General Chemistry I Reginald Stanton, PhD Intermolecular Bonding Sodium sticks to chlorine in a sample of table salt, NaCl(s) or NaCl(l), because sodium in the sample is sodium ion, Na +, and chlorine in the sample is chloride ion, Cl -, and the positively charged cation attracts negatively charged anions. Hydrogen sticks to oxygen in water, H 2 O, because negatively charged electrons of each atom is attracted to the positively charged protons in the nucleus of the other atom. Ionic Bonding The first example is of ionic bonding. The bonds between sodium ion and chloride ion in NaCl(s) or NaCl(l) are ionic bonds. Cations and anions are ions, substances with whole units (whole numbers) of positive or negative charge. Ionic bonds are attractions between cations (ions with whole positive charge) and anions (ions with whole negative charge). Any compound consisting entirely of cations and anions is called an ionic compound. The bonds between anions and cations in an ionic compound are ionic bonds. Because the charge of ions is stable and large, ionic bonds are strong! It takes a lot of energy to break an ionic bond: NaCl(s) -> Na + (g) + Cl - (g) )H ~ 756kJ/mol Covalent Bonding The second example is of covalent bonding. The bonds between oxygen and hydrogen in water are covalent bonds. An electron from both hydrogen and oxygen is attracted to the nucleus of the other atom and the end result is the two electrons are shared by each atom. A covalent bond is formed when electrons from two atoms are attracted to the nuclei of the other atom. Any substance held together entirely by covalent bonds is a covalent substance called a molecule. Covalent bonds are also stable and very strong: H-O-H -> H@ + @O-H )H ~ 460kJ/mol Neither of these types of bonding is the focus of this discussion. Ionic nor covalent bonds are responsible for band-aid sticking, paint sticking, ink sticking, dirt sticking or other such sticking. The type of bonding discussed below, is a bonding between units of substance in a non-ionic sample and not between the atoms that make up a single unit of the substance; not ionic or covalent bonds. Such bonds are called intermolecular bonds. Intermolecular bonds are the consequence of permanent partial charges in a molecule, and/or temporary partial charges in a molecule, and/or induced partial charges in a molecule; the consequence of partial charges. Water molecules in a sample of water, H 2 O(s) or H 2 O(g), stick together because the hydrogens of one water molecule have a partial positive charge and are attracted to oxygen s of other water molecules because all of the oxygen s have a partial negative charge. Butter in a sample of butter have lipid molecules that stick together because all of the lipids have fluctuating, temporary partial positive and negative charges and these charges induce temporary charges in neighboring molecules.
I. INTERMOLECULAR BONDS/INTERACTIONS The three main types of intermolecular bonds are: Dipole-dipole Dipole-ion Dispersion Bonds between oppositely charged atoms of permanent dipoles; like two water molecules. The charges of dipoles are partial units of charge. Hydrogen ion has a +1 charge. The hydrogen atoms of water also have a positive charge, but the charge is partial, it is much less than +1. Partial charges are represented by a lowercase delta; *. *- is a partial negative charge; *+ is a partial positive charge. Bonds between a dipole and an oppositely charged ion (whether simple or polyatomic ions doesn t matter). Partial charges of a dipole are attracted to whole, opposite charges of an ion. Bonds between units of substances that are the result of temporary and/or induced dipoles. Only temporary partial charges are involved. A. Dipole-Dipole Bonding In some molecules, atoms bound together by covalent bonds are widely different in electronegativity. The consequence of this is that the more electronegative atom will have a stronger pull on the electrons in the bond than the atom with the lower electronegativity. In other words, the more electronegative atom will pull the negative charge closer to its nucleus. Now, since its not completely pulling the electrons away, this collecting of extra negative charge around the more electronegative atom will only result in a space that is partially charged; that atom will have a partial negative charge: *- And since negative charge is partially pulled away from the atom with the lower electronegativity, that atom will have a partial positive charge: *+ So, the molecule will have poles; albeit poles of partial charges. The takeaway here is that 1) the poles are a function of the character of the atoms, it is built in and not a consequence of external influence, and therefore, completely stable (permanent), and 2) the poles are composed of partial, not whole charges, and therefore much more weakly attracting than ionic or covalent bonding interactions. We say such molecules are dipoles. H - O - H *+ *- *+ The water molecule is a dipole. The electronegativity of oxygen is greater than that of hydrogen so electrons in the bond holding the atoms together stay closer to the oxygen than to the hydrogen. The result is partial charges on the oxygen and hydrogen atoms; hence, water is a dipole (it has poles; it has positive and negative ends). The partial charges on a dipole are the result of electronegativity differences between atoms in a unit of substance, hence, they are permanent. They never disappear. What happens when one water molecule comes near another water molecule? The small positive charge on the hydrogens of one become attracted to the small negative charge on the oxygen s of the other. A bond is formed between the two molecules because of the attraction between the oppositely charged atoms of each. This type of bond between two units of substance is called an intermolecular bond; a bond between molecules.
