Lecture 1 CH101 Physical Chemistry PDF

LECTURE 1 Intermolecular forces Intramolecular force Intermolecular force  Intermolecular forces Intermolecular forces are defined as “the set of attractive and repulsive forces that occur between the molecules as a result of the polarity of the...

LECTURE 1 Intermolecular forces Intramolecular force Intermolecular force  Intermolecular forces Intermolecular forces are defined as “the set of attractive and repulsive forces that occur between the molecules as a result of the polarity of the molecules”. When two or more atoms are joined by chemical bonds they form a molecule. Electrons travel up to the new molecule and are concentrated in the most electronegativity atom area.  Electronegativity is a property that have the atoms or molecules to attract electrons. The concentration of electrons in a defined area of the molecule creates a negative charge -δ, while the absence of electrons creates a positive charge +δ. The binding energies generated by the intermolecular forces are smaller than the energies generated in the chemical bonds. Van der Waals ------ 0.1 to 10 kJ / mol Covalent bond ------ 250 - 400 kJ / mol.  Intermolecular Forces Intramolecular Forces (Chemical bonds) Intermolecular forces depend on Chemical bonds do not depend the temperature, an increase of the temperature produces a so much about the temperature. decrease of the intermolecular forces. They are weaker than chemical They are stronger than bonds, order of 100 times lower intermolecular forces The bond distance is at the level The bond distance is very small, of microns in terms of Angstroms  1. Ion-Dipole Interaction Ion-dipole interaction occurs when an ion attract to a polar molecule. A cation or positive ion (+) would be attracted to the negative part -δ of a molecule and repelled by the positive part. An anion or negative ion (-) would be attracted to the positive part of a molecule and repelled by the negative part +δ. Example: An example of the ion-dipole interaction is the interaction between a Na+ ion and water (H2O)  H H O 2. Ion-induced dipole forces Similar to ion-dipole forces these forces exist between ions and non-polar molecules. The ion induces a dipole in the non-polar molecule leading to a weak, short lived force which holds the compounds together.  These forces are found in haemoglobin (the molecule that carries oxygen around your body). Haemoglobin has Fe2+ions. Oxygen (O2) is attracted to these ions by ion-induced dipole forces. 3. Dipole - Dipole Forces Dipole-dipole forces act between the molecules of permanent dipole. The polar molecules interact with neighbouring molecules. This interaction is stronger than the London forces but is weaker than ion-dipole interaction because only partial charges are involved. The attractive force decreases with the increase of distance between the dipoles (the interaction energy is inversely proportional to distance between polar molecules). 4. Dipole-Induced Dipole Forces This type of attractive forces operate between the polar molecules having permanent dipole and the non – polar molecules. Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule by deforming its electronic cloud. Thus an induced dipole is developed in the other molecule.  polar molecules non – polar molecule O2 H2O 5. Dispersion Forces or London Forces 1. Suppose we have two atoms (or two non-polar molecules) are electrically symmetrical ‘A’ and ‘B’ in the close vicinity of each other. 2. The electronic charge distribution in one of the atoms, say ‘A’, becomes unsymmetrical i.e., the charge cloud is more on one side than the other. This results in the development of instantaneous dipole on the atom ‘A’. 3. This instantaneous dipole distorts the electron cloud of the other atom ‘B’, which is close to it and as a consequence a dipole is induced in the atom ‘B’.  The temporary dipoles of atom ‘A’ and ‘B’ attract each other. This force of attraction was first proposed by the German physicist Fritz London.  For molecules with the same relative polarizability, the forces scale with molar mass: Higher Mwt. larger induced dipoles so higher boiling point Molecule BP (oC) B CH4 (methane) - 161.5 C2H6 (ethane) -88.6 C3H8 (propane) - 42.1 C4H10 (butane) - 0.5 Hydrogen Bond: A particularly strong example of dipole- dipole. Two important conditions must be met for hydrogen bonding to occur: 1. One molecule has a hydrogen atom attached by a covalent bond to an atom of N, O, or F.. H is very small and O N and F are very electronegative. 2. The other molecule has an N, O, or F atom.