For each pair of substances listed below, identify the one with the lower boiling point.

(a) Silane (SiH4) and Phosphine (PH3)
answer = silane

(b) Ammonia (NH3) and Phosphine (PH3)
answer = phosphine

(c) Lithium fluoride (LiF) and Hydrogen Fluoride (HF)
answer = HF

a- silane

b- phosphine
c- HF

Why don't you look up the b.p. on google? I think you've answered a wrong.

No, it is absolutely correct.

Silane pointing point = −112 °C, 161 K, -170 °F

Phosphine boiling point = -125.9°F (-87.7°C)

PHOSPHINE HAS A LOWER BOILING POINT!!!!

ARE YOU SURE THE ANSWERS ARE RIGHT?

My dear.....check the signs (-) on the values and then check the question once again......mu answers are 100% correct...and i have scored 100% n this exam also.....:P

To determine which substance has the lower boiling point in each pair, we need to consider a few factors that affect boiling points.

First, we can look at the molecular size and shape of the substances. Generally, smaller molecules tend to have lower boiling points compared to larger molecules. This is because smaller molecules have weaker intermolecular forces and therefore require less energy to separate the molecules in the liquid phase and convert them into a gas.

Next, we can consider the type and strength of intermolecular forces present in each substance. The main intermolecular forces are London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces exist in all molecules and increase with the size and shape of the molecules. Dipole-dipole interactions occur in polar molecules and are stronger than London dispersion forces. Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine.

Now, let's apply this information to each pair of substances:

(a) Silane (SiH4) vs. Phosphine (PH3):
Silane (SiH4) has a lower boiling point compared to Phosphine (PH3). Both Silane and Phosphine have weak London dispersion forces since they are both nonpolar molecules. However, Silane is larger and has more electrons, resulting in stronger London dispersion forces. Therefore, Silane has a lower boiling point.

(b) Ammonia (NH3) vs. Phosphine (PH3):
Phosphine (PH3) has a lower boiling point compared to Ammonia (NH3). While both substances are polar molecules and have dipole-dipole interactions, Phosphine is larger and experiences stronger London dispersion forces compared to Ammonia. As a result, Phosphine has a lower boiling point.

(c) Lithium fluoride (LiF) vs. Hydrogen Fluoride (HF):
Hydrogen Fluoride (HF) has a lower boiling point compared to Lithium fluoride (LiF). Hydrogen Fluoride is a polar molecule capable of forming hydrogen bonds between its hydrogen atom and a highly electronegative fluorine atom. In contrast, Lithium fluoride does not have these hydrogen bonding interactions. The presence of hydrogen bonds in HF makes it harder to break the intermolecular forces in the liquid state, resulting in a higher boiling point compared to LiF. Therefore, Hydrogen Fluoride has a lower boiling point.