Strength of an Acid

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Mike Matthews 1D
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Joined: Fri Sep 29, 2017 7:04 am

Strength of an Acid

Postby Mike Matthews 1D » Wed Dec 06, 2017 3:32 pm

So in the book on page 482, it says that "the greater the electronegativity of A, the stronger the acid HA." However, it then goes on to say that in group 17, the acid strengths are HF<HCl<HBr<HI. I'm confused by this because isn't Fluorine more electronegative than Iodine? Therefore shouldn't HF be the stronger acid? Could someone please explain this to me.

Catherine Yang 3G
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Re: Strength of an Acid

Postby Catherine Yang 3G » Wed Dec 06, 2017 3:57 pm

I just know that HI is the strongest acid out of the four because it releases the most hydrogen ions into the solution. The pH is determined by [H+], and stronger acids have a lower pH. That means that stronger acids will release more hydrogens than weaker acids, and HI will release more hydrogens than fluorine because fluorine is smaller and its hold on the electrons in hydrogen is tighter because of its small size.

Radha Patel 1D
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Re: Strength of an Acid

Postby Radha Patel 1D » Wed Dec 06, 2017 4:00 pm

HF is not a strong acid because Fluorine is highly electronegative and also smaller in shape. HF is a weak acid because it does not disassociate as easily in water (the H-F bond is harder to break). As you go down the halogens, the atom sizes become bigger and the bond between the element and H become weaker. These are more likely to disassociate in water.

Courtney Cheney 3E
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Re: Strength of an Acid

Postby Courtney Cheney 3E » Wed Dec 06, 2017 4:07 pm

I got really confused to! But the book goes on to explain that "Acid strengths of binary acids across a period correlate with electron affinities; acid strengths down a group correlate with bond strength."

Courtney Cheney 3E
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Re: Strength of an Acid

Postby Courtney Cheney 3E » Wed Dec 06, 2017 4:08 pm

Since HF, HCl, HBr, and HI are in the same group, we have to relate them through bond strength and since the H-F bond is stronger than the H-I bond, HI is a stronger acid. This is all due to the fact that the weaker the bond between the A and H, the more easily the H dissociates from the A.

Erik Khong 2E
Posts: 50
Joined: Fri Sep 29, 2017 7:07 am

Re: Strength of an Acid

Postby Erik Khong 2E » Thu Dec 07, 2017 9:53 pm

The strength of an acid increases across the periodic table with electronegativity but also downwards with the size of elements.


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