Intermolecular Forces L01 PDF

Intermolecular forces Dr Manal N ALSoub  What determines if a substance is a solid, liquid, or gas? Intermolecular Forces: (inter = between) between molecules 2 Intermolecular forces...

Intermolecular forces Dr Manal N ALSoub  What determines if a substance is a solid, liquid, or gas? Intermolecular Forces: (inter = between) between molecules 2 Intermolecular forces There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces We have already looked at intramolecular bonds (ionic, covalent, polar covalent, metallic) Intermolecular forces (IMF) can be divided into attraction and repulsion forces. The three IMFs are: 1) H-bonding, 2) dipole - dipole, and 3) London forces (in order of decreasing strength)  Intermolecular forces The force is repulsive when the molecules are brought close enough together that the outer charge clouds of the molecules touch, and this causes the molecules to repel each other. The repulsive forces are necessary so that the molecules do not destroy each other  INTERMOLECULAR FORCES The physical properties of melting point, boiling point, vapor pressure, evaporation, viscosity, surface tension, and solubility are related to the strength of Intermolecular forces between molecules. The amount of "stick togetherness" is important in the interpretation of the various properties listed above. In pharmacy 1- Understanding of the properties of gases, liquid, and solids. 2- Understanding of interfacial phenomena. 3- Flocculation of suspensions 4- Stabilization of emulsion 5- Compaction of powders in capsules, and the compression of granules to form tablet.  Intermolecular forces The attractiveforces can be divided into two types: °Cohesive forces: this term is used when like molecules attract each other Adhesive forces: this term is used when different 0 molecules attract each other Attractive forces are divided into two groups: 0 The0 weak forces of attraction are: Van der Waals forces, Ion-dipole forces , and Hydrogen bonds. The strong forces include the Ionic and Cova lent (coordinate type) bonds. 8 Gases: The average kinetic energy of the gas molecules is much larger than the average energy of the attractions between them. Liquids: the intermolecular attractive forces are strong enough to hold the molecules close together, but without much order. Solids: the intermolecular attractive forces are strong enough to lock molecules in place (high order). Are they temperature dependent? 9 10  1. IONIC FORCES The forces holding ions together in ionic solids are electrostatic forces. Opposite charges attract each other. These are the strongest intermolecular forces. Ionic forces hold many ions in a crystal lattice structure. IONIC FORCES  Ion - Ion Interactions Two oppositely-charged particles flying about in a vacuum will be attracted toward each other, and the force becomes stronger and stronger as they approach until eventually they will stick together and a considerable amount of energy will be required to separate them. They form an ion-pair, a new particle which has a positively-charged area and a negatively-charged area. DEFINITION: The formation of an IONIC BOND is the result of the transfer of one or more electrons from a metal onto a non-metal.Metals, with only a few electrons in the outer energy level, tend to lose electrons most readily Ion - Ion Interactions in the Gas Phase  Covalent Bond Electrons are shared between two or more nonmetal atoms in a molecule. There are no ions! Electron clouds of adjacent atoms overlap and merge to form new bonding orbitals (a process called hybridization).  METHANE H H H C H C H H H H atom needs four each atom needs one Carbon shares all 4 of its electrons to complete electron to complete electrons to form 4 single its outer shell its outer shell covalent bonds H H DOT AND CROSS DIAGRAM H C H H C H H H Coordinate Covalent Bond A coordinate covalent bond is a bond formed when both electrons of the bond are donated by one atom: A + :B A coordinate covalent bond is not essentially different form other covalent bonds; it involves the shari ng of a pair of electrons.. between two atoms. Ex: formati \ on of... ·-.... ·-...  Co-ordinate (Dative Bonds) E.g. Ammonium ion  METALLIC BONDING Involves a lattice of positive ions surrounded by delocalised electrons Metal atoms achieve stability by “off-loading” electrons to attain the electronic structure of the nearest noble gas. These electrons join up to form a mobile cloud which prevents the newly-formed positive ions from flying apart due to repulsion between similar charges. Atoms arrange in regular close The outer shell electrons of each atom packed 3-dimensional crystal lattices. leave to join a mobile “cloud” or “sea” of electrons which can roam throughout the metal. The electron cloud binds the newly- formed positive ions together.  Intermolecular interaction Polarity and electronegativity of molecules: In some cases, one of the atoms in bond in a molecule a has the ability to attract shared electrons to itself. covalent Intermolecular interactions include:./ Van der Waals forces. i) Dipole-dipole interaction ( keesom). ii) Dipole - induced dipole interaction ( Debye). iii)Induced dipole - Induced dipole interaction (london)../ lon-diople interactions../ Ion-induced dipole interactions../ Hydrogen bonds.  