Use your understanding of phases, bonding, solids and solutions to explain each

Question

Use your understanding of phases, bonding, solids and solutions to explain each of the following. Why does the barometer fall when it is ping to rain? Water is a liquid at room temperature but H_2S is a gas even though it is a larger molecule with a similar shape. What creates this difference? Why do we add CaCl_3 to icy roads instead of the cheaper NaCl? While adding heat to melt a solid a student notices that the temperature as the substance melts. Metals are good conductors of electricity but ionic solids are insulators. However, when the ionic solid is melted it conducts electricity. Lead (MM = 2072) has a density of 11.30 g/cm^3 and has an edge length of 495.7 pm. What type of crystal lattice does it form?

Explanation / Answer

2)

Reason: Water is liquid because of hydrogen bonding which is not present in H2S

Explanation:

The relatively large electronegativity difference between oxygen and hydrogen (about 2.3) is the reason why the partially positively charged (try to think why they are partially positively charged) hydrogens are attracted to the lone pairs on the oxygen atoms of other water molecules.

The relatively smaller electronegativity difference between sulphur and hydrogen (about .5) is too small to polarise the covalent bond to a required extent. Secondly, the lone pairs on the sulphur atom are delocalised in the sp3sp3 orbitals and the dd orbitals - an option unavailable to oxygen. So, the “negative charge” of the lone pairs is also distributed and the interaction is lowered.

So, the hydrogen bonding in water causes the water molecules to be associated with each other due to intermolecular forces of attraction, whereas in the case of hydrogen sulphide, these forces of attraction area much weaker.

So, lesser energy is required to overcome the forces of interaction between the hydrogen sulphide molecules than those between water molecules. This energy is available at room temperature and hence, hydrogen sulphide is a gas, while water is still a liquid.

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