Question

hydrogen bonding and boiling point
molecular compounds that have low molar
mass are usually gases at stp. for example,
amonia (nh₃) has a molar mass of 17.0 g/mol
and boils at -33°c. water has a similar molar
mass of 18.0 g/mol, but it has a boiling point
of 100°c, so it is liquid at stp. the difference
is due to the fact that water has stronger
hydrogen bonds and forms more bonds per
molecule, which raises the temperature at
which it turns into gas. the reason that water
forms more hydrogen bonds is because its
oxygen atom has two lone pairs of electrons,
while the nitrogen atom in ammonia has only
one pair. this results in two hydrogen bonds
per molecule of water versus one per molecule
of ammonia. the individual hydrogen bonds in
water are also slightly stronger because oxygen
is more electronegative than nitrogen, which
results in a larger partial positive charge on the
hydrogen atoms in water molecules.
hydrogen bonds affect boiling point water has a much higher boiling
point than ammonia because water forms stronger hydrogen bonds with
neighboring molecules than ammonia does.
water (h₂o)

• low molar mass
• strong hydrogen bonds
• high electronegativity of o
• both hs involved in hydrogen

bonding

• low vapor pressure
• high boiling point
• liquid at stp

ammonia (nh₃)

• low molar mass
• weak hydrogen bonds
• lower electronegativity of n
• only one h involved in hydrogen

bonding at any one time

• high vapor pressure
• low boiling point
• gas at stp

34 sep construct an explanation hydrogen fluoride (hf) has a lower
boiling point than water (h₂o), even though fluorine (f) has a greater
electronegativity than oxygen (o). why do you think that is?

Explanation:

Hydrogen bonding affects boiling points. Water (\(H_2O\)) forms more hydrogen bonds per molecule than hydrogen fluoride (\(HF\)). In \(H_2O\), each oxygen atom has two lone pairs of electrons, allowing it to form two hydrogen bonds per water molecule. In \(HF\), each fluorine atom has three lone pairs, but only one hydrogen atom per \(HF\) molecule is available for hydrogen bonding. So, the number of hydrogen bonds per molecule is higher in water. Also, the overall intermolecular forces (due to more hydrogen bonds in water) require more energy to break, leading to a higher boiling point for water compared to \(HF\) despite fluorine having higher electronegativity.

Answer:

Water forms more hydrogen bonds per molecule than \(HF\). In \(H_2O\), each molecule can form two hydrogen bonds (because oxygen has two lone pairs), while in \(HF\), each molecule can form only one hydrogen bond (as there is only one hydrogen atom per \(HF\) molecule available for hydrogen - bonding with the lone pairs of fluorine). More hydrogen bonds mean stronger intermolecular forces in water, requiring more energy (higher temperature) to convert it from liquid to gas (boil), so water has a higher boiling point than \(HF\).