Spectroscopic Methods of Analysis PDF

SPECTROSCOPIC METHODS OF ANALYSIS Specific objectives 4.1. explain the nature of electromagnetic radiation; (Calculation using the equation E=hv = hc/λ are required); 4.2. state the approximate wavelength ranges of the X-ray, UV/VIS, IR and rad...

SPECTROSCOPIC METHODS OF ANALYSIS Specific objectives 4.1. explain the nature of electromagnetic radiation; (Calculation using the equation E=hv = hc/λ are required); 4.2. state the approximate wavelength ranges of the X-ray, UV/VIS, IR and radiofrequency regions of the electromagnetic spectrum; 4.3. recall that the energy levels in atoms and molecules are quantized. 1nm = 1.00 x 10-9 m 1MHz = 1 x 106 Hz ELECTROMAGNETIC RADIATION consist of waves that have electrical and magnetic fields of energy vibrating in particular directions (transmitted through space). Each of these types of radiation has specific ranges of wavelengths and frequencies. WAVELENGTH is the distance between one wave peak and the next peak of waves radiation, it is measure in meters (m) FREQUENCY is the number of waves produced (passing a given point) in one second. It is measure in hertz (Hz). SPECTROSCOPY is defined as the measurement of electromagnetic radiation absorbed, scattered or emitted by atoms, molecules or other chemical species. Light is a form of electromagnetic radiation, but visible light is only one fraction of the electromagnetic spectrum which consist of other types of radiation as gamma-rays, ultraviolet, visible, infrared, microwaves and radiowaves.  TYPES OF SPECTROSCOPY  ULTRAVIOLET (UV) spectroscopy uses electron transitions to determine bonding patterns.  INFRARED (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group.  MASS SPECTROMETRY (MS) fragments the molecule and measures the masses.  NUCLEAR MAGNETIC RESONANCE (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. SPEED, FREQUENCY AND WAVELENGTH The speed of electromagnetic radiation is related to frequency and wavelength by the equation: c=fλ where c is speed in ms-1 f is frequency in hertz, 1Hz = 1s-1 λ is the wavelength in m Note: The symbol v is often used for frequency when electrons are being considered. SPEED, FREQUENCY AND WAVELENGTH Since electromagnetic radiation can be treated as waves the following relationships apply: ∆E = hν ν =c /λ Where E is energy/J; h is Planck’s constant (6.63×10−34 J·s), ν is frequency/Hz, c is the speed of light (2.998 X 108 m s-1) and λ is wavelength of the radiation/m. Wavelength is inversely proportional to the wave frequency THE ELECTROMAGNETIC SPECTRUM.  THE ELECTROMAGNETIC SPECTRUM. short wavelength, UV: valence long wavelength, high energy, electronic IR: molecular low energy, high frequency excitation vibrations low frequency  ENERGY QUANTA Electrons in atoms only have certain fixed values of energy. This values are called QUANTA. When chemical species are exposed to electromagnetic radiation they absorb some frequencies and move from a ground state (E1) to an excited state configuration (E2).  ENERGY QUANTA The energy transition E1 → E2 correspond to the absorption of energy that is equal to the energy of the wavelength (frequency) of electromagnetic radiation absorbed. When an electron falls back to the ground state again, it gives out a quantum of energy as radiation. We can also think of this energy as a particle called a photon.  ENERGY QUANTA A molecule can only absorb a particular frequency of electromagnetic radiation if there exist an energy transition within the molecule where: (E2 - E1) = ∆E = hv The energy difference involved is given by the equation: ∆E = hv This equation can be used to calculate the energy emitted when radiation of a particular wavelength or frequency is emitted from a previously excite atom. An atom or molecule contains electronic energy levels that relate to the arrangement of electrons in various atomic or molecular orbitals along with vibrational and rotational sublevels associated with each electronic level. THE DIFFERENT ENERGY LEVELS IN A MOLECULE. Energy Electronic energy levels Vibrational energy levels Rotational energy levels The energy transition that results when chemical species absorb electromagnetic radiation depends on the energy of the radiation and therefore the type of electromagnetic radiation absorbed. ELECTROMAGNETIC TRANSITIONS occur in the UV/VIS region of the electromagnetic spectrum. VIBRATIONAL AND ROTATIONAL TRANSITIONS occur in the infrared region and INFRARED spectroscopy is used to measure these transitions. The visible hydrogen emission spectrum lines in the Balmer series. H-alpha is the red line on the right. The two leftmost lines are considered to be ultraviolet as they http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Emission_spectrum-H.svg/757px-Emission_spectrum-H.svg.png have wavelengths EMISSION SPECTRUM OF HYDROGEN less than 400nm. WORK EXAMPLE Calculate the energy of an electron transition which emits radiation of frequency 1.01 x 1012 Hz Plank constant = 6.63 x 10-34Js Substituting into the equation ∆E = hv = 6.63 x 10-34 x1.01 x 1012 = 6.70 x 10-22J If we are asked for the value per mole of electrons, we multiply the value by the Avogadro number, 6.02 x 1023. 6.70 x 10-22 x 6.02 x 1023 = 403 J.mol-1 KEY POINTS  Electromagnetic radiation can be regarded as waves that have a characteristic frequency and wavelength.  Speed = frequency x wavelength.  The spectrum of electromagnetic radiation ranges from radio waves (1o7Hz) to gamma- rays (1019Hz).  Energy levels in atoms are quantized- they can only have certain energy values.  The energy associated with a photon is given by E=hv is the speed of light and h is Plank constant.  CHEMISTRY FOR CAPE. Pages 352-353 1. a. State the relationship between the energy, frequency and wavelength of a wave. b. Compound X absorbs light of frequency 156.8 MHz Calculate the wavelength of the light absorbed. λ=2.998x108/156.8 X 108 Hz = 1.91m c. Calculate the frequency of the radiation of wavelength 200nm υ =2.998x108/2.00 x 10-7 m =1.499x1015Hz 2. a. Differentiate between electronic, vibrational and rotational energy levels. b. Explain what is meant by energy levels in atoms and molecules are quantised.  HOMEWORK #1 3. What energy is associated with the following transitions: a. 928 MHz E=hγ= 6.6326x10-34x9.28x108=6.155x10-25J b. 740 nm c. 2 x 108 Hz Identify the part of the electromagnetic spectrum where transitions (a)-(c) occurs. 4. State the approximate wavelength ranges of the following: a. X rays b. Infrared radiation c. Radiofrequencies