Question

Molecule A has hydrogen bonds; Molecule B is non - polar. Which of the molecular liquids will have the higher boiling point? Why? Also, which of the molecular liquids will have the lower vapor pressure? Why?

Answer 1

Ans- The strongest type of dipole-dipole force is the hydrogen bond. While it includes the word, "bond" in its name, this is NOT a chemical bond, as it is still much weaker than either an ionic bond or a covalent bond. "Hydrogen bonds" are the type of intermolecular force found in water, and they give water some very impressive properties, including a very high specific heat, and the ability to dissolve many ionic compounds. The greater strength of a "hydrogen bond" intermolecular force comes from the combination of a molecule's strong dipole moment and the very small size of the hydrogen atom. In water for instance, the smaller hydrogen atom of one water molecule is able to get its slight positive charge very close to the slight negative of the oxygen atom on another water molecule, and the short distance between the relatively large partial charges makes the attraction stronger than in other dipole-dipole attractions.

In order for a compound to exhibit hydrogen bonding, its molecule must have one or more hydrogen atoms covalently bonded to an atom with an electronegativity of 3.0 or greater (N, O, F or Cl). (Please note: this covalent bond between the H atom and N, O, F or Cl is NOT the "hydrogen bond.") The strong polarity of the bond between H and the highly electronegative other atom gives H a relatively large amount of partial positive charge, and the other atom a relatively large amount of negative charge. For F and Cl, the only compounds that will have this are HF and HCl--that is it. For N and O, there are many compounds that will have this, but in each of them the N or the O will have at least one non-bonding pair of electrons, so the molecular geometry around the N or O will always be unsymmetrical, so the molecule will always have a dipole moment. Again, it is the strong dipole moment coupled with the small size of the H atom that creates the strong "hydrogen bond" attractive force.

all molecules and atoms have some amount of attraction for each other. With nonpolar compounds and elements, the attraction results from a momentary, or "instantaneous" dipole moment that occurs when the electron cloud of an atom or molecule is distorted, or "dispersed" by the repulsion from the electron cloud of another atom or molecule. This instantaneous dipole occurs only during the very short period of time just before, during and just after the collision of one molecule or atom with another.

The bromine molecule, Br₂, is nonpolar, as it consists of two identical (and therefore equally electronegative) atoms. A single, isolated bromine molecule will have a perfectly symmetrical "cloud" of electrons around it. However, when two bromine molecules collide with each other, their electron clouds will change shape (dispers) as they repel each other, causing uneven distributions of negative charge within each molecule. The result is that one molecule's negative charge shifts away from the other molecule, allowing the positive charge of one of its nuclei to "shine" through and attract the electrons of the other molecule:

As soon as the molecules bounce away from each other, their electron clouds regain their original symmetrical shape, and the molecules lose their instantaneous dipole moment.

In order to determine the relative strength of a substance's intermolecular forces, the first thing to determine is whether the substance is polar or non-polar--if it is polar, the substance will exhibit dipole-dipole forces, which are stronger than London forces. Then, if the substance is polar, determine whether it will have hydrogen bonds, as these are the strongest type of dipole-dipole force.

Once the relative strength of the intermolecular forces are known, this information can be used to predict other properties, such as the relative melting points of a set of substances.

Example: Which of the following substances has the lowest melting point? Which one has the highest melting point?

PH₃          NaCl          H₂O

Solution: The compound with the lowest melting point will be the one with the weakest forces being broken when the solid becomes a liquid. When ionic compounds melt, it is ionic bonds that are being broken; when other compounds melt, it is intermolecular forces being broken, and ionic bonds are much stronger than intermolecular forces. The compound with the lowest melting will therefore be the molecular compound with the weakest intermolecular forces. Both PH₃ and H₂O are molecular compounds, but, as was shown in the examples above, PH₃ has no dipole moment, so it will exhibit only London forces, while H₂O does have a dipole moment, and will therefore exhibit dipole-dipole forces. Dipole-dipole forces are stronger than London forces, so PH₃ will melt at a lower temperature than H₂O. NaCl is ionic, and ionic bonds are much stronger than even the hydrogen bonds in water, so NaCl will have the highest melting point.The tendency of the particles of a liquid substance to break free from the surface of the liquid in this manner is measured using a quantity called the equilibrium vapor pressure or just vapor pressure. Vapor pressure is a property of the liquid, not of the vapor. A liquid's vapor pressure is defined as the pressure exerted by the particles of the substance that have evaporated from the liquid when the vapor and liquid are in equilibrium with each other. Equilibrium is reached when the rate at which the liquid evaporates equals the rate at which the vapor condenses; another way to put it is that equilibrium is reached when the number of particles leaving the surface of the liquid to become vapor equals the number of particles of the vapor that are recondensing to become part of the liquid again:

Because vapor pressure is related to the average kinetic energy of the liquid particles, and the average kinetic energy of the particles is related to the temperature of the liquid, the vapor pressure is related to the temperature: as the temperature of the liquid increases, the vapor pressure also increases. As the temperature is increased, the vapor pressure eventually equals the pressure of the atmosphere above the liquid--that is, the tendency of the liquid particles to leave the liquid and become vapor becomes so high, that the vapor being produced can push the atmosphere back. In fact, the liquid then forms bubbles of vapor inside the liquid, instead of just forming vapor at the surface. This is the boiling point of the liquid. HENCE H20 HAVE HIGHER VAPOUR PRESSURE THAN PH3