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This question already has an answer here:

Is this statement true or false?

HCl is a stronger acid than HF because fluorine is more electronegative than chlorine.

I'm thinking this is true, because since fluorine is more electronegative than chlorine, it has a higher ionic energy and therefore a stronger bond between hydrogen than chlorine does with hydrogen. In result, leading HCl to dissociate more easily. Can you please clarify.

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marked as duplicate by ringo, ron, Jon Custer, bon, M.A.R. ಠ_ಠ May 8 '16 at 7:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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You are correct, HCl is a stronger acid than HF. Fluorine is both more electronegative and smaller than chlorine. Because fluorine is more electronegative, the bond between it and the hydrogen is more polar, meaning that the proton would need to overcome a larger coulomb force to separate from the fluorine. In addition, fluorine is a smaller atom, meaning that the addition of an electron has a larger effect on the net charge of the atom. This both makes the fluoride anion (the conjugate base) less stable and it also - once again - increases the coulomb force between the proton and the fluorine. In addition, the coulomb force is even stronger because there is a smaller distance between the two atoms. As a general rule of thumb, as you move down the halogens, the acids become stronger and they form more stable conjugate bases.

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It is because of three reasons:

  1. Stability of fluoride is less than chloride, as stability of conjugate base decides acid strength.
  2. Smaller bond dissociation energy of HCl than that of HF.
  3. Hydrogen bonding in HF which effects proton releasing tendency.
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