Graphic illustration of the acidity of EHn molecules

 
Periodic table Group 14 Group 15 Group 16 Group 17
Formula

Electron Density Map (calculated)		 CH4 NH3 H2O HF
electronegativity
acidity		 H 2.1 C 2.5
not acidic in water		 N 3.0
not acidic in water		 O 3.5
v weak acid		 F 4.0
weak acid

increasing acidity  can be correlated with
increasing polarity of E-H bond (red more negative - blue more positive)


Proton transfer from EH involves
E-H bond breaking: E–H E + H
formation of E– and H+ : E + H E– + H+

Relative acidity of EHn molecules can be correlated with
polarity of E-H bond (important for C-H, N-H, O-H, H-F)
more polar more acidic (HF > H2O)

strength of E-H bond (important if E in different rows)
longer E-H bond more acidic (HI > HBr > HCl > HF)
 HCl 
strong acid
HBr
stronger acid
HI
even stronger acid
Dr Sheila Woodgate
© The University of Auckland 2002