States of Matter Differ By Intermolecular Distance The state of a substance at a given temperature and pressure is determined by two factors: Thermal energy of the molecules Intermolecular forces (called Van der walls forces) between molecules
Intermolecular forces are the attractive forces that exist between molecules. These are not the same as intramolecular forces (i.e. bonds) which exist within a molecule. Intermolecular forces are much weaker than intramolecular forces. – Consider water – Water would have to be heated to thousands of degrees to break the O-H covalent bonds. – However, the intermolecular forces can be overcome simply by boiling the water (100 o C) which separates the molecules. Nonetheless, intermolecular forces are very important and dictate many of the important physical characteristics of molecules Intermolecular Forces
Intermolecular Forces: Coulombic Attractions As you recall, ionic compounds are solids at room temperature. There are ion-ion attractions in ionic compounds. The coulombic force that holds ions together is very strong. Coulombic attractions are the strongest of all intermolecular forces. Therefore, all ionic compounds have very high melting/boiling points, and are solid at room temperature.
Dipole-Dipole Forces Polar molecules attract one another. This type of intermolecular force is called dipole-dipole attraction. δ + δ - δ + δ - Covalent bond: Very Strong Dipole-dipole interaction: Polar molecules will orient themselves in a way to maximize these attractions. The strength of these attractions increases with increasing polarity. Polar molecules have higher melting/boiling points than non polar ones.
Intermolecular Forces: Dipole-Dipole Forces The magnitude of the melting/boiling temperatures of various substances reflect how strongly the molecules attract one another. The more strongly the molecules attract, the harder it is to separate them. Hence, the higher the melting/boiling temperatures. Recall polarity from chapter 8. Any molecule with a net dipole is polar. H Cl δ - δ + Partial positive character Partial negative character
Ion-Dipole Interactions When salts are dissolved in water, the water dipoles are oriented around the ions. This strong interaction explains why adding salt to water raises its boiling temperature, or why salting snow causes it to melt. SolutionBoiling Temp. ( o C)Boiling Temp. ( o F) Pure Water Satd. NaCl (aq)
Hydrogen Bonding There is a special, unusual, very strong type of dipole-dipole interaction known as hydrogen bonding Hydrogen bonding is a dipole interaction that exists only between the H atom in an H—F, H—O, or H—N bond on one molecule and an adjacent lone pair on another F, O, or N atom in another molecule Because hydrogen atoms are so small, the large partial positive charge induced on H when bonding with these highly electronegative elements is highly concentrated. Therefore, it strongly attracts neighboring N, O, and F atoms, and can approach them very closely.
O HH δ+δ+ δ+δ+ δ-δ- O HH δ+δ+ δ+δ+ δ-δ- O HH δ+δ+ δ+δ+ δ-δ- O HH δ+δ+ δ+δ+ δ-δ- O HH δ+δ+ δ+δ+ δ-δ-
Hydrogen Bonding Causes Abnormalities in Boiling Point Trend Predicted Boiling Temp of Water (-90 o C) Actual Boiling Temp of Water (100 o C) Without hydrogen bonding, all water would be gas (steam) at normal room temperature!!!
For example, consider ethanol and dimethyl ether (C 2 H 6 O, MW 46 g/mol) ethanol BP: 78.3 o C dimethyl ether BP: o C Hydrogen Bonding The –OH bond in ethanol is susceptible to hydrogen bonding, whereas the C-O-C bond in dimethyl ether is not. The strong attraction between these –OH groups results in the massive difference in boiling point between these substances.
Structure and Density of Ice Water is one of the few compounds that is less dense in its solid phase than its liquid phase. This is due to hydrogen bonding. In liquid water, 80% of the atoms are H-bonded. In ice, 100% are H- bonded. To maximize H-bonding, the water molecules in ice spread out. Therefore, we have the same mass of water, with a larger volume. Since ρ=(mass/volume), ρ decreases.
