Why is the boiling point of carbon tetrachloride higher than chloroform?

Answer 1

Both chloroform and carbon tetrachloride have the same tetrahedral molecular geometry - there are 4 atoms attached to a central carbon atom. In chloroform, there are 3 chlorine atoms and 1 hydrogen atom. In carbon tetrachloride, all 4 atoms surrounding the central carbon are chlorine atoms. So the difference between the two is simply replacing the 1 hydrogen atom with another chlorine atom. In essence, by doing this, the density of the compound is increased, due to the increase in mass (remember a chlorine atom has an Atomic Mass of 35 and hydrogen an atomic mass of 1). The density of chloroform goes from 1.48 g/mL to 1.58 g/mL when you replace chloroform's hydrogen with that chlorine atom. Since there is an increased mass in a given volume (1 mL), it takes just a little more energy (thermal) to get carbon tetrachloride atoms from the liquid state to the gas state, which is why CCl4 has a bp of around 76 (while chloroform's bp was around 62). -------------------------------------------------------------------------------------------------------------- Boiling points are based on intermolecular forces. Stronger the forces, lower the vapor pressure, higher the B.pt. Chloroform has mostly dispersion forces and very, very weak dipole-dipole. The reason Carbon tetrachloride has a higher boiling point is because the combined effect of all the dispersion forces are stronger than the intermolecular forces that exist in chloroform. Dispersion forces increase with increasing molecular weight and # of electrons.