Account for the following observations. (Section 25.3) (a) The boiling point of ammonia is highe

step 1 Hello everyone. Let us see the following question. We have to account for the following observat

Account for the following observations. (Section 25.3) (a)...

Key Concepts

Crystal Structure and Lattice Packing

The arrangement of molecules in the solid phase, or crystal lattice, critically determines the density and phase properties of a substance. For instance, the open, tetrahedral network structure of ice, established by hydrogen bonding, leads to a lower density compared to liquid water. Conversely, substances with non-hydrogen-bonded molecules, like methane, tend to adopt crystal structures that allow for more efficient packing. This results in a denser solid form relative to the liquid state, highlighting how molecular arrangement and lattice organization underlie the density anomalies observed upon phase changes.

Molecular Polarizability and Dispersion Forces

Molecular polarizability refers to the ease with which the electron cloud around a molecule can be distorted, which directly affects the strength of dispersion forces. In molecules with larger atomic radii or more diffuse electron clouds, these forces become stronger, influencing melting and boiling points. This concept helps explain trends such as why the melting point of hydrogen selenide is higher than that of hydrogen sulfide, as greater polarizability results in enhanced attractive interactions in the condensed phases.

Intermolecular Forces

Intermolecular forces encompass all the attractions between molecules, including hydrogen bonds, dipole–dipole interactions, and dispersion (van der Waals) forces. The relative strength and type of these forces dictate physical properties such as boiling and melting points. In cases where hydrogen bonding is absent or weak, as seen in larger molecules with greater polarizability (e.g., phosphine or hydrogen selenide), dispersion forces and the molecular framework become more influential, leading to different phase transition behaviors.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to a highly electronegative atom (such as nitrogen or oxygen). This bond creates a significant positive charge on the hydrogen atom, which can attract lone pairs on electronegative atoms in neighboring molecules. In substances like ammonia and water, hydrogen bonding is responsible for anomalously high boiling and melting points and unusual density behaviors, as it strongly influences how molecules associate and arrange themselves in both liquid and solid phases.

Transcript

0:00 Hello everyone.

00:01 Let us see the following question.

00:02 We have to account for the following observation.

00:04 Observation a is the boiling point of ammonia is higher than that of phosphine.

00:10 So, ammonia is nh3.

00:13 Phosphine is ph 3.

00:15 When compared to the boiling point, ammonia has a higher boiling point to that of phosphine.

00:21 So we know the simple logic.

00:23 The electro negativity of an atom is inversely proportional.

00:30 To its atomic size.

00:33 So, we know that phosphine being bigger atom, its electronegativity is less.

00:38 So, nitrogen having highest electronegativity will exhibit stronger hydrogen bonding.

00:43 Due to stronger hydrogen bonding, ammonia has higher boiling point.

00:49 So, the reason is stronger hydrogen bonding.

00:52 So let us see option b.

00:54 The melting point of hydrogen selenide, that is h2 s .e, is higher than that of hydrogen sulfide h2 s.

01:04 So the melting point now we are talking about melting point of hydrogen cyanide is greater than the melting point of hydrogen sulfide.

01:13 Now bigger the size depends on the vander wall force.

01:17 So now we have to talk about vanderval force which directly depend on the atomic size as well as atomic mass.

01:32 Now among the amount of the water, you know, along sulfur and selenium, selenium has highest atomic mass, hence it has got highest higher vanderwall force.

01:44 Due to that, its melting point is highest...

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