H *+ H *+ \ \ O *- -H *+ CCCCCO *- -H *+ The dashed line (CCCCC) represents the attraction between hydrogen and oxygen. This is that new type of bond. This special intermolecular bond is called a hydrogen bond; h- bond. The bond is NOT ionic and it is NOT covalent! It is, in fact, not intramolecular (bonds between atoms in the same unit of a substance). These bonds are intermolecular bonds (bonds between different units of substance)! These dipoledipole bonds are bonds between two dipoles; two atoms with permanent, partial charges. Hydrogen Bonding One special type of dipole bond is a hydrogen bond. H H-O-HCCCCC O *- *+ H Hydrogen bonds (h-bonds) are due to the weak attraction of dipoles; a hydrogen with a partial positive charge and reasonably electronegative atom with a partial negative charge. A hydrogen bond is a dipole bond where one of the dipoles is a partially, positively charged hydrogen. H-BONDS are stronger than any other intermolecular interactions. This means that the properties of matter regulated by molecular attraction are heavily influenced by H-BONDS, especially for polar compounds. H-bonds are responsible for many of the seemingly strange properties of water: * water expands when it freezes {ordered array produced} * liquid water is denser than ice {array broken} * the BP of water is 200C higher than expected * very high surface tension * very viscose * excellent solvent * highest heat capacity of any liquid The effect of hydrogen bonding in water is illustrated by the relationship between the density and temperature of water. (For the chart on the left, density ranges from 0.955 to 1.000g/mL; temperature ranges from 0 to 100 C) At freezing temperatures, the distance between molecules is essentially fixed. As the temperature increases and some h-bonds break, the distance between the molecules actually decreases. Its like the structure collapses a bit so the density increases. The consequence is the density of ice is less than the density of liquid water; ice floats in water. This physical property of water is a consequence of its intermolecular bonding character.
B. Dipole-Ion Bonding Some samples contain mixtures of ions and polar molecules. In these samples, ions may form intermolecular bonds with the partially charged dipoles. An intermolecular bond between an ion and a dipole is called an ion-dipole (or dipole-ion) bond. Consider salt water. Salt water contains H 2 O molecules whose hydrogen and oxygen atoms are permanently, partially charged, positively charged sodium ions (Na + ), and negatively charged chloride ions (Cl - ): *- H-O-H Na + Cl - *+ *+ The cationic sodium ions are attracted to the oxygen s and the anionic chlorine s are attracted to the hydrogens. Ion-dipole bonds are formed between water and sodium as well as between water and chloride ions. These bonds are stronger than dipole-dipole bonds because the magnitude of charge on the ions is larger than the partial charges of dipoles; their attraction to opposite charge is greater. Notice, like dipole-dipole bonds, ion-dipole bonds are not characterized by the environment. The charges of all the players are permanent and built-in. The introduction of disruptive (or productive) agents into the environment can alter the degree of the interactions, but the interaction exists in the absence of any entities other than the ions and the dipoles in the sample. C. London/Dispersion Forces Finally, we have bonds between units of substance that are not intrinsic. They depend upon the i) composition of the environment and/or ii) the distribution of electrons around an atom. Lets take the first case. i. Induced Dipoles If there are charged units of substance in a sample, they will cause the distribution of charge on other entities to be shifted. This is because electrons are mobile. So, if a charged entity comes close to another unit of substance, it will attract or repel the electrons of that other entity, causing them to be moved. For instance, when a cation get close enough to another entity, it will pull that entity s electrons toward itself. This movement of negative charge creates partial charges on the other entity. These partial charges are only due to slight shifts in the positions of electrons, and so they are weak. Furthermore, everything is moving, so the charge distributions are constantly changing. The process of causing changes to an entity s electron distribution that results in the creation of weak partial charges is called induction. Charged substances induce charges on other substances. And since everything has electrons and protons, induction is ubiquitous. But never lose sight of the fact that bonds between induced entities are weak and fleeting.
