Section 6 Intermolecular Forces PDF

Section 6 Intermolecular Forces 1  Figures and tables in these notes are from the CHEM1500: Chemical Bonding and Organic Chemistry LibreText collection Available at: https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM1 500%3A_Chemical_Bonding_and_Organic_Chemi...

Section 6 Intermolecular Forces 1  Figures and tables in these notes are from the CHEM1500: Chemical Bonding and Organic Chemistry LibreText collection Available at: https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM1 500%3A_Chemical_Bonding_and_Organic_Chemistry/06%3A_Intermolec ular_Forces_and_Liquids_and_Solids The original sources are: Sections Original Source Available from 6.1 – 6E OpenStax General Chemistry https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry _(OpenSTAX) 2 6.1.1 Forces Between Molecules Two major types of forces: 1. Intramolecular forces (bonding forces): Ionic Covalent Metallic 2. Intermolecular forces: Dipole-dipole forces Hydrogen bonding van der Waals forces London dispersion forces Substances with similar types of intermolecular forces dissolve in each other: “like dissolves like” 3 Summarizing Intermolecular Forces 4  Intermolecular Forces (Figure 6.1.4) The attractions between molecules are not nearly as strong as the intramolecular attractions (control chemical properties) that hold compounds together. They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities. These intermolecular forces as a group are referred to as van der Waals forces. 5  6.1.2. Dispersion Forces Dispersion (London) forces: instantaneous dipole induces a dipole in a nearby particle, giving weak intermolecular force (Figure 6.1.5) the ease with which the charge distribution in a molecule can be distorted to induce a momentary dipole is called polarizability dispersion forces: weak but exist between all particles and dominates when other stronger forces are absent; increases with polarizability (ie. ↑ as particles get larger)6 Dispersion Forces cont… In general, as theofvolume The strength of the dispersion electron forces tendscloud increases, to increase with its polarizability increasing size also increases. Larger atoms have larger electron clouds, which are easier to polarize. Table 6.1.1: Melting and Boiling Points of the Halogens Trends in polarizability: 1. down a group? Increases (larger n  larger size) 2. L to R across a period? Decreases (Zeff increases, size decreases) Try the following App for interactions between various atoms except for between two O atoms: https://phet.colorado.edu/en/simulation/atomic-interactions 7 Dispersion Forces cont… The strength of dispersion forces tends to increase with increasing size Larger atoms have larger electron clouds, which are easier to polarize. 6.1.1 8 Dispersion Forces cont… the shapes of molecules also influences the magnitudes of dispersion forces (Figure 6.1.6) The increased surface area in n-pentane leads to higher polarizability and hence stronger dispersion forces. 9 6.1.3 Dipole-Dipole Attractions Molecules that have permanent dipoles are attracted to each other. – The positive end of one is attracted to the negative end of the other and vice-versa. – These forces are only important when the molecules are close to each other. (Figure 6.1.8) 10 6.1.4 Hydrogen Bonding The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds. Hydrogen bonding arises from the high electronegativity of nitrogen, oxygen, and fluorine. (Figure 6.1.9) 11 Hydrogen Bonding cont… In general, an increase in the MW results in an increase in the BP. E.g. follow the trend from H2O 100oC H2Te through H2S, but H2O is quite an anomaly. HF 19.5oC Why? NH3 -33oC We see similar anomalies for HF and NH3 due to H- Bonding. (Figure 6.1.10) 12 Hydrogen Bonding cont… Much weaker than ordinary chemical bonds Very important in biological systems (Figures 6.1.11 and 6.1.12) Also, when hydrogen is bonded to one of those very electronegative elements (F, N, O), the hydrogen nucleus is exposed. 13 Which Have a Greater Effect: Dipole-Dipole Interactions or Dispersion Forces? If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force. If one molecule is much larger than another, dispersion forces tend to determine its physical properties. 14 Example: CH3CN CH3I Polarity 3.9 D 1.62 D Mass 41 a.m.u. 142 a.m.u. b.p. 354.5 K 315.6 K Compare the intermolecular forces at play in these two molecules. Which substance has the greater overall attractive force? CH3CN; smaller mass but greater polarity leads to higher boiling point 15 Another Example: CH3F CCl4 Polarity 1.8 D 0D Mass 34 a.m.u. 152 a.m.u. b.p. -78 oC 76.5 oC Compare the intermolecular forces at play in these two molecules. Which substance has the greater overall attractive force? CCl4; smaller polarity but larger mass. Here it is the dispersion forces which dominate and give CCl4 a much higher bp. 16  Summarizing Intermolecular Forces Examples: ion-dipole hydrogen dipole-dipole dispersion ionic bonding forces bonding forces forces only NaCl, NH4NO3 KBr in H2O H2O, NH3, HF H2S, CH3Cl Ar(l), I2(s) 17 Examples: For each pair, identify the dominant intermolecular forces in each substance and select the substance with the higher boiling point. (a) CH3Br, CH4 CH3Br (dipole dipole versus dispersion) (b) CH3CH2CH2OH, CH3CH2OCH3 CH3CH2CH2OH (hydrogen bonding versus dipole) (c) CH3OH, CH3CH2OH CH3CH2OH (both can H-bond, but more London forces in CH3CH2OH) (d) MgF2, PF3 MgF2 (ionic bonding, crystalline salt) 18  Suggested Exercises 6.E.1 – 6.E.4 6.E.7 6.E.9 6.E.11 – 6.E.19 6.E.21 19