Page 13 - PRE-U STPM CHEMISTRY TERM 1

Chemistry Term 1 STPM
9 The frequency of the radiation emitted is given by Plank’s
equation: Exam Tips
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Exam
∆E = hf
where ∆E = energy difference between the two levels concerned
10 All transitions from higher energy levels to level n = 1 emit 2009/P1/Q3
radiations with frequencies in the ultraviolet region, giving rise to CHAPTER
Lyman Series. 2

11 Transitions from higher levels back to level n = 2 emit radiations
with frequencies in the visible region, giving rise to Balmer Series.
This is summarised in the diagram below:

n = 6
n = 5
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n = 4 Exam T ips
n = 3
Paschen series
The intervals between levels/
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orbits gets smaller with
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n = 2 increasing value of n.
Balmer series
n = 1
Lyman series
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Frequency Tips
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Example 2.2
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The difference in energy between the first and second levels in
–1
the hydrogen atom is 985 kJ mol . Calculate the frequency of the Since ∆E is given in terms of
radiation emitted when an electron falls from the second level to kJ mol , h will have the value
–1
–1
the first. of 3.99 × 10 –13 kJ mol s and
not 6.63 × 10 –34 Js.
Solution
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Exam
∆E = 985 kJ mol –1
This frequency corresponds
to the ultraviolet region.
Using the equation:
∆E = hf
–13
985 = (3.99  10 ) f
∴ f = 2.47  10 Hz
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02 Chapter 2.indd 43 3/26/18 3:14 PM