London Dispersion Forces With nonpolar molecules, there are no dipoles, so we would not expect to see dipole-dipole interactions. Despite this, intermolecular interactions have still been observed. For example, nonpolar gases like Helium can be liquified, but how can this happen? What force brings the He atoms together? Fritz London, a physicist, proposed that the motion of electrons in a nonpolar molecule can create instantaneous dipoles
Lets take a Helium atom. At some moment in time, the electrons are spread out within the atom However, because electrons are constantly moving, electrons can temporarily end up on the same side of the atom, creating an instantaneous dipole. e-e- e-e e-e- e-e- δ-δ- δ+δ London Dispersion Forces
These dipoles induce other dipoles on neighboring atoms, the cycle repeats indefinitely. This leads to the formation of a condensed phase. e-e- e-e- δ-δ- δ+δ e-e- e-e e-e- e-e e-e- e-e- δ-δ- δ+δ e-e- e-e- δ-δ- δ+δ e-e- e-e- δ-δ- δ+δ London Dispersion Forces repel attract
London Dispersions The ease of the electron distortion is called polarizability. Bigger molecules are more polarizable, so they are more prone to instantaneous dipoles. Hence, London dispersion forces increase with increasing molar mass because heavier atoms/molecules have more electrons, and are therefore are more polarizable. All substances have dispersion forces. C 12 H 26 C 18 H 38 C 5 H 12
Boiling Points Increase With Increasing Strength of London Dispersion Forces
Show to the right are two isomers of pentane, n-pentane and neopentane. Both have the formula C 5 H 12 (same mass), but the boiling points are very different. Why? The long, straight shape of n- pentane allows for more atoms to participate in LD interaction, as compare to the compact neopentane molecule. n-pentane BP: 36.1 o C neopentane BP: 9 o C Other Things to Consider
Like Substances Mix Polar substances are soluble in polar substances. Nonpolar substances are soluble in nonpolar substances. Polar and nonpolar substances DO NOT MIX (ex. oil and water) E.G. Oil and water; Dipole-dipole and H-bond forces between water molecules are MUCH stronger than the London dispersion forces between oil and water Water molecules would rather associate with other water molecules than to associate with oil. Salts are soluble in water and other highly polar solvents, but insoluble in nonpolar and weakly polar solvents.
Which of the following substances mix? O 2 and CH 3 CH 2 CH 2 CH 3 (butane) SO 2 and CCl 4 H 2 O and CH 3 OH K 2 S and H 2 O H 2 O and CH 3 CH 2 NH 2
Imagine a body of liquid. The liquid molecules in the bulk have attractive intermolecular forces on all sides, pulling in all directions. These interactions minimize the energy of these molecules. Molecules at the surface have less interactions, which makes them less stable due to their higher potential energy. The net force on the surface molecules is inward, and they cluster close together to minimize surface area. Intermolecular Forces At Work: Surface Tension
The surface tension of a liquid is the energy required to increase its surface area by a unit amount (J/cm 2 or mJ/m 2 ). The surface tension of a liquid is what causes its surface to create a “skin” that resists penetration, or assume a droplet shape. Intermolecular Forces At Work: Surface Tension
Viscosity is a measure of a liquid’s resistance to flow and is a temperature dependent property (e.g. Maple syrup is more viscous than water). Stronger intermolecular forces lead to higher viscosity. C.I.R.L.: Viscosity is an important property of motor oil. It must be thick enough to lubricate and protect the engine, but thin enough not to clog the components. Multi-grade oils allow drivers to keep the same oil all year long without a loss of performance. Ex. An oil with a 10W-30 rating has a viscosity of 10 (thin) in the winter and 30 (thick) in the summer. How? Intermolecular Forces At Work: Viscosity
Closely related to surface tension, capillary action is the ability of a liquid to flow against gravity up a tube. This is caused by a combination of intermolecular forces between neighboring liquid molecules (cohesive forces), as well as intermolecular forces between the liquid molecules and the molecules of the surface of the tube (adhesive forces). If the adhesive forces are stronger than the cohesive forces, the liquid rises. Intermolecular Forces At Work: Capillary Action
A consequence of surface tension and capillary action is the formation of a meniscus (the shape formed by a liquid in a tube). In systems with strong adhesion forces, the liquid molecules rise up the sides of the tube, forming a concave meniscus. In systems with strong cohesion forces, the liquid molecules crowd around the interior to maximize their interactions, creating a convex meniscus. Intermolecular Forces At Work: Capillary Action
Summary We can arrange the intermolecular forces by relative strength: 1.Coulombic attraction 2.Ion-dipole/Hydrogen Bonding 3.Dipole-Dipole 4.*London Dispersion It is important to note that molecules with large nonpolar portions and polar end groups will not mix with polar solvents (ex. CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH) because the LD forces dominate. The list above is a general trend. For very large molecules, LD forces can exceed the strength of dipole-based forces.
Example Arrange the following in order of increasing boiling point: – H 2 O, Xe, CH 4, NaCl (aq), NOCl, CH 3 OH, NOBr To do this, we must consider the intermolecular forces that exist between the molecules. Those with the strongest forces between them will be the hardest to separate, and thus, have the highest boiling temperature. – DRAW THE LEWIS STRUCTURES (where applicable) – Separate polar and nonpolar molecules, then sort by strength of attraction CH 4 < Xe < NOCl < NOBr < CH 3 OH < H 2 O < NaCl (aq)