Lecture10, 10-20-06 Hydrogen Bonding, Liquids

8/8/2019 Lecture10, 10-20-06 Hydrogen Bonding, Liquids

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First Mid-term ExamWed, October 25

Locations for exam:Chem 1171: Most students, exceptions belowPSBN 4606: Students whose last name begins

with the letters A-C If you are unfamiliar with the room location,

make sure you locate the room prior to theexam time

Come 5 minutes early and relax prior to the

exam.


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Coverage of Material Mid-TermExam 1, 10-25-06

Chapter 13, Bonding Concepts

Chapter 14, Covalent Bonding, Orbitals Chapter 16, Liquids and Solids

Sections 16.1 16.7, pp 765-801Lecture notes for today and Monday will be posted

this afternoon.

Information sources used for exam will be1) Lectures2) Text Reading Material

3) Assigned Homework Problems


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CLAS Review Session I got back my room request, and the time was changed

to 6:30-9:30 (though again, it won't necessarily take that long). The

day is still the same; Monday the 23rd of October (2 days before

the midterm). The review will be held in LSB 1001, and seating will be first

come first served, with preference given to those enrolled.

-- Kyle Jacoby [email protected]


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Camels in the Desert. What do you see?


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Intermolecular ForcesIntermolecular Forces

Hydrogen BondingHydrogen Bonding Special case of dipole-dipole forces.

By experiments: boiling points of compounds with H-F, H-O,and H-N bonds are abnormally high. Intermolecular forces are abnormally strong. H-bonding requires H bonded to an electronegative element

(most important for compounds of F, O, and N). Electrons in the H-X (X = electronegative element) lie

much closer to X than H.

H has only one electron, so in the H-X bond, the + Hpresents an almost bare proton to the - X. Therefore, H-bonds are strong.


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H-Bonding

Occurs when Hydrogen is attached to a

highly electronegative atom.

N-H N- O-H N- F-H N-

N-H O- O-H O- F-H O-

N-H F- O-H F- F-H F-

+ -Requires Unshared Electron Pairs of Highly

Electronegative Elements


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Hydrogen Bonding in

Water Molecules


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Structure of Ice

Observe the orientation of the

Hydrogen Bonds


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Why Does Ice Float?

D2O(s)H2O(s)


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Intermolecular ForcesIntermolecular Forces

Hydrogen BondingHydrogen Bonding Hydrogen bonds are responsible for:

Ice Floating Solids are usually more closely packed than liquids; therefore, solids are more dense than liquids.

Ice is ordered with an open structure to optimize H-bonding. Therefore, ice is less dense than water. In water the H-O bond length is 1.0 . The OH hydrogen bond length is 1.8 .

Ice has waters arranged in an open, regular hexagon. Each + H points towards a lone pair on O. Ice floats, so it forms an insulating layer on top of lakes, rivers,

etc. Therefore, aquatic life can survive in winter.


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The Boiling Points of the Covalent Hydrides of theElements in Groups 4A, 5A, 6A, and 7A


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DNA Double Helix-Held Together with

H-Bonds


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base: thymine(pyrimidine)

sugar: 2-deoxyribose

monophosphate

no 2-hydroxyl

(5 to 3)

5

3

base:adenine(purine)

1

2

4

3 linkage

5 linkage

Three Components of DNA Structure


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Pyrimidines used in Base Pairs,

DNA

6-membered rings only


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Purines used in Base Pairs, DNA

Fused 5 and 6 member rings


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DNA Base Pairing

A-T pairing

2 H-Bonds

G-C pairing

3 H-bonds


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A-T and G-C Base Pairs Hold theDNA helices together


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Base Pairs Double Helix


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Hydrogen-Bondings Role in DNA Structure


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Intermolecular Forces SummaryIntermolecular Forces Summary

Intermolecular Intramolecular


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Some Properties of LiquidsSome Properties of Liquids

ViscosityViscosity Viscosity is the resistance of a liquid to flow.

A liquid flows by sliding molecules over eachother. The stronger the intermolecular forces, the

higher the viscosity.

Surface TensionSurface Tension Bulk molecules (those in the liquid) are

equally attracted to their neighbors.


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Surface TensionSurface Tension


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Surface TensionSurface Tension Surface molecules are only attracted inwards

towards the bulk molecules. Therefore, surface molecules are packed moreclosely than bulk molecules.

Surface tension is the amount of energyrequired to increase the surface area of aliquid.

Cohesive forcesbind molecules to eachother.

Adhesive forcesbind molecules to a surface.


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Surface TensionSurface Tension


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Surface Tension Mercury


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Surface TensionSurface Tension Meniscusis the shape of the liquid surface.

If adhesive forces are greater than cohesiveforces, the liquid surface is attracted to itscontainer more than the bulk molecules.

Therefore, the meniscus is U-shaped (e.g. waterin glass). If cohesive forces are greater than adhesive

forces, the meniscus is curved downwards. Capillary Action: When a narrow glass tubeis placed in water, the meniscus pulls the

water up the tube.


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Water (left) has a downward curving meniscus.Mercury (right)has an upward curving meniscus


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Capillary ActionCapillary Action

The tendency of certain liquids to rise in aThe tendency of certain liquids to rise in anarrow tube.narrow tube.

There is aThere is a

competitioncompetition

betweenbetween

adhesive andadhesive and

cohesivecohesiveforces.forces.


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1) Write your perm number inthe first 7 columns of the IDnumber section.

2) Mark the bubbles in the first 7

columns with your permnumber

3) Use a soft pencil to assure thereading machine will read your

perm number

To receive attendance credit you must mark yourcorrect perm number in the first seven columns of theID number box. This is worth 5 points credit

Lecture10, 10-20-06 Hydrogen Bonding, Liquids

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