Chemical Ideas 6.2What happens when

radiation intracts with matter?

c = λ ν

υcλ

c

c

νλ

c = speed of light = 3.00 x 108 m s-1

λ = wavelength m lambda

ν = frequency Hz ( or s-1 ) nu

c

Use m

for λ

c = λν problems….

1. Work out λ if v = 8.19 x 1013 Hz

c = 3.00x108 m s-1

Rearranging again….

hE

h

E

E

h

E = hν problems …

• h = 6.63 x 10-34 J Hz -1

• Planck constant

2. If the frequency is 3.5 x 1013 Hz, what is the energy of each photon?

3. If E = 4.6 x 10-17 J per photon, what is the frequency (ν) of the radiation?

visible ultra violet

E increases

What happens to electrons?

1

234

• Electrons absorb energy

• Electrons are excited

• Electrons move to higher energy level

• E = h v

Molecules are made of more than one atom but they still have

electrons.• Electronic

transitions

Vis + UV

Molecules move !

There are vibrational energy levels as well as electronic

energy level ..

I.R.

There are rotational energy levels as well as electronic

energy level ..

microwave

• Electronic transitions

Vis + UV

• Rotational transitions

• Vibrational transitions

Microwave

I.R.

Note the range in Table 1, P130

Change .. ΔE in joules Part of e.m.s.

in vibrational energy level

1 x 10-20 to

1 x 10-19

infrared

C-F C-Cl C-Br

What could happen to a Cl2

molecule in the Atmosphere?

Visible

U.V. Photodissociation

radicals

Electronic

transitions

U.V.

+Ionisation

Now …

• Read CI 6.2 for yourself

• Complete Notes 3, 4 and 5

• Try Q1, 2 and 4 from CI 6.2. Only do Q4 if you are doing AS maths!