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Strong vs. Weak Acids

Posted: Sat Dec 07, 2019 7:09 pm
by Rebecca Remple 1C
Hi all,

I'm preparing for the final by reviewing the rules for strong and weak acids. I understand that bond length in HF and HI makes HI a stronger acid, because it is more likely to lose a proton. However, why is HOCl stronger than HOI? I read through the explanation in the review worksheet, but I'm having a hard time visualizing this concept. How does Cl lead to a more evenly distributed formal charge? I'd really like to understand the concepts behind this important rule. Please let me know your thoughts. Thank you and good luck on the final!

-Rebecca

Re: Strong vs. Weak Acids

Posted: Sat Dec 07, 2019 7:19 pm
by Ryan 1K
The structure of oxyacids involves the H atom bonding to the O atom and then to the Cl, I, or whichever atom is remaining. Since the H is not directly bonded to the Cl or I, the bond length is not relevant. Instead, the electronegativity is more important. This is because the Cl or I atom pulls electrons closer to itself and away from the O atom. In turn, it also pulls the electrons farther away from H and closer in the direction of the O and Cl atom. As a result, since the electrons are closer to the O atom and the Cl atom, the H becomes even more highly positive and the O-H bond becomes weaker since the O is pulling all the shared electrons toward itself, making it easier for H2O molecules to take the H+.

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Re: Strong vs. Weak Acids

Posted: Sat Dec 07, 2019 7:21 pm
by Chem_Mod
Hi, HOI and HOCl are oxoacids. Look at stability of conjugate base ClO-. The conjugate base is more stable when the electron density is distributed over more atoms versus concentrated on the oxygen only. Cl is more electronegative than I, so it can withdraw more electron density from the Oxygen. This helps delocalize/redistribute the negative charge, stabilizing the conjugate base. Think about this: if the oxygen has a greater negative charge concentrated on it, then it will be more likely to regain a proton to neutralize that negative charge, which means the conjugate base is not as stable

Re: Strong vs. Weak Acids

Posted: Sun Dec 08, 2019 9:12 am
by Rebecca Remple 1C
Chem_Mod wrote:Hi, HOI and HOCl are oxoacids. Look at stability of conjugate base ClO-. The conjugate base is more stable when the electron density is distributed over more atoms versus concentrated on the oxygen only. Cl is more electronegative than I, so it can withdraw more electron density from the Oxygen. This helps delocalize/redistribute the negative charge, stabilizing the conjugate base. Think about this: if the oxygen has a greater negative charge concentrated on it, then it will be more likely to regain a proton to neutralize that negative charge, which means the conjugate base is not as stable

Ryan Chew 1C wrote:The structure of oxyacids involves the H atom bonding to the O atom and then to the Cl, I, or whichever atom is remaining. Since the H is not directly bonded to the Cl or I, the bond length is not relevant. Instead, the electronegativity is more important. This is because the Cl or I atom pulls electrons closer to itself and away from the O atom. In turn, it also pulls the electrons farther away from H and closer in the direction of the O and Cl atom. As a result, since the electrons are closer to the O atom and the Cl atom, the H becomes even more highly positive and the O-H bond becomes weaker since the O is pulling all the shared electrons toward itself, making it easier for H2O molecules to take the H+.

Thank you both so much for your help! Your explanations make a lot more sense. Good luck on your finals :)

-Rebecca