Question
Asked Feb 6, 2020
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Assign boiling points to the given substances based on intermolecular forces.

A 100
B 20
C -33


B: HF
A: H2O
C: NH3

Why does  NH have a lower boiling point than HF?

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Expert Answer

Step 1

The boiling points to the given substances are to be assigned on the basis of intermolecular forces-

A

100

B

20

C

-33

B: HF
A: H2O
C: NH3

Why NHhave a lower boiling point than HF is to be explained.

Step 2

The boiling point determines the strength of intermolecular forces between their molecules.

Thus, the stronger the intermolecular force, higher is the boiling point.

Now, 

Intermolecular interactions are the forces that act between the neighbouring molecules.

In general, there are four types of intermolecular forces-

London-dispersion forces: It is the weakest force that exists between all the molecules. It is temporary attractive force that results due to movement of electrons.

Dipole-dipole interactions: The polar molecules are dipoles i.e., one end has a negative charge while the other end has positive charge. When one dipole comes in vicinity of another dipole, the negative end of one dipole attracts the positive end of the other dipole and vice-versa.

Hydrogen bonding: when the hydrogen atom in a molecule is bonded to a highly electronegative atom (O, N or F), then this hydrogen atom forms a hydrogen bond with another electronegative atom with lone pairs.

Ionic-forces: This type of interaction occurs between two ions or ionic compounds.

  • The order of strength of these intermolecular forces are:

London dispersion forces < dipole-dipole forces < hydrogen bonding < ionic forces

Step 3

Now, all HF, H2O and NH3 are capable of forming hydrogen bonds.

H2O has 2 hydrogen atoms bonded to oxygen atom and two lone pairs. Thus, water molecules are capable of forming an average of 4 hydrogen bonds.   

Chemistry homework question answer, step 3, image 1

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