Intermolecular Forces

Transcription

1 Intermolecular Forces

2 5/14/12 Do now: find a piece of metal at your table and describe three properties about it Work on bonding types Homework: work on pg Due Wednesday Blazertime: finish organic compounds if not done will be checked off tomorrow OR work on homework Lab on Thursday PDB melting point lab

3 Current standards we are working on: 20. Compare the atomic radius, electronegativity, ionization energy and/or electron affinity of a set of elements or ions. Demonstrate your knowledge of periodic trends and group characteristics 21. Identify a chemical bond as ionic, covalent or metallic in a compound. Draw Lewis Structures for various compounds, predict the molecular geometry and determine if it is polar or nonpolar. 22. Predict the type of Intermolecular Force between two molecules to be Ion-dipole, dipole-dipole, H-bond or Dispersion. Describe how intermolecular forces, Molecular Weight, and Shape of molecules affect its physical and chemical properties.

4 Probe on a scratch paper for me to turn in What makes a given material have more viscosity (High viscosity = molasses!, low viscosity = water) Explain your thinking.

5 REVIEW Chemical Bonds Three basic types of bonds: Ionic Electrostatic attraction between ions Covalent Sharing of electrons Metallic Metal atoms bonded to several other atoms

6 Bond types compared Bond type Atoms involved Electrons role Bond strength characteristics Ionic Metal and non metals Transferred electrons create ionic charge attraction Covalent Non metals Electrons shared equally (non polar) or unequally (polar) Metallic Metals only Delocalized electron cloud Strong bond due to electrostatic attraction Fairly strong bond due to nuclei both sharing electrons Fairly weak bonding by itself Always a solid crystal structure High melting /boiling points Brittle Can be a solid liquid or gas Lower melting /boiling points, squishier Non conductor Solid, Malleable and ductile, shiny, conducts electricity

7 Bond types compared Bond type Atoms involved Electrons role Bond strength characteristics Ionic Metal and non metals Transferred electrons create ionic charge attraction Covalent Non metals Electrons shared equally (non polar) or unequally (polar) Metallic Metals only Delocalized electron cloud Strong bond due to electrostatic attraction Fairly strong bond due to nuclei both sharing electrons Fairly weak bonding by itself Always a solid crystal structure High melting /boiling points Brittle Can be a solid liquid or gas Lower melting /boiling points, squishier Non conductor Solid, Malleable and ductile, shiny, conducts electricity

8 Why are ionic bonds stable? The main reason is the attraction between ions of unlike charge. This attraction draws ions together releasing energy and causing the ions to form a solid array or lattice. We measure this by measuring the lattice energy(what it takes in terms of energy to rip the lattice apart)

9 Solubility and Ionic compounds Consider what happens when you take an ionic compound (salt) like NaCl and place it in water. Most ionic compounds dissolve into ion form to interact with the polar water molecule.

10 Covalent Bonding In these bonds atoms share electrons. Between nonmetals There are several electrostatic interactions in these bonds: Attractions between electrons and nuclei Repulsions between electrons Repulsions between nuclei

11 Polar Covalent Bonds Although atoms often form compounds by sharing electrons, the electrons are not always shared equally. Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. Therefore, the fluorine end of the molecule has more electron density than the hydrogen end.

12 Electronegativity: The ability of atoms in a molecule to attract electrons to itself. On the periodic chart, electronegativity increases as you go from left to right across a row. from the bottom to the top of a column.

13 REVIEW Polar Covalent Bonds When two atoms share electrons unequally, a bond dipole results. The dipole moment is the overall combination of the bond dipoles in a molecule

14 Metallic bonding Bonding resulting from attraction between metal atoms and the surrounding sea of electrons Atoms are delocalized free to move about any of the metals orbitals Good conductors of electricity(electron flow) Shiny (electrons jump up and fall back down easily emitting the same light frequencies) Pounds flat (maleability) Draws into a wire (ductility)

15 Bond type by electronegativity Electronegativity difference 0 Between 0 and 1.9 >2.0 Bond type Non polar covalent Polar covalent Ionic

16 5/15/12 Do now: 5/15 Draw a lewis dot structure of a polar molecule 5/14find a piece of metal at your table and describe three properties about it Homework: work on pg Due Wednesday Questions today? Introduce molecular forces organic compounds check off today Handout Lab for Thursday PDB melting point lab do standard pre-lab. Make series of labeled diagrams

