Honors Chem Block Name POGIL 7 KEY Intermolecular Forces In chemistry we talk a lot about properties of substances, since the object of chemistry is substances and their properties. After learning different types of bonding, you already know that molecular structure of substances and three-dimensional arrangement of particles inside influences their properties. Then how would you explain that water (H2O) has a boiling point of 100 o C and it is a liquid at room temperature while hydrogen sulfide (H2S), which has a similar structure and the same shape of the molecule, has a boiling point of -60 o C and it is a gas at room temperature? Another question: how would you explain that chlorine is a gas at room temperature while iodine is a solid at room temperature, although both of them belong to family of halogens and have similar molecular structure? All these questions can be answered by analyzing intermolecular forces. Intermolecular Forces are forces of attraction or repulsion between molecules. They can only be found in molecular substances (elements or compounds). You will learn different types of intermolecular forces: I Hydrogen bonding; II Dipole dipole forces; III London dispersion forces I Hydrogen Bonding (still intermolecular forces, do not confuse with covalent bonds to hydrogen atoms) In order to understand the hydrogen bonding better, answer a few questions: 1. How would you estimate the dipole moment inside of water molecule? High? Low? What numerical data can you use to support your answer? If we look at the electronegativity values of H and O, it is apparent that the molecule has a high dipole moment due to the fact that the charges are not distributed evenly (oxygen is partially negative and hydrogen is partially positive). The EN difference is large: 3.5-2.1 = 1.4 2. How would you estimate the attraction between water molecules based on the dipole moment? Strong? Weak? The attraction between water molecules should be very strong, since water molecules are very polar. The partial charges are high and close to ionic. Bigger charges => stronger attraction 3. How would you estimate the amount of energy needed for a water molecule to overcome the attraction and separate from other molecules? Large? Small? Explain. The energy needed to separate water molecules should be large due to strong attractions. (This is why water has to be heated to such a high temperature for boiling for example). 1
Hydrogen bonding can be considered an extreme case of dipole dipole attraction forces, since all molecules that make hydrogen bonding are polar. Hydrogen bonding, however, is much stronger attraction than any of the intermolecular forces, including dipole dipole forces. That is why this is the only attraction called bonding. Hydrogen bonding is only possible if a molecule has hydrogen atoms connected to fluorine or oxygen or nitrogen (FON), the most electronegative atoms. Hydrogen bonding is formed when fluorine or oxygen or nitrogen atoms of one molecule attract hydrogen atoms of another molecule. It looks like two extremely electronegative atoms get connected to each other through hydrogen atoms. That is why it is called hydrogen bonding. The pictures below show hydrogen bonding between water molecules. You can see how oxygen atoms from different molecules are pulling hydrogen atoms. However, only one of the oxygen atoms is really bonded with the hydrogen atom, the second one is only strongly attracted to it. Real bonds are always depicted with dashes; hydrogen bonding is depicted with dotted lines. When you take a biology class, you will learn that hydrogen bonding is very important for some biomolecules. For example it connects loops in the secondary structure (helix) of protein molecules, which is very important for their function. It also holds double helix of DNA molecules together. Exercise question Which substances have hydrogen bonding between molecules (circle the formulas)? H-I H-F Now you are ready to explain why water, as opposed to H2S, is a liquid at room temperature with a very high boiling point. Let s break your answers and explanations into small steps. 2
Water is a volatile substance (constantly evaporates, even at low temperatures). Water vapors are always present above the surface of liquid water. If temperature is higher, more vapors there are above water, if the temperature is lower, less vapors are present. Vapors create pressure (molecules collide with each other). More vapors more pressure (molecules collide more often). It is true about any volatile liquid. 4. Think about vapors above water at room temperature. Do you expect vapor pressure to be high or low (hint, consider intermolecular attractions)? Explain. The vapor pressure above water surface should be low. Water has strong dipole-dipole forces and hydrogen bonds between molecules. Due to these strong attractions, it takes a lot of energy to phase change liquid water to vapor (separating the molecules). Another way to think about it is that with water, the kinetic energy needed for the molecules to break the IM (intermolecular) attractions and escape the liquid (vaporize) is relatively high. When you heat liquid, it generates vapors and boiling only starts when the vapor pressure becomes high enough to be equal to the atmospheric pressure (air pressure). Then the atmospheric pressure is not pushing on the surface of liquid strong enough to keep molecules down, and they can take off and fly easily. Liquid starts boiling. 