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Chemistry 11AP Notes Fall 2013 Ms Butler Intermolecular Forces Intermolecular forces- forces of attraction and repulsion between molecules that hold molecules, ions, and atoms together. These depend on the distance between the atoms/molecules – they will grow weaker as the distance between particles increases, and stronger the closer the particles are to one another Intramolecular - forces of chemical bonds within a molecule Observable Properties Solids keep their shape and volume and are virtually incompressible.They often have crystalline shapes. Liquids retain their volume when placed into a container, but conform to the shape of the container. They are fluid and are able to flow. Liquids are nearly incompressible. Gases are easily compressed, but expand spontaneously to fit whatever container they are in. Molecules are tightly packed and highly ordered. Very strong attractions between molecules hold them in place so they are effectively locked in position. Molecules lightly packed but with little order. They are able to move past each other with little difficulty. Intermolecular attractive forces are relatively strong. Widely spaced molecules with much empty space between them. Motion is random and with very weak attractions between the molecules. Boiling Point and Electronegativity Boiling Point - the temperature at which the liquid form of a compound vaporizes into a gas. - the attractive forces in the liquid must be overcome to become a gas. - boiling point of 0 = no attractive force Trends: Increasing attractive forces Compound NaCl H2O BrCl Ar Boiling Point 1413oC 100oC 5oC -186oC Increasing boiling point Stronger intermolecular forces result in higher boiling points. The dipole moments increase with the polarity of a bond (the greater the difference in EN between the atoms, the stronger the dipole-dipole attractions.) The strength of London forces increases with the number of electrons. Chemistry 11AP Notes Fall 2013 Ms Butler Electronegativity: - The attractive force that an atom exerts on an electron - Closer e- the nucleus = more electronegative - More protons, more positive, more attractive to eElectronegative Difference: - Ionic Bonds >1.7 - Polar Covalent Bonds 0.5-1.7 - Slightly polar covalent above 0 to 0.5 - Non-polar or pure covalent – 0 - Electrostatic Attraction (common in every non-covalent molecular force) = when opposite charges with permanent dipoles attract bonding IONIC Ionic Bond (strongest bond) a complete transfer of one or more e- occurs when one interacting atom is much more electronegative than the other. o one gives up e- (cation) and one takes e- (anion) making ions that are electrostatically attracted. It takes a lot of energy to break ionic bonds and to turn the molecule into a gas Cl Na EN 0.9 3.0 ∆EN = 2.1 Ionic Bond Strength: strongest bond Trend: The greater the charge difference, the stronger the attraction DIPOLE-DIPOLE - sharing of electrons in a bond is covalent but not completely even (Polar Covalent) ∆EN = 3.0-2.8 = 0.2 Slightly Polar Br Cl Cl Br Chemistry 11AP Notes Fall 2013 Ms Butler Trends: Electronegative Difference. Polarity is amplified when electronegativity difference is large and minimized when the difference is small. Bond Length (primarily atomic radii of the bonding partners) Increasing bond length decreases dipole HYDROGEN BONDING A dipole-dipole interaction where the H atom is non-covalently attracted to an electronegative atom The H must have a large positive delta and the other atom must have a source of electrons to be attracted to H Usually negatively charged F, O, N is attracted to the positively charged H Very important biologically Strength: a H bond is a weak bond and is dynamic In solid phase H-bond is static H-bonds between molecules continually form and break in liquid. Every water molecule forms 3.4 H bonds with other water molecules Trend: The more electronegative the atom attracted to H, the stronger the bond LONDON DISPERSION FORCES London Dispersion Forces/ Van der Waals Forces The attraction between instantaneous electric dipoles on adjacent molecules Present in all molecules because everything has an electron cloud Strength: Weak because temporary Trends: Strength increases with # of electrons in molecule strength depends on the polarizability [ease with which their electron clouds can be distorted] Molecules with lesser EN and larger radii have higher polarizability. Higher polarizability = stronger attraction. Chemistry 11AP Notes Fall 2013 Ms Butler Summary of Intermolecular Attractions Type of Substance that Exhibit Intermolecular Attraction Attraction Dipole-Dipole attractions Hydrogen bonding London dispersion forces Ion-dipole attractions Ion-induced dipole attractions Occurs between molecules that have permanent dipoles (i.e. polar molecules) Occurs when molecules contain N-H, O-H or F-H bonds All atoms, molecules, and ions experience this type of attraction. They are present in all substances (and are the only type of force in non-polar molecules). Occurs when ions interact with polar molecules Occurs when an ion creates a dipole in a neighbouring particle, which may be another molecule or ion. Relative Strength to a Covalent Bond 1-5% 5-10% Depends on the sizes and shapes of molecules. For large molecules, the cumulative effect of many weak attractions leads to a stronger net attraction. ≈ 10% depending on the ion charge and polarity of the molecule Variable, depends on the charge on the ion and the polarizability of its neighbour Physical Properties Affected by Intermolecular Forces - - Viscosity: is the resistance of a liquid to flow. Viscosity increases with stronger intermolecular forces, and decreases with increasing temperature. Surface Tension: Water will bead up on a waxed surface. This is due to an “imbalance” of intermolecular forces at the surface of the liquid. In the interior of the liquid, the molecules are surrounded by other, similar molecules, but at the surface they are only attracted to the sides and inward. This works to reduce the surface area, attempting to create the shape with the minimal surface area, a sphere. The surface tension is the amount of energy required to stretch or increase the surface area. Molecules with high IMF also have high surface tensions Vapor Pressure: At any temperature, some molecules in a liquid have enough energy to escape (become a gas/vapour). As the temperature rises, the fraction of molecules that have enough energy to escape increases. Chemistry 11AP Notes Fall 2013 Ms Butler As more molecules escape the liquid, the pressure they exert increases. The liquid and the vapour reach a state of dynamic equilibrium: liquid molecules evaporate and vapour molecules condense at the same rate. • The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. • The normal boiling point is the temperature at which its vapor pressure is 760 torr. • Vapor Pressure is not affected by surface area. • If the intermolecular forces are strong, the vapor pressure will be relatively low. If the forces are low, the vapor pressure will be relatively high. ethyl ether (C4H10O) Pvapor (25oC) = 520 torr ethyl alcohol (C2H6O) Pvapor (25oC) = 75 torr The relatively weak dipole-dipole forces and London dispersion forces between molecules results in a much higher vapor pressure compared to ethyl alcohol. Although dipole-dipole forces and London dispersion forces also exist between ethyl alcohol molecules, the strong hydrogen bonding interactions are responsible for the much lower vapor pressure compared to ethyl ether. - Compressibility: is a measure of a substance’s ability to be forced into a smaller volume. In a liquid and solid, there is very little space in which to crowd other molecules.