17 REVIEW Polar Covalent Bonds When two atoms share electrons unequally, a bond dipole results. The dipole moment is the overall combination of the bond dipoles in a molecule area of positive charge area where electrons go negative charge

18 REVIEW Polarity Just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. Red high electron density Blue low electron density

19 REVIEW Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. Red high electron density Blue low electron density

20 REVIEW Polarity

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

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

23 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

24 Intermolecular Forces Inter (between); Intra (within) The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together. They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

25 Summarizing Intermolecular Forces Bond strength for comparable sized molecules

26 Targeted notes Look on pages to find info about the terminology you might need to know more about: hint dispersion forces are on pg 205 last paragraph

27 Types of Intermolecular Forces Ion-dipole interactions- between aqueous ion and dipole (polar molecule) Dipole-dipole interactions- between two polar molecules Hydrogen bonding- unique type of dipoledipole interaction with an H attached to an N, O or F atom London dispersion forces- between two nonpolar molecules

28 5/16/12 Do now: 5/16 What kind of intermolecular forces would be between ethanol and water? (draw lewis structures look for polarity and look at the IM chart I gave you yesterday) 5/15 Draw a lewis dot structure of a polar molecule 5/14 find a piece of metal at your table and describe three properties about it Homework: work on pg Due today organic compounds check off today Lab for Thursday PDB melting point lab do standard pre-lab. Make a series of labeled diagrams Hand out the IM practice problems

29 5/17/12 Have lab out you must have labeled diagrams of procedures done 5/16 What kind of intermolecular forces would be between ethanol and water? (draw lewis structures look for polarity and look at the IM chart I gave you yesterday) 5/15 Draw a lewis dot structure of a polar molecule 5/14 find a piece of metal at your table and describe three properties about it Homework: work on pg Check on Friday organic compounds check on Friday Lab for Thursday PDB melting point lab do standard pre-lab. Make a series of labeled diagrams HAVE LAB OUT WiTH LABELED DIAGRAMS OF PROCEDURES

30 PDB Post lab Make a graph using the directions and color coding Conclusion: just answer questions 1-8 SKIP #7 No conclusion paragraph needed FYI : melting/freezing point of PDB is 53.5 C

31 5/18/12 5/18 - How long did you measure the cooling for in the lab? 5/16 What kind of intermolecular forces would be between ethanol and water? (draw lewis structures look for polarity and look at the IM chart I gave you yesterday) 5/15 Draw a lewis dot structure of a polar molecule 5/14 find a piece of metal at your table and describe three properties about it Homework: work on pg Check on Friday organic compounds check on Friday Post Lab for Thursday PDB melting point Work on IM forces practice sheet Due next Wednesday

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37 Summarizing Intermolecular Forces Bond strength for comparable sized molecules

38 5/18/12 Have lab out you must have labeled diagrams of procedures done 5/16 What kind of intermolecular forces would be between ethanol and water? (draw lewis structures look for polarity and look at the IM chart I gave you yesterday) 5/15 Draw a lewis dot structure of a polar molecule 5/14 find a piece of metal at your table and describe three properties about it Homework: work on pg Check on Friday organic compounds check on Friday Lab for Thursday PDB melting point lab do standard pre-lab. Make a series of labeled diagrams HAVE LAB OUT WiTH LABELED DIAGRAMS OF PROCEDURES

39 Ion-Dipole Interactions The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

40 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.

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

42 London Dispersion Forces 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.

43 London Dispersion Forces 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.

44 London Dispersion Forces 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.

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

46 London Dispersion Forces 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.

47 Factors Affecting London Forces 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.

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

49 Kids on the school yard scenario Kids = electrons Ice cream van = + side of instantaneous dipole The I love math van = - side of instantaneous dipole

50 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 will likely determine its physical properties.

51 Which Have a Greater Effect: Dipole-Dipole Interactions or Ion-Dipole? If two molecules are of comparable size and shape, Ion-dipole interactions will likely be the dominating force. An ion has a full charge while a dipole polar molecule has a partial charge

52 How Do We Explain This? Boiling point! 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.

53 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.

54 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.

55 Is there an H interacting with an N, O or F on another molecule? Summarizing Intermolecular Forces Bond strength for comparable sized molecules

56 Is there an H interacting with an N, O or F on another molecule? Summarizing Intermolecular Forces Bond strength for comparable sized molecules