1. Advanced Higher Chemistry Unit 1 The Electromagnetic Spectrum

2. Electromagnetic Radiation  Radiation such as x-rays, microwaves and radio signals is known as electromagnetic radiation.  Electromagnetic radiation can be considered in terms of waves which have wavelengths of between 10 4 and 10 -14 metres.  These waves travel in a vacuum at a constant speed of 3 x 10 8 ms -1. This value is called the velocity and is given the symbol ‘c’.

3. The Electromagnetic Spectrum At the low energy end, the waves are further apart than the length of a football pitch. At the high energy end, the waves are so tightly packed that they are closer together than the size of an atom.

4. Electromagnetic Radiation Electromagnetic radiation can be specified by its wavelength and its frequency. Electromagnetic radiation can be specified by its wavelength and its frequency. Wavelength ( λ) is the distance between adjacent crests of a wave. It is usually measured in nanometres (nm). Wavelength ( λ) is the distance between adjacent crests of a wave. It is usually measured in nanometres (nm). 1nm = 10 -9 m Frequency (f or Ʋ) is the number of wavelengths that pass a fixed point in one second. It is measured as 1/t and has the units s -1. Frequency (f or Ʋ) is the number of wavelengths that pass a fixed point in one second. It is measured as 1/t and has the units s -1. s -1 is now more commonly known as Hertz (Hz). s -1 is now more commonly known as Hertz (Hz).

5. Wavelength and frequency The wavelength of wave A has double the value of wave B. Since both waves travel at the same velocity (3 x 10 8 ms -1 ) then twice as many wavelengths of wave B will pass every second than wave A i.e. the frequency of wave B is twice that of wave A.

6. velocity = wavelength x frequency velocity = wavelength x frequency c f c f (m s -1 ) (m) (s -1 ) (m s -1 ) (m) (s -1 )

7. Wavenumber  The wavenumber ( ⊽) is the number of waves in unit length i.e. the number of waves in 1m or 1cm.  ⊽ = 1 / λ  The units for wavenumber are m -1 or cm -1.

8. Energy Electromagnetic radiation can be thought of as waves or as particles of energy known as photons. Electromagnetic radiation can be thought of as waves or as particles of energy known as photons. These photons have a definite size and therefore a definite amount of energy. These photons have a definite size and therefore a definite amount of energy.

9. Energy of a Photon For one photon : For one photon : Energy = Planck’s x frequency constant constant E= h  f i.e. because h is a constant, E is directly proportional to f orE = hc λ Planck’s constant = 6.63 x 10 -34 J s Planck’s constant = 6.63 x 10 -34 J s 1kJ = 1000J 1kJ = 1000J

10. Energy For one mole of photons: For one mole of photons: Energy = Avogadro x Planck’s x frequency Energy = Avogadro x Planck’s x frequency constant constant constant constant E = L h f or E = Lhc λ

11. Energy Calculations The red line in the hydrogen spectrum has a wavelength of 656 nm. Calculate the energy, in kJ mol -1, for one mole of photons. The red line in the hydrogen spectrum has a wavelength of 656 nm. Calculate the energy, in kJ mol -1, for one mole of photons.NOTE Because c is given in ms -1,  must be converted to m. E = Lhc / E = Lhc / = 6.02 x 10 23 x 6.63 x 10 -34 x 3 x 10 8 656 x 10 -9 = 1.82 x 10 5 J mol -1 = 182 kJ mol -1

12. Exercise  Copy worked examples 1.1. and 1.2 from pages 4 & 5 of Advanced Higher Chemistry Calculations.  Complete the problems on pages 6 and 7 using the worked examples to help you.  Complete pages 1 – 3 of your workbook.