Answer to Question #299558 in General Chemistry for Zaza

Question

Considering the observed IMF and strength of those forces, complete the following questions with your best predictions.

1) Rank the following molecules from lowest to highest boiling point: H2, Br2, and F2. Explain your prediction based on IMF.

2) Predict which of the following has the highest boiling point: HF or HBr. Explain your prediction based on IMF.

3) Consider water, H2O. Predict how water’s boiling point compares to HBr and HF? Explain your prediction based on IMF.

4) Look up the boiling points of H2, H2O, Br2, F2, HBr, and HF. Were your predictions correct? Explain.

Answer 1

1) H₂ < F₂ < Br₂

Bigger molecules will have stronger London dispersion forces. So Br₂ has the strongest forces, and H₂ will have the weakest. Correspondingly, Br₂ will have the highest boiling point and H₂ will have the lowest boiling point.

2) HBr < HF

HBr and HF are hydrogen halides. F, Cl, Br, and I belong to the group of halogens.

Talking about the boiling point, it depends upon the hydrogen bonding present between the molecules.

Now, if we consider the above molecules, i.e. HI, HBr, and HCl. In these molecules, the dominating interactions would be van der Waals forces. As among these three halides, HI has the larger size or molecular mass.

Then we know, larger the size, or molecular mass of hydrogen halides, greater will be the van der Waals forces, higher the boiling point. The presence of Hydrogen bonding is negligible in these halides.

If we talk about the HF, it has strong hydrogen bonding, thus has the highest boiling point.

Therefore, HF exists in the liquid state, and HI, HBr, and HCl exists in the gaseous state.

We can conclude, the order of boiling point followed is HF > HI > HBr > HCl.

Don’t get confused between the boiling points of HF, and HI. The confusion can occur why HF has a higher boiling point than the HI. The reason is the presence of strong hydrogen bonding in the HF, as hydrogen intermolecular interactions are greater than the van der Waals forces. So, HF has a higher boiling point than the HI.

3) Of the three compounds, HBr should have the lowest boiling point because it has

the weakest IMFs (HBr is dipole-dipole, HF and H₂O are hydrogen bonds). Between

water and HF, I would expect HF to have a lower boiling point because it has less

mass than water and should be easier to boil. Water also has two opportunities to

hydrogen bond with the oxygen, so it will be more difficult to separate the molecules into the gas phase. So HBr < HF < H₂O

4) H₂O: 100°C

Br₂: 59°C

F₂: -188°C

H₂: -253°C

HBr: -66°C

HF: 19.5°C

Br₂ and F₂ are nonpolar, so they have low boiling points, and F₂ should be lower than Br₂ because of its smaller size/mass (it is less polarizable). Br₂ has a higher boiling

point than HF because of its large mass and size, which makes it more polarizable. H₂O has the highest boiling point because it has the strongest/most IMFs. HF has a

higher boiling point than HBr because of hydrogen bonds.