Chemistry Unit 2: Assignment 1 Module 2: Analytical methods and separation techniques.

Spectroscopic methods of analysis

What is spectroscopy? Spectroscopy is a technique which involves using instruments to examine radiation emitted, absorbed or reflected by chemicals (electromagnetic radiation), giving information about their molecular structure. The information obtained from spectroscopy is called a SPECTRUM.

What is a spectrum? A spectrum is a plot of the intensity of energy detected versus the wavelength or frequency of the energy.

Objective 4.1: explain the nature of electromagnetic radiation; Calculations using the equation are required.

Electromagnetic radiation Electromagnetic radiation can be descried in terms of photons which are: - massless - travels in a wave-like pattern - moves at the speed of light - possess energy Each photon contains a certain amount of energy and all electromagnetic radiation consists of these photons.

Radio waves, visible light, X- rays, and all the other parts of the electromagnetic spectrum are fundamentally the same thing, electromagnetic radiation.

Electromagnetic radiation can also be described as the emission and transmission of energy in the form of ELECTROMAGNETIC WAVE. -An electromagnetic wave has the same wavelength ( ) and frequency (v) and hence the same speed (c), but they travel in mutually perpendicular planes.

Frequency (v) is measured in cycles per second (which is called a Hertz). Wavelength ( ) is measured in meters. Energy (E) is measured in electron volts. The speed of light (c) in a vacuum (empty space) is 3 x 10 8 ms -1.

The electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency.

The speed of the wave (distance travelled per unit time) must be the product of the wavelength (distance between maxima) and the frequency (number of maxima passing per unit time):  (1)

Planck gave the name quantum to the smallest quantity of energy that can be emitted (or absorbed in the form of electromagnetic radiation. The energy E of a single quantum of energy is given by:  (2) Where h is Planck's constant with the value 6.63 x J.s

U sing equation 1 and 2, where we substitute 1 into 2 we can get the expression for a quantum of energy: c = λ v (1) (3) E= hv (2) Sub 3 into 2 for a Quantum of energy E= h c/ λ Thus the shorter the wavelength (the higher the frequency) the more dangerous they are since would posses a greater quantum of energy.

Objective 4.2: State the approximate wavelength ranges of the X- ray, UV/VIS, IR and radiofrequency regions of the electromagnetic spectrum; relative energies and dangers associated with exposure to high energy wavelengths.

The electromagnetic spectrum Diagram showing the wavelengths, frequency and an example of the approximate size of the wavelengths.

Note: The lower the frequency the longer the wavelength, for example radio waves and infra red. The higher the frequency the shorter the wavelength the more dangerous they are, for example gamma rays and X rays.

Typical Wavelengths X- Rays m Ultra violet m Visible 5 x m Infra-red m Radio waves m to 10 3 m Pictures Sources e.g. X-ray tubes. Very dangerous! Very hot objects, sun, sparks, mercury lamps. Dangerous! Hot objects, fluorescent substances, lasers. Warm or hot objects, sun, irons, fires, grills, toasters. Radio transmitters including radar and TV transmitters Microwave ovens. Detector Photographic film Photographic film, causes sun tan, makes fluorescent substance glow. Eyes, photographic film SkinMobile phone, radio or TV set

Dangers associated with exposure too high energy wavelengths The exposure of high energy wavelengths are dangerous and poses treats to human lives i.e. (the shorter the wavelength or the higher the frequency the more dangerous it is to humans).

X rays- has a high frequency and short wavelength therefore it is very dangerous and causes cancer, damages living tissues resulting in birth defects and mutations. Ultra violet (UV) rays- also has a high frequency and short wavelength and is also dangerous. UV rays causes tissue effects, from as slight as sunburn to as major as skin cancers. The UV rays could also damage the retina of the eyes causing impaired vision!

Objective 4.3 Recall that energy levels in atoms and molecules are quantized.

Energy levels! T he electrons in free atoms can be found only in certain discrete energy levels! These energy levels are associated with the orbitals or shells of electrons in an atom for example the hydrogen atom.

Bohr predicted: W hen energy is added to an atom, an electron in the ground state (lowest energy state) absorbs a quantum of energy and moves to an orbit with a higher energy level (excited state) which is further away from the nucleus. This is known as ATOMIC EXCITATION!

Bohr prediction cont’d: T he excited electron in the orbit with a higher energy level cannot maintain its position for a long time and therefore emits a quantum of energy, returning to the lower energy level. This is known as ATOMIC DE-EXCITATION! Note: the energy absorbed or emitted is equivalent to the difference in energy levels.

Nuclear motions in energy levels Each energy level has discrete nuclear motion: Vibrational: the nuclei can move relative to one another. Rotational: the entire molecule can rotate in space. Electronic energy: Increases the energy of one (or more) electrons in the molecule. These movements vibrational and rotational ONLY occurs in molecules NOT atoms.

Atoms within a molecule (e.g. HCl) can rotate and vibrate about an axis through the centre of gravity of the molecule. Electronic, vibrational and rotational energy types quantised and are associated with a specific region of the spectrum of electromagnetic radiation.

The table below shows the regions of the electromagnetic spectrum associated with the energy levels! Energy (level) type: Associated region of electromagnetic spectrum: Electronic Visible and ultra- violet radiation Rotational Microwave radiation VibrationalInfrared radiation

T he energy difference between electronic (E), vibrational (V) and rotational (R): E>V>R

References! Chemistry in context Chemistry for Cape AS Chemistry. Carol Hibbert For further information read Chemistry for Cape by Susan Maraj.