Physical Chemistry IIa Dr Williams

• De Broglie Equation E = hv  E = mc^2   λ = h/p
• Example of particle behaving like wave = diffraction of an electron beam on nickel surface
• Wave property arises due to interference between scattered waves
• Compton scattering equation Δλ = (λ2 – λ1) = (h/mc)(1-cosѳ)
• Compton scattering – shows light as a particle when electromagnetic radiation strikes electrons it scatters from them and its frequency is shifted = light = photon with momentum transferred during collision
• E = mc2 = pc
• When waves combine in phase = constructive interference where total amplitude is sum of individual waves
• When waves combine out of phase = destructive interference where a peak meets a trough sum will be 0 if individual amplitudes are equal
• Two slit diffraction experiment makes interference pattern of peaks and troughs
• Superposition = when wave form different sources meet at same point in space resulting amplitude of waves = sum of two displacements of individual waves node displacement at 0 antinode displacement at max
• Amplitude = maximum displacement
• Wave equation = y = Asin(Ф - 2π/λ)  Ф = 2πvt
• Particles collide conserve momentum and kinetic energy m1v1i = m2v2i = m1v1 +m2v2 1/2m1v1i2 + 1/2m2v2i2 = 1/2m1v12 + 1/2m2v22
• The photoelectric effect: observations from light being photons with 𝐸=ℎ𝑣
• Light particles (photons) are involved in collisions = transfer of energy
• Spectroscopy = evidence of wave as particles : The appearances of atomic and molecular spectra are defined by the absorption or emission of a photon (of energy 𝐸 = ℎ𝑣) resulting in a transition between energy levels
• Wave particle duality particles can sometimes behave like waves and waves can sometimes behave as particles
• Particle features = defined position masses velocities momenta
• Wave features = frequency wave number wavelength
• Photons have wave and particle properties
• Limitations of Bohr model = 1) only works for hydrogen and one electron ions as no electron-electron repulsion effects 2) does not explain fine structure of spectral lines vibronic transitions 3) assume electron moves in circular orbits but in quantum mechanics particles not allowed to have definite trajectories
• Bohr model has nucleus and stable electron orbits
• Electron energy equation = Eelectron = -2.178 x 10^-18 J x Z^2/n^2
• Lyman series = n1 =1
• Balmer series = n1 = 2
• Paschen series = n1=3
• Rydberg series ( adaptation from Balmer to make it work for all lines ) = 1/λ = R x (1/n^21 -1/n^22) n1<n2
• Bohr principles - Electrons are located in stable orbital paths around the positively charged nucleus ( predicted circular orbits) • these orbits can only exist at certain radii from the nucleus ( restricts the electron-nucleus distance and the electron energy to a number of allowed values = quantisation)
• evidence of waves as particles = photoelectric effect observations from light being photons with E = hv light particles photons are involved in collisions = transfer of energy
• Atomic emission spectra= H atoms excited in an electric arc emit discrete lines of EM radiation as relax from excited state to ground state discrete lines with definite wavelength black background coloured lines Rutherford model = no restriction on allowed energies of orbiting electrons Rutherford fail to explain emission spectra
• Particles discrete have momentum and undergo collisions particles follow definite trajectory specifies position as a function of time x(t) have momentum and kinetic energy
• Waves are a disturbance which propagates through a medium = continuous phenomenon diffracted in pair of slits = sinusoidal wave
• Electromagnetic radiation = oscillating magnetic and electric fields perpendicular aligned at 90 degrees
• EM radiation frequency increases wavelength decreases from radio to gamma waves
• red light wavelength 700nm
• purple light 400 nm
• Classical mechanics predicts precise trajectory for particle exact position r and velocity p = mv of particle mass = m can be known simultaneously r velocity and momentum are vectors magnitude and direction v = vx,vy,vz
• Particles and waves are distinguishable phenomena, with different, characteristic properties and behaviour3) In classical mechanics, any type of motion (translation, vibration, rotation) can have any value of energy associated with it.Classical mechanics predicts that any type of motion (e.g. translational, rotational or vibrational) can have any value of energy associated with it – there is a continuum of energies
• if an object is heated, it will emit radiation with broadrange of frequencies (from EM spectrum).The main emitted wavelength decreases as the temperature the object is heated to increases. Metal to glow “red” and “ hot” as it heated explain Black body – a hypothetical object perfect absorber of radiation (no reflect). results disagreed with the experiment calculations = classical physics. Planck solution= energy could only be absorbed or emitted in small ‘packets’ which were multiples of frequency and a new constant now known as Planck’s constant

  
• Plancks equation E = nhv