"Example 1"

 

Determine the energy, in J of a photon of red light, correct to four significant figures, given that the wavelength λ. = 650.0 nm.

 

h = 6.626 x 10^ -34 J s; c = 2.998 x 10 ^8 m s^-1

 

SOLUTION

ΔE = hv = hc/λ

ΔE = 6.626 X 10 ^-34 J s x 2.988 x 10 ^8 m s^-1

                               650.0 x 10^-9m                        =  3.056 x 10 ^-19 J

 

EXAMPLE 2

 

Calculate the first ionization energy, in KJ mol ^-1, for hydrogen given that its shortest wavelength line in the Lyman series is 91.16 nm.

 

h= 6.626 x 10 ^-34 J s; c = 2.998 x 10^ 8 m s ^-1; N A mol ^-1

 

SOLUTION

 

The shortest wavelenght line in the lyman serie scorresponds to a transition of n = ∝ to n = 1

 

IE1 = hv = hc/λ

 

IE1  = 6.626 x 10 ^-34 J s x 2.998 x 10^8  m s^ -1

                              91.16 x 10 ^-9 m                        = 2.179 x 10 ^-18 J

 

Expressed in kJ mol ^ -1

 

IE1  = (2.179 x 10 ^-18 J ) x (6.022 x 10^23 mol ^-1 ) = 1.312 x 10 ^ 6 mol ^-1

= 1312 kJ mol ^-1