Lecture Chemistry, The Central Science, 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten

Transcription

1 Chemistry, The Central Science, 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten Lecture 1101 John D. Bookstaver St. Charles Community College Cottleville, MO

2 Molecular Comparison of Gases, Liquids, and Solids Temperature and Determine the Physical State of a Substance

3 States of Matter The fundamental difference between states of matter is the distance between particles.

4 States of Matter Vibration, Rotation, Translation

5 States of Matter Because in the solid and liquid states particles are closer together, we refer to them as condensed phases.

6 States of Matter Since they flow, we refer to gases and liquids collectively as fluids.

7 The States of Matter

8 The States of Matter The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: the kinetic energy of the particles; the strength of the attractions between the particles. The Phases of Matter

9 Ion-Dipole Dipole-Dipole London Dispersion Hydrogen Bonding

10 The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.

11 They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

12 These intermolecular forces as a group are referred to as van der Waals forces.

13 van der Waals Dipole-dipole interactions Hydrogen bonding London dispersion forces

14 Ion-Dipole Interactions Ion-dipole interactions (a fourth type of force), are important in solutions of ions. The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents. Structure of Solutions Odyssey #59

15 Dipole-Dipole Interactions 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 viceversa. These forces are only important when the molecules are close to each other.

16 Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point.

17 Dipole-Dipole Interactions Explain the following. The boiling point of F 2 is C whilst the boiling point of HF is 19.5 C?

18 London Dispersion Why Does/How Can Helium Liquefy? While the electrons in the 1s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom.

19 London Dispersion At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side.

20 London Dispersion Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.

21 London Dispersion London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.

22 London Dispersion These forces are present in all molecules, whether they are polar or nonpolar. The tendency of an electron cloud to distort in this way is called polarizability. Fish are thankful that oxygen is soluble in water!

23 Dipole Induced Dipole

24 Factors Affecting London The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane). This is due to the increased surface area in n-pentane.

25 Factors Affecting London The strength of dispersion forces tends to increase with increased molecular weight. Larger atoms have larger electron clouds which are easier to polarize.

26 How Do We Explain This? The nonpolar series (SnH 4 to CH 4 ) follow the expected trend. The polar series follows the trend from H 2 Te through H 2 S, but water is quite an anomaly.

27 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.

28 Hydrogen Bonding Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.

29 Hydrogen Bonding Ice Odyssey #51

30 Dimer of Acetic Acid

31

32 Exercise 11.1 Of Br 2, Ne, HCl, HBr, and N 2, which is likely to have (a) the largest intermolecular dispersion forces, (b) the largest dipole dipole attractive forces?

33 Exercise 11.2 In which of the following substances is significant hydrogen bonding possible: methylene chloride (CH 2 Cl 2 ), phosphine (PH 3 ), hydrogen peroxide (HOOH), or acetone (CH 3 COCH 3 )? Why?

34 Exercise 11.3 (a) Identify the intermolecular attractions present in the following substances, and (b) select the substance with the highest boiling point: CH 3 CH 3, CH 3 OH, and CH 3 CH 2 OH.