03 Physical
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Physical Chem
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Quantum theory
- Energy can be transferred between systems in discrete amts only
- Radiation (light) has a particle character
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Electron has a wave character
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Planks Law
The internal modes of atoms and molecules can posses only certain energies
the modes are quantized
E=nhv
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Light as a particle
The photoelectric effect
- No electrons are ejected regardless of the intensity of the radiation unless its frequency exceeds a threshold value characteristic of the metal
- The kinetic energy of the ejected electron increases linearly with the frequency of the incident radiation but is independent of the intensity of radiation
- Even at low light intensities, electrons are ejected immediately if the frequency is above the threshold
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According to conversation of energy,
where ½meV^2 is the kinetic energy of the ejected electron
hv is the energy of the photon
theta is the work function ( characteristic of the metal ) - energy required to remove an electron from the metal “to infinity and beyond”
hv
no photoejection where h is 6.626 x 10^-34
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Electron as a wave
Davisson-Germer experiment
- Diffraction of electrons by a single crystal of Ni
- Diffraction is the interference caused by an object in the path of waves
- Depending on whether the interference is constructive or destructive , the result is a region of enhanced or diminished intensity of the wave
- particles are wave-like - Wave-Particle Duality
Any particle travelling with a linear momentum p=mv should have a wavelength lambda.
particle with high linear momentum has short wavelength
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Matter wave and de Broglie's relation
Matter wave is expressed as
The de-broglie relation implies that the wavelength of a matter wave should decrease as the particles speed increases
Large particles only manifest their particle nature and never their wave nature because their momenta will be so high due big mass making their wavelengths very small
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Dynamics of microscopic systems
According to classical mechanics a particle has specific trajectory
position and momenta are specified at each instant
According to quantum mechanics a particle cannot have specific trajectory
The wavefunction determines the probability distribution , darker the area higher the probability of finding the particle
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Schrodinger's Equation
The schrodingers equation for a single particle of mass m moving with energy E in one dimension is
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Physical Significance of wave function
The born interpretation
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Uncertainty principle
It is impossible to specify simultaneously , with arbitrary precision, both the momentum and the position of a particle
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Application of quantum physics
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Atomic Structure
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Hydrogenic atoms
A hydrogenic atom is a one-electron atom
Schrodinger equation can be solved for them and their structures can be discussed exactly
Spectrum of atomic hydrogen
Boundary condition
- The wavefunction must not become infinite anywhere
- it must repeat itself ( like the particle on the surface of a sphere )
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Quantum numbers
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Spectral transition and rules
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Tutorial Quantum + Atomic + Practice qs
- Chemical bonding
Molecular orbital theory
show 1s2 also
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Molecular spectroscopy
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Vibrational spectroscopy
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Electronic transitions
sigma to anti sigma in saturated
n to anti sigma in saturated with lone pairs
pi to anti pi in unsaturated
n to anti pi in unsaturated with lone pairs
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Chemical kinetics
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Tutorial Chemical bonding + spectro + kinetics
Correction :- B2H6 is IR active because of selection rule
In quantum mechanics, a selection rule is a criterion that determines whether a particular physical process or transition is allowed or forbidden. It specifies the conditions under which a transition can occur between two energy states of a system. In the context of spectroscopy, selection rules determine which types of transitions are observable in a particular spectroscopic technique, such as infrared (IR) spectroscopy.
IR spectroscopy is based on the absorption or emission of infrared radiation by molecules. It involves transitions between different vibrational energy levels of the molecules. The selection rule for IR activity is known as the electric dipole selection rule. According to this rule, for a molecule to exhibit an infrared absorption, the vibrational mode must result in a change in the molecular dipole moment.
Now, let's consider the molecule B2H6, which is called diborane. Diborane consists of two boron atoms (B) and six hydrogen atoms (H). In this molecule, the B-B bond is symmetrical, and the H atoms are arranged symmetrically around the boron atoms. Due to this symmetry, some vibrational modes of B2H6 do not result in a net change in the dipole moment of the molecule and, therefore, do not produce infrared absorption. These modes are said to be "IR inactive."
However, there are vibrational modes in B2H6 that do result in a change in the dipole moment and satisfy the electric dipole selection rule. For example, the bending vibrations of the BH3 groups and the stretching vibrations of the B-H bonds involve changes in dipole moment and are therefore IR active. These modes can be observed in the infrared spectrum of B2H6.
In summary, the selection rule for IR activity requires a change in the molecular dipole moment for a vibrational mode to be observable in the infrared spectrum. B2H6 exhibits IR activity for certain vibrational modes that involve changes in the dipole moment, such as the bending and stretching vibrations
Correction :- Beneze - pi to anti pi and sigma to anti sigma
Acetone:- n to anti pi , pi to anti pi , n to anti sigma , sigma to anti sigma
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Thermodynamics
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