonitates (v) acid give out hydrogen on reaction with Magnesium.

Hydrogen can also be prepared in the laboratory in the following ways,

1. By the action of Sodium hydroxide on zinc metal or aluminium.

Zn_(s) + 2NaOH_(aq)    →       Na₂ZnO_2(s) + H_2(g)

2Al_(s) + 2NaOH_(aq) + 2H₂O_(l)    →    2NaAlO_2(s) + 3H_{2 (g)}

2. Action of steam on heated metals e.g. Iron, Magnesium.

3Fe_(s) + 4H₂O_(g)       →   2NaOH_(aq) + H_2(g)

PREPARATION OF DRY HYDROGEN

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