If a photon of 254 nm is used, the excess energy (in J) over that needed for dissociation is 2.93 10^-19 J.
The bond-dissociation energy is defined as measure of the strength of a chemical bond which is represented as A−B. It can also be defined as the standard enthalpy change when the bond A−B is cleaved by homolysis to give two fragments A and B, that are usually radical species.
Energy of photon with λ 254 nm is given as
E = hC \ λ
= 6.626 × 10-³⁴ JS × 3 × 10 8 ms -1 /254 × 10 -9 m
= 7.83 10^-19 J
As we know that,
dissociation energy of HI = 295 10³ J. mol-¹
= 295 × 10³ J / mol -¹ / 6.023 × 10²³ mol -¹
= 4.898 10^-19 J.
Hence excess energy over dissociation
= 7.83 10^-19 J – 4.448 10^-19 J
= 2.93 10^-19 J
Thus, we concluded that the excess energy (in J) over that needed for dissociation is 2.93 10^-19 J.
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