Matter, Motion, and Entropy: A Review

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CHM 102 Sinex

What is the difference between

an atomic and molecular substance?

Atomicsingle atom

HeArFe

Molecularmultiple atoms

N2CO2

C6H12

Describe the types of motion

• Translational

• Rotational

• Vibrationalmotion that changes the shape of the molecule – stretching, bending, and rotation of bonds

whole atom or molecule changes its location in three dimensional space

whole molecule spins around an axis in three dimensional space

Motion of

whole molecul

e

Motion within molecule

Which type of motion contributes the most to the entropy of a substance?

Which type of motion is the cause of collisions?

Which type of motion influences the orientation for an effective collision?

Translational motion

Translational motion

Rotational motion

Matter Mode State Type Translatio

n Rotation Vibration

GasAtomic Yes No No

Molecular Yes Yes Yes

Liquid

Atomic Yes No NoMolecula

r Yes Yes Yes

SolidAtomic No No Yes

Molecular No No Yes

Types of Motion and the States of Matter

Vibration Minimum number of

atoms

How atoms are connecte

d

What changes?

Stretching 2 Bond length

Bending 3 Bond angle

Internal rotation 4 Torsion

angle

Types of Vibrations

In solids such as the carbon tetrahedron from the diamond

structure, how would you describe the motion of the atoms?

As the carbon atoms vibrate, all the bond lengths vary about an average value.

Describe all the modes of vibration in each of the

molecules.N2

CO2

C4H10

stretching only

stretching, both symmetric and asymmetric, and bending

stretching, bending, and internal rotation

Which molecule has the highest entropy?

What hinders internal rotationin a molecule?

• Double or triple bonds• Bulky groups in certain positions on

some molecules• Ring formation - cyclic molecules• Sometimes hybrid orbitals (lone pairs

of electrons) on single bonds such as hydrogen peroxide

• Molecules bound in a crystalline solid

Examples are shown on the following three sides!

Twisting vibration around double bond in ethene, C2H4

Two methyl groups (-CH3) on biphenyl

Click to view Chime structure of lowest energy

state

Click to view a movie of the

vibration

Chime Guide

Restricted internal rotation or twisting vibration in ring

Hydrogen peroxide, twisting motion along single bond

C6H12Click here for Chime structure – you will need to right click on the Chime structure and click on animation

Click here for Chime structure – you will need to right click on the Chime structure and click on animation

H2O2 has a permanent dipole moment = 2.0 D

Molecules in solid CO2 – note the tight packing of

moleculesHow would larger molecules ever move around?

Hexane, C6H12, in the solid state – note the straight chain arrangement of the molecules

Space-filled

From:Roland Boese per. comm.

Chime structure

Does the phase make a difference in biphenyl?

Both rings in thesame plane

40o angle between ringsstill allows pi clouds to interact

solid gas

Chime structure Chime structure

What is the cause of the entropy of solid carbon monoxide?

Vibrations and arrangement of the molecules of CO

Stotal = Strans + Srot + Svib

Strans > Srot >> Svib

Why do solids have low entropies?

How do impurities in a solid influence the solid’s entropy?

Impurities cause an increase in entropy.

Here is a crystal of the mineral zircon. Click on the image, and radioactive decay will occur. How does entropy change?

The decay particle causes a zirconium ion to recoil and it disrupts the structure which slowly recovers. In the end, the crystal structure becomes more disordered or S > 0.

Source: http://www.esc.cam.ac.uk/movies

ZrSiO4

Proteins – long chains of amino acids

How does the entropy change as the chain takes the form of a helix?

helix*

chain*

S < 0 due to limited internal rotation in hydrogen-bonded helix (secondary structure of protein)

coiling of 10 glycine chain

*Chime structures