ii. Temporary Dipoles Regardless of any influences in the environment, the electrons of everything are constantly, randomly moving. This means that even for nonpolar entities, the negative charge is always on the move. At any moment, the charge distribution will be uneven, asymmetric, and the unit will have partial charges. And as soon as an entity has partial charges, it can bond to things with opposite charges. This explains why even atoms of neon, Ne, an inert Nobel gas, can stick to each other. From moment to moment the electrons are spread unevenly around the nucleus and each atom will have partial charges. Temporary dipoles are dipoles that result from the uneven distribution of electrons in all substances. They only last for an instant and they are very, very weak. However, because everything has moving electrons, they are everywhere, constantly. So even though on a per interaction basis, they are the weakest type, there are so many that they contribute significantly to the physical properties of substances. For example, the only type of bonding you ll find in lard is induced and temporary dipoles. However, lard is a solid at room temperature; its melting point is higher than the melting point of water, even though the dipole bonds between water molecules is much stronger. The rationale is that there are so many more of these dispersion forces than there are hydrogen bonds, that the weak bond numbers are a more significant factor. II. PHYSICAL PROPERTIES AND INTERMOLECULAR BONDING Many of the physical properties of substances are dependant upon how hard or how easy it is to break bonds between units of substance in a sample. Because these bonds are intermolecular in samples of molecules, those physical properties are a function of the character of the intermolecular bonding within the sample. These properties include freezing/melting point, boiling/condensation point, viscosity, and surface tension just to name a few. The basic idea is, the higher the strength of the intermolecular bonds, the larger the magnitude of the physical property. A. Boiling Point Boiling is the process of maximum rate of conversion of a liquid to a gas: H 2 O(l) º H 2 O(g) Boiling/Vaporization Boiling point is the temperature where a sample of substance boils. Well, that s the non-technical definition. The technical definition is the temperature where the vapor pressure is equal to the ambient pressure. At that temperature, we observe what we call boiling; the maximum generation of vapor and bubbles. This temperature is also the point at which we observe maximum condensation of a gas (the reverse process); the condensation point. H 2 O(g) º H 2 O(l) Condensation In order to boil, the bonds between the units of substance in the sample must be broken. The stronger the bonds, the more energy is required to cause boiling (gotta us a hotter fire). Substances with strong intermolecular bonds have a higher boiling point that substances with weaker intermolecular bonds.
H *+ H *+ \ \ O *- -H *+ CCCCCO *- -H *+ H *+ H *+ \ \ S *- -H *+ CCCCCS *- -H *+ Water has a much higher boiling point (100 C) than hydrogen sulfide (-60 C) because the electronegativity difference between oxygen and hydrogen is greater than that between sulfur and hydrogen. The partial charges of H 2 S are smaller than those of H 2 O. The bonds between water molecules is greater; it takes more energy to break them. Hydrogen sulfide has a much higher boiling point (-60 C) than methane (-161.5 C); CH 4. Methane only has dispersion forces and these temporary interactions are much weaker than the dipole bonds between H 2 S molecules. H 2 S has a higher boiling point than CH 4. Boiling is an endergonic process because bonds must be broken; energy must be supplied to boil. It takes more energy to boil a cup of water than it does to boil a cup of hydrogen sulfide. The reverse of boiling is condensation. And, since it is the reverse, condensation is an exergonic process because bonds are formed; energy is given off when samples condense. More energy is given off when water condenses than H 2 S. H 2 O(l) º H 2 O(g) )H ~ 40.7kJ/mol H 2 S(l) º H 2 S(g) )H ~ 20kJ/mol H 2 O(g) º H 2 O(l) )H ~ -40.7kJ/mol H 2 S(g) º H 2 S(l) )H ~ -20kJ/mol B. Melting Point Melting is the process of maximum rate of conversion of a solid to a liquid: H 2 O(s) º H 2 O(l) Melting Melting point is the temperature where a sample of substance melts. It is the temperature of the maximum rate of conversion of a solid to a liquid. This temperature is also the point at which we observe maximum freezing of a liquid (the reverse process); the freezing point. H 2 O(l) º H 2 O(s) Freezing Again, when we look at melting, the bonds between the units of substance in the sample must be broken. The stronger the bonds, the more energy is required to cause melting. Substances with strong intermolecular bonds have a higher melting point that substances with weaker intermolecular bonds. Melting is an endergonic process. The reverse of melting is freezing; an exergonic process. H 2 O(l) º H 2 O(g) )H ~ 6.0kJ/mol H 2 S(l) º H 2 S(g) )H ~ 2.4kJ/mol H 2 O(g) º H 2 O(l) )H ~ -6.0kJ/mol H 2 S(g) º H 2 S(l) )H ~ -2.4kJ/mol
