Question

Which of the following pairs of acids are incorrectly ordered?

stronger acid                weaker acid

a) HF                               HI

b) PH3                            NH3

c) HF                               NH3

d) H2S                            H2O

e) H2O                           NH3

Answer 1

a) is incorrect. HF is weaker acid than HI.HF has high bond energy which prevents it's easy dissociation and thus,HF is a weak acid.the HF bond is much stronger than HI bond because of the larger electronegativity difference in the former ,so HI will be more acidic then HF . As it can easily donate H+ ion as comparative to HF

b) - correct PH3 is stronger acid than NH3 beacuse NH3 acts as a stronger  base than PH3  due to the presence of lone pair on nitrogen atom which are available for donation. PH3 is a stronger acid than NH3 because N-H bonds are about twice as strong as P-H bonds, so it’s easier for PH3 to lose a proton

c) correct . again the same, NH3 acts as base because of lone pair.

d) coorect .  The S-H bond is weaker than the O-H bond, meaning that H2S more easily donates a proton and is therefore more acidic.also one can say that S is much larger atom than O, so the H–S bond is much longer and weaker than H–O; hence H2O is weaker acid than H2S

e) correct  H2O is more acidic than NH3 due to the more electronegative O present in H2O furthermore the lone pair of NH3 can easily be donated which makes it more basic than water. NH3 is stronger base than water which also means that acidic character in water is more than thatin NH3