Intermolecular Forces of Attraction

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1 Name Unit Title: Covalent Bonding and Nomenclature Text Reference: Pages Date Intermolecular Forces of Attraction Intramolecular vs. Intermolecular So far in our discussion of covalent bonding, we have been discussing intramolecular forces. Intramolecular forces are forces within a molecule that hold atoms together, that is, covalent bonds. Today we are going to look at intermolecular forces. Intermolecular forces are forces between molecules that hold molecules to each other. These intermolecular forces are collectively referred to as van der Waals forces. Van der Waals forces are much weaker than covalent bonds. Importance of Intermolecular Forces The strength of the intermolecular forces can be used to determine whether a covalent compound exists as a solid, liquid, or gas under standard conditions. Solids have the strongest intermolecular forces of attraction between their particles. The intermolecular forces of attraction between the molecules of liquids are not as strong as those found between the particles of a solid. Gases have the weakest intermolecular forces of attraction between their particles. The strength of the intermolecular forces of attraction can also be used to compare boiling and melting points. The more strongly the molecules are attracted to each other, the higher the boiling and melting points. Types of Intermolecular Forces London Dispersion Forces London dispersion forces exist in all covalent molecules, however; they are the most noticeable between nonpolar molecules and the nonbonding atoms of noble gases. London dispersion forces arise from the motion of valence electrons. o From the probability distributions of orbitals, it is concluded that the electrons are evenly distributed around the nucleus. owever, at any one instant, the electron cloud may become distorted as the electrons shift to an unequal distribution. o It is during this instant that a molecule develops a temporary dipole. This temporary dipole introduces a similar response in neighboring molecules, thus producing a short-lived attraction between molecules. o In general the larger the electron cloud, the more likely the molecule is to form temporary dipoles. London forces are the weakest type of intermolecular forces of attraction. 1

2 Dipole-Dipole Forces Dipole-dipole forces of attraction exist between polar molecules. o Polar molecules contain uneven distributions of charge. o The negative dipole of one molecule is attracted to the positive dipole of another molecule. Example of Dipole-Dipole Forces - Cl Cl is a polar molecule. The hydrogen end of the molecule forms the positive dipole because it has the lower electronegativity. The chloride end of the molecule forms the negative dipole because it has the higher electronegativity. The chloride end of the molecule is attracted to the hydrogen end of a neighboring molecule. δ+ δ- δ+ δ- Cl Cl δ- δ+ δ- δ+ Dipole-dipole forces of attraction are stronger than London dispersion forces. ydrogen-bonding ydrogen-bonding is a special type of dipole-dipole force. Since no electrons are shared or transferred, hydrogen bonding is not a chemical bond. ydrogen bonding exists between where the very electronegative elements of nitrogen, oxygen and fluorine are covalently bonded to hydrogen. ydrogen bonding occurs between hydrogen and the unbonded electron pairs of nearby N, O, or F molecules. Examples of hydrogen bonding. o ydrogen bonding occurs in pure substances. The hydrogen bonding is represented by a dotted line. o O F N hydrogen fluoride and ammonia water and ammonia ydrogen bonding is about ten times stronger than ordinary dipole-dipole forces. 2 C l Cl O water ammonia ydrogen bonding can also occur in mixtures. N O Dipole-dipole forces N F F N hydrogen fluoride

3 Identifying the Types of Intermolecular Forces of Attractions The chart below can help you identify the types of intermolecular forces of attraction exhibited by a substance. Reminder: London Dispersion Forces are exhibited by all covalent molecules. You Try It 1. List the intermolecular forces of attraction in order of increasing strength. 2. What type of intermolecular forces of attraction would be exhibited by each of the following substances? Justify your answer. The first one has been done for you. (int: Draw the Lewis Structure for the molecule in order to help you determine the polarity of the molecule.) a. N 3 b. London dispersion forces, dipole-dipole, hydrogen bonding N 3 exhibits London dispersion forces because all covalent molecules exhibit London dispersion forces. N exhibits dipole-dipole forces because it is a polar 3 molecule. N 3 exhibits hydrogen bonding because it is a polar molecule in which hydrogen is bonded to a nitrogen, oxygen, or fluorine atom. In this case hydrogen is bonded to nitrogen. CO 2 3

4 c. I d. Be 2 Comparing Boiling Points Two factors that affect boiling point are the mass of the substance (molar mass) and the strength of the intermolecular forces of attraction. The stronger the intermolecular forces of attraction the higher the boiling point. Examine the table below. Boiling Points of alogens Name Formula Physical State at Molar Mass Boiling Point Room Temperature (g/mol) (K, at 1 atm) fluorine F 2 gas chlorine Cl 2 gas b romine Br 2 liquid iodine I 2 solid What relationship exists between the mass of the halogens and the boiling point? Arrange the halogens 2. in order of increasing intermolecular strength of attraction. Justify your answer. 4

5 The graph below is a plot of the boiling points of the hydrogen compounds in the groups headed by fluorine (F, Cl, Br, and I), oxygen ( 2 O, 2 S, 2 Se, 2 Te), nitrogen (N 3, P 3, As 3, Sb 3 ), and carbon (C 4, Si 4, Ge 4, Sn 4 ). Use the graph below to answer the following questions. 1. Which group of elements has the lowest boiling points for each period? Why do they have the lowest boiling points for each period? 2. Notice in each of the other three groups that the first compound ( 2 O, N 3, and F) in each group has a significantly higher boiling point than the other elements in their groups. What accounts for this phenomenon? 3. With the exception of O, N, and F, why do the boiling points generally 2 3 increase within a group? 5

6 You Try It 1. Determine whether each of the following would more likely be formed by polar or nonpolar molecules. a. a solid at room temperature b. a liquid with a high boiling point c. a gas at room temperature d. a liquid with a low-boiling point 2. Considering what you have learned about forces between atoms and molecules, why do you think all of the elements in group 18 exist as gases at room temperature? 3. Arrange the following according to increasing boiling point: 2 O, 2 S, CO 2. Justify your ranking. 4. Arrange the following according to increasing boiling point: C 4, CI 4, CF 4. Justify your ranking. 5. int. Suggest a F, N 3, and 2 O all exhibit hydrogen bonding. 2 O, however, has stronger intermolecular forces of attraction and therefore a higher boiling po possible explanation as to why 2 O has stronger hydrogen bonding. 6