lecture1(quantm chem) ks

Chemistry I (CHEM C141)

K. Sumithra

Chemistry-I (CHEM C141)The Scope and Objective

•The electronic structure of atoms and molecules

•Chemical bonding

•Molecular Spectroscopy

•Phase and chemical equilibrium

•Chemical kinetics.

CHEM C141 Chemistry I•Quantum theory

•Atomic structure and spectra

•Chemical bonding

•Spectroscopy of various types

CHEM C141 Chemistry I•Thermodynamics and thermochemistry

•Statistical thermodynamics

•Phase equilibria

•Mixtures

CHEM C141 Chemistry I•Chemical reaction thermodynamics

•Chemical Kinetics

Main Text:

‘The Elements of Physical Chemistry’, P.W. Atkins & Julio de Paula, Fifth edition (Oxford University Press, Oxford 2009).

Reference :

(i) ‘Physical Chemistry’, G. M. Barrow, Fifth Edition, Special Indian Edition (Tata McGraw Hill Publishing Company Limited, New Delhi 2007).

(ii) ‘Physical Chemistry’, D. W. Ball, First Edition, Indian Edition (Thomson, 2007).

Component Duration Wt % Date/Time

dd/mm/yy

Remarks

Test I 50

min

20% 4.9.2010 Closed book

Test II 50

min

20% 9.10.2010 Open book

Test III 50 min 20% 20.11.2010 Closed book

Comprehensive

examination

3 hrs 40%

16%

Quiz

3.12.2010 -do-

24% -do-

Tutorial Hour

•A review of the lectures •Discussion and interactions•Clarification of doubts •Problem solving

Origin of Quantum Mechanics

Physics in the late 19th Century1.Classical mechanics

Newton’s law applies universally

Any motion arbitrary energy2.The world is deterministic.3.Light is an electromagnetic waveWaves and particles : Distinct concepts

Invalid Assumptions of Classical Physics

1. Any type of motion can be excited to a state of arbitrary energy: Oscillation of a pendulum

Invalid Assumptions of Classical Physics

2. Particle travels in a trajectory or path, with a precise position and momentum at each instant.

Invalid Assumptions of Classical Physics

3. ‘Waves’ and ‘Particles’ are distinct concepts.

These are excellent assumptions at the macroscopic level, but break down when one considers the microscopic level:

Behaviour of very small entities such as electrons, atoms, molecules, etc.

• Need to understand detailed structure of atoms and molecules

Timeline:•Dalton : Concept of atoms (1803)•Avogadro: Concept of molecules (1811)•Thomson: Concept of electron (1897)

Experiments that defied Classical Physics

Black body radiation

Photoelectric effect

Line spectra of atoms

Heat capacity of solids

Interference

Diffraction

Conclusion

Newton’s laws do not apply to

the microscopic world!

Classical mechanics fails

Outcome New Rules

QUANTUM MECHANICS!-a new philosophy

R. P. Feynman : Quantum Mechanics deals with “Nature as she is – absurd”

Stephan Hawking : Quantum mechanics is the basis of modern developments in chemistry, Molecular biology, and electronics, and the foundation for the technology that has transformed the world in the last fifty years

Quantum Mechanics deals with “Nature as she is – amazing!”

"And anyone who thinks they cantalk about quantum theory withoutfeeling dizzy hasn't yet understoodthe first thing about it."

Niels Bohr.

Quantum Mechanics

•Describes rules that apply to electrons in atoms and molecules •Non-deterministic, Probabilistic

-Explains unsolved problems of the late 19th Century-Explains bonding, Structure and reactivity in Chemistry

Electromagnetic spectrum

Much of a person's energy is radiated away in the form of infrared energy.

Thermal RadiationBlackbody Radiation

Blackbody radiation

T

Common observation with heated bodies; Red blue

Black body radiation•Any dense object can radiate energy-

•Graphite 96% Absorption/Emission

•Surface coated with lamp black/ Platinum black

Properties of a perfect blackbody

A blackbody is a perfect absorber or emitter of radiation

The distribution of absorbed or emitted radiation depends only on the absolute temperature, not on blackbody material

Black body radiation

•Perfect black body

•Pin hole in a container Fery

Black body radiation

Problem: To account for the spectral distribution of the power emitted by a black body

Intensity

Wavelength

Radiates at all wavelengths in principle! Curve touches X axis at infinity

Major experimental observations

Not all wavelengths of light are emitted equally

At any temperature, the intensity of emitted light → 0 as the wavelength → 0

It increases to some maximum intensity Imax at some wavelength

Black body radiation- Features1. Wien’s Displacement Law

maxT = 2.99 mm K (Constant)

max

T

T

Common observation with heated bodies; Red blue

Shorter wavelengths contribute more to the energy density as temperature is raised and

the color shifts t

(T λmax = 2.9 mm K)Estimation of the Temperatures of Stars

Maximum Emission of the Sun occurs at λmax = 490 nm. What is the surface temperature?

The Surface Temperature of Sun T = 2.9mmK/ λmax

= (2.9 x 10-3 m K) / (490 x 10-9 m) = 5918 K

Application of Wien’s Displacement Law

Black body radiation : features

2. Stefan-Boltzmann Law:

Emittance M = aT4

(Power emitted per unit Surface Area)

‘a’ =5.67 x 10-8 Wm-2K-4 Rapid increase with increasing temperature

Stephan-Boltzman Law ; Emittance

T =1000K, Emittance, M= 5.7Wm-2

What is the emittance at 3000K ?T=3000K, M= 34 x 5.7Wm-2

Emittance : Area under the curve

Rapid increase with increasing temperature

M3000K = 81 x M1000K

Attempted explanation of black body radiation with classical mechanics

Rayleigh-Jeans law

Main assumptions

Electromagentic radiation was regarded as waves in jelly like ‘ether’

Ether could oscillate at any frequency so waves could exist in it of any wavelength

Rayleigh-Jeans Theory

Expt

Black body radiation : Rayleigh-Jeans formula

Energy density d is the energy per unit volume associated with radiation of wavelength from to +d, and is proportional to the

emittance :

Rayleigh-Jeans formula : d =k

d

Consequences

Works at long wavelengths (low frequencies) but fails badly at short wavelengths( high frequencies) As λ decreases, ρ increases without going through maximum

Oscillations of short wavelength areOscillations of short wavelength arestrongly excited at room temperaturestrongly excited at room temperature

k

dd =

Rayleigh-Jeans Formula:

d =

The function rises without bound as decreases

•Even cold objects would emit UV and visible!

Black body radiation : UV Catastrophe

k

d