solutions and intermolecular forces

Intermolecular Forces

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 209 1-7 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

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.

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.

Chemical Bonds

• Three basic types of bonds: – Ionic

• Electrostatic attraction between ions

– Covalent • Sharing of electrons

– Metallic • Metal atoms bonded to

several other atoms

REVIEW

Bond types compared Bond type Atoms involved Electrons role Bond strength characteristics

Ionic Metal and non metals

Transferred electrons create ionic charge attraction

Strong bond due to electrostatic attraction

Always a solid crystal structure High melting /boiling points Brittle

Covalent Non metals Electrons shared equally (non polar) or unequally (polar)

Fairly strong bond due to nuclei both sharing electrons

Can be a solid liquid or gas Lower melting /boiling points, squishier Non conductor

Metallic Metals only Delocalized electron cloud

Fairly weak bonding by itself

Solid , Malleable and ductile, shiny , conducts electricity

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)

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.

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

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.

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.


• When two atoms share electrons unequally, a bond dipole results.

• The dipole moment is the overall combination of the bond dipoles in a molecule

REVIEW

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)

Bond type by electronegativity

Electronegativity difference

• 0

• Between 0 and 1.9

• >2.0

Bond type

• Non polar covalent

• Polar covalent

• Ionic

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 209 1-7 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


• When two atoms share electrons unequally, a bond dipole results.

• The dipole moment is the overall combination of the bond dipoles in a molecule

REVIEW

area of positive charge area where electrons go negative charge

Polarity

• Just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

REVIEW

Red high electron density Blue low electron density

Polarity

By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

REVIEW

Red high electron density Blue low electron density

Polarity REVIEW

States of Matter

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

REVIEW

States of Matter

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

REVIEW

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

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.

Summarizing Intermolecular Forces

Bond strength for comparable sized molecules

Targeted notes

• Look on pages 203-207 to find info about the terminology you might need to know more about:

– hint dispersion forces are on pg 205 last paragraph

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 dipole-dipole interaction with an H attached to an N, O or F atom

• London dispersion forces- between two nonpolar molecules

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 209 1-7 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

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)



• Homework: work on pg 209 1-7 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

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

5/18/12

• 5/18 - How long did you measure the cooling for in the lab?




• Homework: work on pg 209 1-7 Check on Friday

• organic compounds check on Friday

• Post Lab for Thursday PDB melting point

• Work on IM forces practice sheet Due next Wednesday

5/18/12

• Have lab out you must have labeled diagrams of procedures done




• Homework: work on pg 209 1-7 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

Ion-Dipole Interactions

• The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

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 vice-versa.

– These forces are only important when the molecules are close to each other.

Dipole-Dipole Interactions

The more polar the molecule, the higher is its boiling point.

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.


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.


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.


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


• 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.

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.

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.

Kids on the school yard scenario

• Kids = electrons

• Ice cream van = + side of instantaneous dipole

• The I love math van = - side of instantaneous dipole

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.

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

How Do We Explain This? Boiling point!

• The nonpolar series (SnH4 to CH4) follow the expected trend.

• The polar series follows the trend from H2Te through H2S, but water is quite an anomaly.

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

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.



Is there an H interacting with an N, O or F on another molecule?

solutions and intermolecular forces

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