Intermolecular forces : Van Der Waals Forces And Hydrogen Bonds
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Intermolecular forces are forces of attraction and repulsion between molecules of matter. A study of these Intermolecular forces has been an important part of development of physical chemistry in the 20-th century. The following are explanations about intermolecular forces dealing with Van Der Waals forces and hydrogen bond.
1. Van Der Waals forces
Because atoms and molecules were surrounded by electrons which electrically charged. Then the moleculs subtance might be in an repulsion of each other, but it never happens. This phenomenon has been explaned by a dutch physicist. Johannes Diderik Van Der Waals. Acording to him, molecules in the subtance does not in repulsion of each other because between the molecule there is a weak attraction force. The force is called Van Der Waals force. Which can hold the molecule to stay in a certain substance.
The occurance of Van Der Waals force was believed because of three force, those are dipole-dipole attraction, induction force, london force or dispertion force.
a. Dipole-dipole atraction
Dipole-dipole attraction occurs between polar molecules. In a polar molecule, one pole of the molecule is positivly charged and the other pole is negativly charged. These two oppositely charged poles from a dipole. The dipole plays role in happening of dipole-dipole attraction between the polar molecule in a substance. The posiive part of each polar molecule atracts the negative part of its neighboring molecules, and the negative part of each polar molecule atracts tha positive part of its neighboring molecules, so dipole-dipole atractions is formed, such as shown the following figure.
The Van Der Waals in polar molecules is produced from the dipole-dipole atractions between the molecule. In this case. The more polar the molecule, the stronger the dipole-dipole atraction between them and the harder it is to separate them. Molecules that are more polar tend to have higer melting points and boiling points than these of molecules that are less polar. Polar molecule which have dipole-dipole atractions, among others are water, methanol and amonia.
b. Induction force
Induction force occurs between a polar molecule and non polar molecule. It is because the polar molecule induces its dipole to the non polar molecule, so the negative charges at the non polar molecule will be acumulated to the certain side of to the non polar molecule near the positive side of polar molecule. This condition can produce a temporary dipole, which produces an atraction between the two molecule called induction force. Consider the following figure ilustrating the occurance of induction force between a polar molecule and a non polar molecul
c. London force
Basically, london force is a weak form of a dipole-dipole attraction. London force occurs when a nonpolar molecules can briefly becomes polar because their electrons are in contstant motion. This motion is usually balanced around a nonpolar molecule. If the molecule is briefly disturbed around a non polar molecule. If the molecule is briefly disturbed, its electrons may accumulate at one part of the moleccule and it produced a positive charge on anther part of the molecule.
Based on the case above, the molecule has a temporary dipole until the electrons rebalance. As long as the molecule has a dipole, the charges can disturb electrons in neighboring molecules, so these neighboring molecules have also a temporary dipole. These temporary dipoles attract each other using a force called london force.
Non polar molecules, such as methane (CH4), and Nitrogen (N2) molecules have only the intermolecular attraction force in the form of london force. These force are very weak, so the non polar substances have low melting and boiling points and usually they are found in the form of gaseous at room temporary.
d. The strenght of Van der Waals Force
Basicalyy, the srength of Van Der Waals forces is influenced by two main factors, these are the complexity of molecular shape and molecular size. The following are the brief explanation about those two factors.
Generally, simple molecules have intermolecular forces which is stronger than that for complex molecules. It is because intermolecular forces, especially Van Der Waals forces acting at a very close distance. The closer intermolecular distance, the stronger the intermolecular attraction. For example, n-butane (C4H10) has the intermoleculer forces which are stronger than thoso for methyl propane (C4H10) indicated by the melting point of n-butane which is larger than that of methyl propane. It is because n-butane has molecular structure which is simpler than that for methyl propane.
Meanwhile, in the relation to the molecular size, large molecules tend to form a temporary dipole which is easter than small molecules. It is because at a molecule which has large size, its elctrones can move freely.
2. Hydrogens bonds
Hydrogen bonds are chemical bonds formed between molecules containing a hydrogen atom bound to a srongly electronegative atom, such as flourine (F), oksigen (O), and Nitrogen (N). Because the electronegative atoms pull the electron from the Hydrogen atom, the atoms form a very polar molecule. Meaning one side is negativly charged and the other side is positivly charged. Hydrogen bonds form between these molecules because the negative sides of the molecules are atracted to the positive sides of other molecules, and vice versa. These hydrogen bonds are stronger than the bonds formed by Van Der Waals force. Hydrogen bonds play an important role in influencing the compound properties. Pay attention to the following explanation.
In everyday life we find water in liquid phase, whereas, the relative molecular mass of water is relativly small ( Mr = 18 gram/mole ) and between those water molecules there are Van Der Waals forces which are very weak. Thus, if at those water molecules there are only the Van Der Waals force, theoritically at very low temperature (about – 75 C). Water will be found in liquid phase at normal temperature in the earth is caused by the presence of hydrogen bonds between those water molecules. These hydrogen bonds can be described as follows.