Which hydride is H2S an example of this



Fig. 1: Spatial structure
of the water molecule
Fig. 2: Geometry
of the water molecule
Fig. 3: Space-filling model
of the water molecule

The molecule of water consists of two hydrogen atoms and one oxygen atom. It is the basis for explaining the properties of water. The water molecule is geometrically angled and corresponds to the AB in the VSEPR theory2E.2-Type. The two hydrogen atoms and the two electron pairs are consequently directed into the corners of an imaginary tetrahedron. The angle enclosed by the two O-H bonds is 104.45 °. It deviates from the ideal tetrahedron angle (~ 109.47 °) due to the increased space requirement of the lone electron pairs. The bond length of the O-H bonds is 95.84 picometers in each case.

In the water molecule, the 1s orbitals of two hydrogen atoms combine with one sp3 hybrid orbital each of the oxygen atom to form two σ bonds (Fig. 4). The four orbitals of the bonding and non-bonding electron pairs are aligned with the corners of a tetrahedron. The electron pairs shown in red in Figure 1 are in the atomic orbitals and the gray electron pairs in the molecular orbitals.

Dipole moment

On the Pauling scale, oxygen has an electronegativity of 3.5, which is 1.4 higher than hydrogen, which has 2.1. Due to the angled geometry of the molecule and the different partial charges of the atoms, it has negative polarity on the oxygen side and positive polarity on the side of the two hydrogen atoms. This causes the dipole moment, which is 1.84 Debye in the gas phase. In contrast to the linear structure of carbon dioxide, this shows that the angular arrangement of the two hydrogen atoms prevents the polar atomic bonds from balancing each other out, i.e. the centers of charge do not coincide. Only then does water have a permanent electrical dipole moment and has many properties that result from this. The VSEPR theory provides an explanation for this angular arrangement based on the two lone electron pairs of the oxygen atom. Due to the different partial charges, the molecule can be set in rotation by certain electromagnetic waves, the microwaves, which lead to the heating of the water.

Hydrogen bond

 

Water molecules interact with each other via hydrogen bonds and thus have pronounced intermolecular attractive forces. It is not a permanent, fixed chain. The bond of the water molecules, which are inconsistently linked via hydrogen bonds, only lasts for a fraction of a second, after which the individual molecules are released from the bond and are linked again in an equally short period of time. This process repeats itself continuously and ultimately leads to the formation of a variable cluster, as shown in the sketch on the right.

Among other things, the small diameter of the hydrogen atom is important for the formation of hydrogen bonds, since this is the only way it can approach the oxygen atom to a sufficient extent. The higher homologues of water, for example hydrogen sulfide H2S, do not form such bonds due to the lower electronegativity difference between the bond partners.

The chaining of the water molecules through hydrogen bonds is the reason for many special properties, for example that water is liquid under standard conditions despite the low molar mass of around 18 g / mol. H2In contrast, S is in gaseous form. The fact that water, due to its density anomaly, has its greatest density at around four degrees Celsius, and thus, for example, ice can float on liquid water, can also be attributed to the hydrogen bonds.

Heavy, semi-heavy and super-heavy water

In addition to "normal" water, there is also "heavy water" (deuterium oxide, D2O), the "half-heavy water" (HDO) and the "super-heavy water" (tritium oxide, T.2O). In these waters, the normal hydrogen atoms (protium, symbol H) have been partially or completely replaced by their heavier isotopes, deuterium (D) or tritium (T). Heavy water differs from ordinary water in terms of its physical and chemical properties. They have a higher melting point, a higher boiling point and a greater density. Due to the particularly large mass difference between protium and tritium or deuterium, the kinetic isotope effect is particularly pronounced here. Consequently, when normal water is replaced with heavier water, the equilibrium position is changed during chemical equilibrium reactions, which can lead to health consequences in the human body, for example.

In nature, the concentrations are (apart from slight fluctuations) distributed over:

  • H216O: 99.73%
  • H218O: 0.20%
  • H217O: 0.04%
  • HD16O: 0.03%
  • HD18O: 0.000'057%
  • D.216O: 0.000'002'3%
  • D.218O: 0.000'000'004'4%
  • D.217O: 0,000,000,000,9%
  • HD17O: 0.000'000'000'000'001'2%

Due to the different spin properties of nuclear spin, deuterated water is used as a solvent for NMR analysis. The hydrogen isotopes of the water molecule become along with the oxygen isotope 18O used as a tracer.

Categories: Water | Hydrogen compound | Oxygen connection