Advanced Higher Chemistry Unit 1 The Electromagnetic Spectrum.

Advanced Higher Advanced Higher ChemistryChemistry

Unit 1 Unit 1

The Electromagnetic SpectrumThe Electromagnetic Spectrum

Electromagnetic RadiationElectromagnetic Radiation

Radiation such as x-rays, microwaves and radio Radiation such as x-rays, microwaves and radio signals is known as electromagnetic radiation.signals is known as electromagnetic radiation.

Electromagnetic radiation can be considered in Electromagnetic radiation can be considered in terms of waves which have wavelengths of terms of waves which have wavelengths of between 10between 1044 and 10 and 10-14-14 metres. metres.

These waves travel in a vacuum at a constant These waves travel in a vacuum at a constant speed of 3 x 10speed of 3 x 1088msms-1-1. This value is called the . This value is called the velocity and is given the symbol ‘c’.velocity and is given the symbol ‘c’.

The Electromagnetic SpectrumThe 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.

Electromagnetic RadiationElectromagnetic Radiation• Electromagnetic radiation can be specified by its Electromagnetic radiation can be specified by its

wavelength and its frequency.wavelength and its frequency.• Wavelength (Wavelength (λλ)) is the distance between is the distance between

adjacent crests of a wave. It is usually measured adjacent crests of a wave. It is usually measured in nanometres (nm). in nanometres (nm).

1nm = 101nm = 10-9-9mm• Frequency (f or Frequency (f or Ʋ)Ʋ) is the number of wavelengths is the number of wavelengths

that pass a fixed point in one second. It is that pass a fixed point in one second. It is measured as 1/t and has the units smeasured as 1/t and has the units s -1-1. .

• ss-1 -1 is now more commonly known as Hertz (Hz).is now more commonly known as Hertz (Hz).

Wavelength and frequencyWavelength and frequencyThe wavelength of The wavelength of wave A has double wave A has double the value of wave B.the value of wave B.

Since both waves Since both waves travel at the same travel at the same velocity (3 x 10velocity (3 x 1088msms--

11) then twice as ) then twice as many wavelengths many wavelengths of wave B will pass of wave B will pass every second than every second than wave A i.e. the wave A i.e. the frequency of wave B frequency of wave B is twice that of wave is twice that of wave A.A.

• velocity = velocity = wavelength x frequencywavelength x frequency

cc ff

(m s(m s-1-1)) (m) (s (m) (s-1-1))

WavenumberWavenumber

The wavenumber (The wavenumber (⊽) is the number of ⊽) is the number of waves in unit length i.e. the number waves in unit length i.e. the number of waves in 1m or 1cm.of waves in 1m or 1cm.

⊽ ⊽ = 1 /= 1 / λ λ The units for wavenumber are mThe units for wavenumber are m-1-1 or or

cmcm-1-1..

Energy Energy

• Electromagnetic radiation can be thought Electromagnetic radiation can be thought of as waves or as particles of energy of as waves or as particles of energy known as photons.known as photons.

• These photons have a definite size and These photons have a definite size and therefore a definite amount of energy.therefore a definite amount of energy.

Energy of a PhotonEnergy of a Photon• For one photon :For one photon :

Energy Energy = Planck’s x frequency = Planck’s x frequency constant constant

EE == h hff

i.e. because h is a constant, E is directly proportional i.e. because h is a constant, E is directly proportional to fto f

oror E E = = hchc λλ

• Planck’s constant = 6.63 x 10Planck’s constant = 6.63 x 10-34-34 J s J s• 1kJ = 1000J1kJ = 1000J

Energy Energy

• For one mole of photons:For one mole of photons:

Energy = Avogadro x Planck’s x frequencyEnergy = Avogadro x Planck’s x frequency

constant constantconstant constant

E = L h E = L h ff

or E =or E = LhcLhc

λλ

Energy CalculationsEnergy Calculations• The red line in the hydrogen spectrum has a The red line in the hydrogen spectrum has a

wavelength of 656 nm. Calculate the energy, in wavelength of 656 nm. Calculate the energy, in kJ molkJ mol-1,-1, for one mole of photons. for one mole of photons.

NOTENOTEBecause c is given in Because c is given in mmss-1-1, , must be converted to must be converted to mm..

• E E = Lhc / = Lhc / = = 6.02 x 106.02 x 102323 x 6.63 x 10 x 6.63 x 10-34-34 x 3 x 10 x 3 x 1088

656 x 10656 x 10-9-9

= 1.82 x 10= 1.82 x 1055 J mol J mol-1-1

= 182 kJ mol= 182 kJ mol-1-1

ExerciseExercise

Copy worked examples 1.1. and 1.2 from Copy worked examples 1.1. and 1.2 from pages 4 & 5 of Advanced Higher pages 4 & 5 of Advanced Higher Chemistry Calculations.Chemistry Calculations.

Complete the problems on pages 6 and 7 Complete the problems on pages 6 and 7 using the worked examples to help you.using the worked examples to help you.

Complete pages 1 – 3 of your workbook.Complete pages 1 – 3 of your workbook.