Page 15 - PRE-U STPM CHEMISTRY TERM 1

Chemistry Term 1 STPM

n = ∞

f
(maximum)

CHAPTER
n = 1
2
15
3.29  10 Hz
Frequency
4 Using the formula:

∆E = hf
–13
15
= (3.99  10 )(3.29  10 ) kJ mol
–1
= 1312.7 kJ mol –1
Example 2.3

The frequencies of the first four lines in the Lyman Series are given Graphical method to
below: calculate the ionisation
15
2.56; 2.92; 3.08; 3.16 ( 10 Hz) energy of hydrogen
(a) Plot a suitable graph to determine the frequency of the last
line in the Lyman Series.
(b) Use your answer to question (a), calculate the ionisation
energy of hydrogen.

Solution
Using the equation:
The Rydberg’s equation for the Lyman series is:
1 1 1
= R  – 
λ H 1 2 n 2
1
Or, f = CR  1 – 
H 1 2 n 2
Rearranging:
1
f = CR – CR  
H H n 2
1
A graph of frequency, f, against will give a straight line where
n 2
the maximum frequency can be obtained by the intercept at the
frequency axis (where n = ∞) and 1 = 0
n 2

15
f (× 10 )/s –1 2.56 2.92 3.08 3.16
n 2 3 4 5
1
0.25 0.11 0.063 0.040
n 2

45

02 Chapter 2.indd 45 3/26/18 3:14 PM