Question

Rank the following compounds in order of decreasing vapor pressure based on intermolecular forces:

a) CH4
b) isopentane C5H12 (2-methyl butane)
c) CH3CH2CH2CH2OH
d) CH3CH2CH2CH2CH3

you can overlap the choices if they are of the same vapor pressure

-if intermolecular force increases, vapor pressure increases
-intermolecular forces such as london dispersion, dipole dipole, ion dipole, hydrogen bonds

my answer was
(a)<- highest vapor pressure
(b) and (d)
(c)<- lowest vapor pressure

but I got it wrong

what is the correct answer?
thanks

Answer 1

General guidance

Concepts and reason

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase (liquid or solid) in a closed system at a given temperature.

The set of repulsive and attractive forces between molecules that result from the polarity is represented as intermolecular force. There are four types of intermolecular forces.

Fundamentals

Non-covalent interactions:

The intermolecular interactions that exist among the molecules that are not covalent bonding interactions are known as non-covalent interactions.

The four types of intermolecular forces are given below:

\uf0a7London dispersion forces: These are weak forces of attraction that exist in the molecule.

\uf0a7Dipole\u2013dipole interactions: These are electrostatic interactions. The molecule having a permanent dipole interacts with other polar molecules using dipole\u2013dipole interactions.

\uf0a7Hydrogen bonding: If the hydrogen atom is directly attached to the electronegative atom, then it exhibits intermolecular hydrogen bonding.

\uf0a7Ion\u2013dipole interactions: The interactions present between the ions and dipole of solvent are known as ion\u2013dipole interactions.

Step-by-step

Step 1 of 2

The vapor pressure of the compounds decreases as the number of carbon atoms increases.

The vapor pressure of the compounds increases as the branching increases.

When the strength of the intermolecular force increases, the molecules will come closer. It requires more energy to break down, so more temperature is needed to boil.

So, the strength of intermolecular force increases with decreasing vapor pressure.

Explanation | Hint for next step

If the strength of the intermolecular force increases, the tendency for a particle escapes from solid or the liquid phase decreases. Therefore, vapor pressure decreases.

Step 2 of 2

a)

Spherical

It has only London dispersion forces.

b) Isopentane (2-methyl butane).

Branched

It has only London dispersion forces.

c)

Linear

It has London dispersion forces, dipole\u2013dipole interactions, and hydrogen bonding.

d)

Linear

It has only London dispersion forces.

The given compounds are in order of decreasing vapor pressure based on intermolecular forces:

a) > b) > d) > c)

Methane has less intermolecular forces compared to other molecules. So, it has the highest vapor pressure.

The branched one has more vapor pressure than that of the linear one. So, it should be placed before linear one in order of decreasing vapor pressure.

Alcohol has the strongest intermolecular forces such as London dispersion forces, dipole\u2013dipole interactions and hydrogen bonding. Due to this the vapor pressure of alcohol is low. It should be placed at the last in order of decreasing vapor pressure.

Answer

The given compounds are in order of decreasing vapor pressure based on intermolecular forces:

a) > b) > d) > c)

Answer only

The given compounds are in order of decreasing vapor pressure based on intermolecular forces:

a) > b) > d) > c)