Relative Strengths of Bond Types Covalent Ionic Intermolecular Forces The stronger the intermolecular interaction, the stronger molecules are held together. When molecules are held together more tightly, we see the evidence when we examine physical properties of the substance  melting and boiling points increase  A. Dipole - Dipole attractions We have seen that molecules can have a separation of charge + – This happens in both ionic and polar bonds (the greater the ΔEN, the greater the dipoles) H Cl Molecules are attracted to each other in a compound by these positive and negative forces + – + – + –  +  –   A- Dipole – Dipole Forces  When a molecule has a permanent dipole moment (molecule is polar), the negative end of one molecule will come closer to the positive end of its neighbor polar molecule creating electrostatic attractions. In order for a molecule to be polar, the atoms must differ in their electronegativity  -  -  - + + +  -  - + +  DIPOLE-DIPOLE FORCES Molecules with larger dipole moments have higher melting and boiling points (hard to break) than those with small dipole moments. Dipole attractions are relatively weak and tend to be liquids or gas at room temperature. 2 3  The surface area of a molecule determines the strength of the van der Waals interactions between Polar molecules have molecules. The larger the surface area, the larger the attractive force dipole-dipole attractions for between molecules, and the stronger the intermolecular forces. one another. HCl----- +HCl- + dipole-dipole attraction 25  B- Dipole – Induced Dipole Forces The electric field generated by a permanent dipole moment gives rise to a dipole moment (induced dipole moment) on a nearby apolar molecule. Introduction – what is instantaneous dipole? Electrons (e-) move at very HIGH speed randomly leads to uneven distribution of e- density Fluctuating & Temporary uneven distribution of e- produce a rapidly varying instantaneous dipole Permanent dipole What is induced dipole? An induced dipole is a temporary dipole that is created due to the influence of neighbouring dipole (which can be a permanent or an instantaneous dipole).  B- Dipole – Induced Dipole Forces - - In the dipole–induced-dipole interaction, the presence of the partial charges of the polar molecule causes a polarization, or distortion, of the electron distribution of the other molecule. As a result of this distortion, the second molecule acquires regions of partial positive and negative charge, and thus it becomes polar. The partial charges so formed behave just like those of a permanently polar molecule and interact favourably with their counterparts in the polar molecule that originally  C- Induced Dipole – Induced Dipole Forces Dispersion Forces Caused by electron motion. Electrons around one molecule momentarily repel electrons a nearby molecule creating a momentary charge difference Canexist between nonpolar molecules as well as polar Weakest intermolecular force but increases as the number of electrons increases  C- Induced Dipole – Induced Dipole Forces Instantaneous dipole-induced dipole He atom 2 produces an instantaneous dipole that induces a dipole in neighbouring He atom 1. 12.2 - Caused by the natural vibrations of the electrons in the bond, there is no permanent dipole moment. - They are the only attractive forces present in nonpolar compounds. - LD force molecules have Low melting and boiling pts - Molecules that have an even electron distribution Are typically 1) single atoms, 2) molecules of the same element 3) symmetrical molecules For example Ne, H2, O2, N2, S8, C3H8, CCl4, CO2  Polar molecules Molecules containing polar bonds are not always polar. Non-polar Polar molecules molecules If the polar bonds are If the polar bonds are arranged arranged symmetrically, asymmetrically, the partial the partial charges cancel out charges do not cancel out and and the molecule is non- the molecule is polar. polar.  Molecules with 3 Atoms Even though the C-O bond is polar, the CO bonds cancel each other out because the molecule is linear the dipole moments are 2 equal and in opposite directions.Therefore CO2 is non-polar. The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the HC dipole vectors are additive and HCN is N polar Electronegative values: H=2.2, C = 2.55, N= 3.04 SO SO2 is a polar molecule because the S-O 2 dipole Moments don’t cancel each other out due to the angle Electronegative values: S=2.58, O = 3.44 Which of the following molecules are polar (have a dipole moment)?H2O, CO2, SO2, and CH4 O S dipole dipole moment polar moment molecule polar molecule H O C O H C HH no dipole moment no dipole nonpolar molecule moment 10.2 nonpolar molecule  Ion–Dipole Interactions Ion–dipole interactions (a fourth type of force) are important in solutions of ions. The strength of these forces is what makes it possible for ionic substances to dissolve in polar solvents. (SOLOUBILITY) This is also important in the use of diuretics. Diuretics increase the volume of urine and remove excess electrolytes and fluid.  An ion-induced dipole attraction is a weak attraction that results when the approach of an ion induces a dipole in a nonpolar molecule by disturbing the arrangement of electrons in the nonpolar species.  Hydrogen bond Hydrogen bonding: cases of very strong dipole-dipole interaction (bonds involving H- F, H-O, and H-N are most important cases). + H-F- --- +H-F- Hydrogen bonding 43 Intermolecular forces and melting/boiling point