To the intermolecular forces are a topic in Chemistry that, of course, can appear on the student’s exam. National High School Examination (And either). Therefore, it is worth checking out the main contents of this subject in this article.
Topics of this article
Definition
intermolecular forces are forces – of varying intensities (weak, medium or strong) – of electrostatic attraction or bonds established between the molecules of a substance that maintain the union between them.
Types of Intermolecular Forces
a) Induced dipole
And the intermolecular force that occurs between molecules of a nonpolar substance. It happens, for example, between molecules of substances such as O2, H2, N2, CO2, CH4.
The union between these molecules occurs when a dipole is created between them. This happens when the electrons of one molecule displace the electrons of another, thus creating a negative and a positive pole, which is transferred from one molecule to another.
Representation of dipole formation in nonpolar molecules
Between these molecules, two poles were formed, and the negative pole of one interacts with the positive pole of the other. As these poles were created, it is a low-intensity intermolecular force.
b) permanent dipole
And the intermolecular force that occurs between molecules of a substance with polar characteristics. Examples are molecules of substances such as HCN, H2O, NH3, CO, CH3Cl.
The union between these molecules happens when the negative pole of one interacts with the positive pole of the other.
Representation of the permanent dipole force between HCl molecules
As the poles already exist, the permanent dipole intermolecular force is stronger than the induced dipole.
c) hydrogen bonds
That intermolecular force occurs between polar molecules that have a hydrogen atom directly bonded to an oxygen, nitrogen, or fluorine atom.
Representation of hydrogen bonds between water molecules
A hydrogen bond is formed when the hydrogen of one molecule interacts with the oxygen, nitrogen, or fluorine of another molecule.
d) dipole ion
This intermolecular force occurs between a polar molecule, which has a nonbonding pair of electrons, and an ion (cation or anion) in solution.
The water molecule, for example, has two pairs of non-bonding electrons on oxygen. If there are ions in the aqueous medium, they approach the water molecule, since there are poles in it.
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Importance
The importance of knowing the intermolecular forces for the enem it is the understanding they bring in relation to the physical states, melting point, boiling point and the solubility of substances.
a) Relation to the melting and boiling point of the substance
The melting point indicates the temperature at which it changes from a solid to a liquid state, and the boiling point indicates the temperature at which it changes from a liquid to a gaseous state.
This passage is directly related to the interaction between the molecules of the substance, because what differentiates one physical state from the other is the level of aggregation between its molecules.
Thus, the more intense the intermolecular force, the higher the melting and boiling points. The less intense the intermolecular force, the lower the melting and boiling points. Thus, we can define the decreasing order of melting and boiling points:
Hydrogen bond > permanent dipole > induced dipole
b) Relationship with solubility
In general, we should know that like dissolves like, that is, polar substance dissolves polar substance and non-polar substance dissolves non-polar substance.
However, there is still the possibility of interaction between solute molecules and solvent molecules. This only occurs if this new interaction is greater than the one that already exists between the molecules of the substance itself.
Example of application of intermolecular force in Enem
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1st Example: (Enem-2011) Human skin, when well hydrated, acquires good elasticity and a soft and smooth appearance. On the other hand, when it is dry, it loses its elasticity and appears dull and rough. To avoid dryness of the skin, it is necessary, whenever possible, to use humectant moisturizers, usually made with glycerin and polyethylene glycol:
The retention of water on the surface of the skin promoted by moisturizers is a consequence of the interaction of the hydroxyl groups of the humectant agents with the moisture contained in the environment through:
a) ionic bonds
b) London forces
c) covalent bonds
d) dipole-dipole forces
i) hydrogen bonds
The answer to this question is hydrogen bonding, as the water molecule has hydrogen bonded to an oxygen atom. The same happens with propylene glycol and polyethylene glycol molecules, a favorable condition for the occurrence of this type of intermolecular force.
By Me. Diogo Lopes Dias