The relative acid strength is determined by the Δ G of the ionisation process
In the following table the above data are given in kJ/mol at the standard condition
In the following table for ionisation step T Δ S° are given in kJ/mol-
(i) The ionisation constant (Ka) is lesser than 1 for-
Text Solution
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(i)
Sol. (i) For the ionisation, Δ H = Δ H dehyd + Δ H disso + Δ H ionis (H) + Δ H eg + Δ H Hyd (H + ) + Δ H Hyd (x – ) for HF, Δ H ionisation = 50 + 575 + 1300 + (–340) –1100 –500
Δ H ionisation = –15 kJ
For HCl , Δ H ionisation = 20 + 425 + 1300 –360 –1100 –370 = –85 kJ
For HBr, Δ H ionisation = 22 + 365 + 1300 –330 –1100 –360 = –103 kJ
for HI, Δ H ionisation = 25 + 300 + 1300 – 300 – 1100 –400 = –175 kJ
For ionisation,
for HF, Δ r G° = Δ r H°– T Δ r S°
Δ r G° = –15 – (–25) = 10 kJ
For HCl, Δ r G° = Δ r H°– T Δ r S°
Δ r G° = –85 – 55 = –140 kJ
For HBr, Δ r G° = Δ r H°– T Δ r S°
Δ r G° = – 103 – 60 = –163 kJ
For HI, Δ r G° = Δ r H°– T Δ r S°
Δ r G° = –175 –63 = –238 kJ
For HF only Δ r G° is positive hence K a < 1
(ii)
Sol. Δ r G° for ionisation indicates, the order of acidic strength is
HI > HBr > HCl > HF
(iii)
Sol.
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