C. Solubility in Water When substances dissolve in water, an aqueous solution is produced; X(aq). Whether or not a substance dissolves in water depends upon whether water can form stronger and/or more bonds with units of the substance than the substance can between units of itself and vice versa. A substance that can t dissolve in water is classified as insoluble. A substance that can dissolve in water is classified as soluble. If the intermolecular bonds between water and the substance are weaker than the bonds between water molecules, the substance will not dissolve in water because it can t separate water molecules; it will be classified insoluble, incapable of dissolving in water. If the intermolecular bonds between water and the substance are weaker than the bonds between substance molecules, the substance will not dissolve in water because the water can t separate molecules of the substance; it will be classified insoluble. Table salt, NaCl(s), is soluble in water. Na + and Cl - make strong ion-dipole bonds with water and a sample of NaCl(s) is completely dissociated into Na + (aq) and Cl - (aq) in water. Olive oil is insoluble in water. The weak dispersion force bonds formed between oil and water are much weaker than the hydrogen bonds between water molecules so the oil can t break up bound water molecules. CaCO 3 (s) is insoluble in water. The ionic bonds between Ca 2+ and CO 3 2- ions are too strong for water to break. Note that oil will dissolve in gasoline or fingernail polish remover (acetone); all of those substances consist almost entirely of dispersion forces. That s because the bonds formed between the oil and the gasoline are on par in strength with those between oil molecules or between gasoline molecules. Bonds between gas molecules are much weaker than those between water molecules; oil can break gas molecule bonds. A good general rule of thumb regarding solubility is like dissolves like. Polar substances dissolve polar substances, and nonpolar substances dissolve nonpolar substances. Now you should understand why. Amphipathic molecules are large organic compounds that have polar and nonpolar components. Examples include substances like fatty acids, soaps, and detergents. Amphipathic molecules have the ability to act as solvent for both nonpolar and polar substances. Detergents dissolves in the water of a wash solution because of their hydrophilic HEAD; they removes oils from dirty fabric because of their hydrophobic TAIL. D. Glue Character Many substances can serve as glue to hold other materials together. The reason they can do this is because the form strong bonds with the materials. Some special glues are called epoxys; these compounds are substances mixed together at bond time to actually make covalent or comparable bonds between two or more substances. Such glues are not commonly used as that type of bonding is rarely required. We focus here on the more prevalent common type of glue. Glue like Elmer s glue, silicon resins, even white board marker inks work by forming intermolecular bonds with other material. This is evident because the jobs are often reversible. If you re careful you can pull bound pieces apart. With a simple eraser, you can remove marker ink from a white board. How well such glue works is a function of a) the character of the material s surface and b) the strength of the intermolecular bond between the glue and the materials to be bound. Bottom line as far as bonding strength; the stronger the intermolecular bonds, the more likely the bound material(s) will stay bound.
C. Viscosity The viscosity of a substance is the amount of force required to spread the substance over a meter square area in a second. Viscosity is the resistance of a material to flow. The higher the viscosity, the harder it is for the material to flow. Syrup has a higher viscosity than water. We say syrup is more viscous. Substances that have a larger number and/or stronger intermolecular bonds have a higher viscosity. D. Surface Tension Surface tension is the resistance of the surface of a substance to be deformed. Think about a balloon. If the balloon doesn t have a lot of air, and you set an object on top of the balloon, the surface will sink where the object is sitting. That sinking of the surface if the deformation of the surface. If you add more air to the balloon, and set the same object in the same place, it will not sink down as far. The deformation of the surface is lessened. The same thing happens to the surface of pure substances. For example, if a fly lands on the surface of a water drop, and you look real closely where the fly s feet are on the surface of the water, you ll see that the water s surface is deformed where the fly s feet are. Technically, surface tension is the amount the surface is distorted by a given force. The surface of any given substance has an area. If a forces is applied to the surface, the surface is distorted, and the area is increased. The amount of energy required to increase the surface a given amount of area is the surface tension. Substances that have stronger intermolecular bonds have larger surface tensions; it takes more energy to distort/increase their surface area. III. SUMMARY A. Bonds that hold the atoms in a substance together are called intramolecular bonds (covalent or ionic bonds). Breaking intramolecular bonds results in a change in the identity of the substance and except for the ionic bonds, is not easily reversed. B. Bonds that hold the units of substance in a sample together are called intermolecular bonds (dipolar and dispersion). Breaking intermolecular bonds results in phase changes, not identity changes. The process can be reversed by simply changing the energy in the surroundings. UNIFYING EXAMPLE: A Band-Aid. Band-aids have a sticky coating that sticks to almost any other solid sample. However, if you stick a band-aid to something, you can reverse the sticking simply by applying energy to the band-aid (just pull it off). However, the atoms in the band-aid are held together by covalent bonds. If you rip the band-aid in half, it is not easily put back together!