In each pair of compounds, pick the one with the higher...

In each pair of compounds, pick the one with the higher vapor pressure at a given temperature. Explain your reasoning. $$ \begin{array}{l}{\text { a. } \mathrm{Br}_{2} \text { or } \mathrm{I}_{2}} \\ {\text { b. } \mathrm{H}_{2} \mathrm{S} \text { or } \mathrm{H}_{2} \mathrm{O}} \\ {\text { c. } \mathrm{NH}_{3} \text { or } \mathrm{PH}_{3}}\end{array} $$

In each pair of compounds, pick the one with the higher vapor pressure at a given temperature. Explain your reasoning.

$$
\begin{array}{l}{\text { a. } \mathrm{Br}_{2} \text { or } \mathrm{I}_{2}} \\ {\text { b. } \mathrm{H}_{2} \mathrm{S} \text { or } \mathrm{H}_{2} \mathrm{O}} \\ {\text { c. } \mathrm{NH}_{3} \text { or } \mathrm{PH}_{3}}\end{array}
$$

Key Concepts

Dispersion Forces

Dispersion forces, also known as London forces, arise from temporary fluctuations in electron density, leading to temporary dipoles in molecules. These forces become stronger with increased molecular size and greater polarizability. In comparing similar molecules, a larger or more polarizable molecule typically exhibits stronger dispersion forces, which results in a lower vapor pressure relative to its counterpart with weaker dispersion forces.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when hydrogen is directly bonded to highly electronegative elements such as oxygen, nitrogen, or fluorine. This strong attraction significantly increases the energy needed to vaporize a substance, thereby lowering its vapor pressure compared to similar molecules that do not form hydrogen bonds.

Vapor Pressure

Vapor pressure is the measure of a substance's tendency to evaporate, defined as the pressure exerted by its vapor when in equilibrium with its liquid or solid form. It is influenced by the strength of the intermolecular forces present; substances with weaker intermolecular attractions tend to have higher vapor pressures because their molecules more easily escape into the gas phase.

Intermolecular Forces

Intermolecular forces are the attractions between molecules, including dispersion forces, dipole-dipole interactions, and hydrogen bonding. The strength of these forces dictates many physical properties, such as boiling points and vapor pressures. Stronger attractive forces mean that more energy is required for molecules to leave the liquid phase, resulting in a lower vapor pressure.

Transcript

00:01 My name is ahmad ali in this question.

00:03 We have a pair of compound and we want to know which one of them has a higher vapor pressure.

00:09 The vapor pressure is related to the substance that scale from its intermolecular force and become a vapor.

00:20 So the vapor pressure is inversely proportional to the intermolecular forces, which i mean here that compound with a lower intermolecular force will be had a higher vapor pressure so first of all we must know which compound has a lower intermolecular force and then we can decide it has a higher vapor pressure then the intermolecular strengths is depend firstly on the hydrogen bonding so the substance with hydrogen bond has a higher intermolecular forces strengths then we look if no hydrogen bond we look to the dispersion force which dispersion force strength is depend on the molecular way or the molar mass of these molecules so let's do this experimentally.

01:35 We have here pr2 and i2.

01:40 Both of them are element substance and they don't contain hydrogen bonds so they only contain dispersion force.

01:52 The dispersion force in iodine is higher than the bromine which means the iodine has a higher intermolecular force because the molar mass of iodine is higher than the bromine.

02:07 That means that the bromine will be had a higher vapor pressure because we say that the vapor pressure is inversely proportional to the intermolecular force.

02:23 So this is the first example.

02:29 The second example we have here is, h2s is h2s and h2o.

02:43 Here we found that h2o had a hydrogen bond while h2o is h2s not...

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