How boiling points change as the number of carbons change

Jasdeep Bains

How boiling points change as the number of carbons change

Aim:

The aim is to find out how the boiling point changes as the number of carbons in an alcohol increases.

Scientific Knowledge:

Molecules exist as distinct, separate collections of matter. The bonds within a molecule are typically quite strong, such that it's usually necessary to heat a molecule to very high energies before the bonds begin to break. For example, water is stable to decomposition to hydrogen and oxygen up to temperatures well above 500 C. In contrast, the forces between molecules tend to be relatively weak. If we chose an arbitrary scale in which the bonds between atoms within a molecule are set at 100, then the forces between molecules range between 0.001 and 15. In other words it generally takes far less energy to separate molecules from one another than it does to take molecules apart. The forces between molecules are called intermolecular forces.

Intermolecular forces are the forces of attractions that exist between molecules in a compound. These cause the compound to exist in a certain state of matter: solid, liquid, or gas; and affect the melting and boiling points of compounds as well as the solubility of one substance in another. Intermolecular forces are generally much weaker than covalent bonds. As the number of carbons increase the intermolecular force increases. However the more branches there are the weaker the intermolecular forces become. Isomers can also affect intermolecular forces; different physical properties of isomers can make the intermolecular forces stronger. The boiling points are determined by the strength of the intermolecular forces. So the stronger the intermolecular force, the higher the boiling point is.

In alcohols, as the carbons increase then the intermolecular force should also increase. This basically means that the alcohols with the most carbons should have higher boiling points then the alcohols with the least carbons. As you get more and more carbons in an alcohol then the boiling point should increase.

Many molecules contain bonds that fall between the extremes of ionic and covalent bonds. The difference between the electro-negativities of the atoms in these molecules is large enough that the electrons aren't shared equally, and yet small enough that the electrons aren't drawn exclusively to one of the atoms to form positive and negative ions. The bonds in these molecules are said to be polar, because they have positive and negative ends, or poles, and the molecules are often said to have a dipole moment. HCl molecules, for example, have a dipole moment because the hydrogen atom has a slight positive charge and the chlorine atom has a slight negative charge. Because of the force of attraction between oppositely charged particles, there is a small dipole-dipole force of attraction between adjacent HCl molecules. The dipole-dipole interaction in HCl is relatively weak; only 3.3 kJ/mol. (The covalent bonds between the hydrogen and chlorine atoms in HCl are 130 times as strong.) The force of attraction between HCl molecules is so small that hydrogen chloride boils at -85.0oC.

A dipole-dipole force exists between neutral polar molecules

Polar molecules attract one another when the partial positive charge on one molecule is near the partial negative charge on the other molecule
The polar molecules must be in close proximity for the dipole-dipole forces to be significant
Dipole-dipole forces are characteristically weaker than ion-dipole forces
Dipole-dipole forces increase with an increase in the polarity of the molecule

Typically, dipole-dipole and dispersion forces are grouped together and termed van der Waal forces (sometimes the hydrogen bonding forces are also included with this group).

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