5. Considering your answer to question #4 and the description of boiling, how would you explain the high boiling point of water? Strong attractions => a lot of energy needed to separate molecules => high boiling point. Water has low vapor pressure at a room temperature, so it needs a lot of energy to generate enough vapors before its vapor pressure equals the atmospheric pressure and it starts boiling. Exercise questions: a) Which substances have higher vapor pressure above its liquid phase? Explain. PH3 or NH3 (Acetone) or (Propionic acid) NH3 has hydrogen connected to nitrogen. It means it has hydrogen bonding. Molecules are attracted stronger, and it is not easy for them to take off and fly; so lower vapor pressure. Propionic acid has hydrogen attached to oxygen, so hydrogen bonding, so stronger attraction, and so lower vapor pressure. b) Which substances have higher boiling point? Explain. PH3 or NH3 or If the vapor pressure is low, a substance should be heated to higher temperatures to produce higher vapor pressure and start boiling. 3
II Dipole Dipole Forces Polar covalent molecules can be called "dipoles", meaning that they have two "poles". One end (pole) of the molecule has a partial positive charge while the other end has a partial negative charge. Use your knowledge of electronegativity and VSEPR theory to list all factors that contribute to molecular polarity In order for a molecule to be polar, the shape should not be symmetrical in terms of charge and the atoms on different ends should have different electronegativity. 6. What kind of force will be observed when the positive end of one molecule is brought near the negative end of the neighboring molecule? When oppositely charged ends of molecules approach, they attract each other. It is a Coulombic (electrostatic) attraction. The polar molecules will orientate themselves so that the oppositely charged ends are closer to each other and further from the same-charge ends. The diagram below depicts forces between polar molecules. Exercise question: chemwiki.ucdavis.edu Which of the following molecules are able to have dipole dipole forces of attraction? Draw Lewis Dot structures; determine their shape and polarity to answer the question. CO2 SO2 SCl4 CCl4.. :O=C=O: :Cl S Cl: / \ :CL: :CL: Linear, nonpolar bent, polar see-saw, polar tetrahedral, nonpolar 4
III London dispersion forces Substances that consist of nonpolar molecules or atoms still exist in liquid and solid phase. The example is dry ice, which is used to ship & freeze biological samples, transport fruits and vegetables or keep ice cream at a freezing temperature. Dry ice is a solid phase of carbon dioxide gas. Its molecules are nonpolar. This clearly indicates that there must be some forces of attraction between nonpolar molecules. These intermolecular forces are called dispersion forces or London dispersion forces, which are the result of induced dipole moments in nonpolar molecules or atoms. 7. Hypothesize on how is it possible to induce a dipole moment in a nonpolar molecule or atom. Atoms and molecules are in a constant motion. The electrons that are inside of them have inertia (since they are particles) and sometimes fall behind creating asymmetrical distribution of charge. This way an instantaneous or temporary dipole gets created. When this dipole approaches an atom or nonpolar molecule, it induces a dipole moment in it by pushing away or attracting electrons. The instantaneous dipole and induced dipole start attracting each other with very weak forces dispersion forces. Dispersion forces exist between all types of particles: polar or nonpolar. The ease of having electrons in atoms or molecules distributed unevenly is called polarizability. Observe the diagram on the left depicting instantaneous and induced dipoles created in atoms and in nonpolar molecules. chemwiki.ucdavis.edu 8. What factors influence London dispersion forces? Discuss this question with your team and write down some ideas. 5
Factors that influence London dispersion forces First factor: the size of an atom or molecule. Bigger particle has more electrons. It means that some of these electrons are on large distances from the nucleus. This makes them less attracted and more lose. So, they can shift to one side of the atom or molecule easier. Bigger particles have higher polarizability. The dispersion forces are stronger between particles that are more polarizable. One way of deciding if a molecule is big or small is to calculate molecular mass. Bigger molecule has bigger mass. When atoms are being compared, atomic mass or number of energy levels can be used to determine which one is bigger. Second factor: the shape of a molecule. Unbranched molecules can come closer to each other than branched ones. So, the forces of attraction between them are stronger. Exercise questions: 1. Explain why oxygen (O2) is a gas at room temperature, while sulfur (S8) is a solid. Their molecules are nonpolar and they have similar atomic structure. O-atoms have two shells; S-atoms have three. O2 molecule is much smaller than S8. So, S8 has electrons that are far away and that is why it has higher polarizability than O2 ability to become polar. So, London dispersion forces between S8 are stronger and more energy needed to make it melt and boil (higher melting point and boiling point). 2. Explain why butane has a boiling point of 1.0 o C; while isobutane, that has the same number of atoms of the same kind and the same molecular mass has a boiling point of 11.7 o C Butane Isobutane Butane has unbranched molecules whereas isobutene has a branch attached to the chain. So, butane molecules can come closer together, the London dispersion forces are stronger, and it is harder to separate them. More energy is needed, so higher boiling point. 6