Sapling #6 week 10

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Melody Haratian 2J
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Sapling #6 week 10

Postby Melody Haratian 2J » Mon Dec 07, 2020 4:26 pm

I was doing the sapling homework and was confused on why HCN is a weak acid instead of being a weak base. Since the nitrogen of HCN has a lone pair, I thought it would be a weak base since it can accept a proton from water. Can someone explain why it is a weak acid instead?
Thanks!

Arezo Ahmadi 3J
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Re: Sapling #6 week 10

Postby Arezo Ahmadi 3J » Mon Dec 07, 2020 4:57 pm

One of the methods that helps me initially determine if an acid is strong or weak is by referring to the seven strong acids that can be found in the textbook. Since HCN is not one of them, that helps me identify it as a weak acid.

Darlene Lien 3E
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Re: Sapling #6 week 10

Postby Darlene Lien 3E » Mon Dec 07, 2020 5:01 pm

Also, most of the time, acids begin with a Hydrogen. So, HCN is a weak acid. Hope this helps!

Samuel Flores 1E
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Re: Sapling #6 week 10

Postby Samuel Flores 1E » Mon Dec 07, 2020 5:13 pm

Hello! We know that according to the Bronsted definition of an acid, a substance is an acid if it is a H+ donor. Now let's look at HCN. We see that it has a H atom attached to the N atom. This H atom, in the presence of a base, can be removed from the HCN molecule as a H+. Therefore, we can say that HCN donated an H+, which signals that HCN is a weak Bronsted acid.

Hope this helps!

Tiao Tan 3C
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Re: Sapling #6 week 10

Postby Tiao Tan 3C » Wed Dec 09, 2020 7:55 am

Hi. I had the same question before and found this response by varlam17 on https://www.reddit.com/r/chemistry/comm ... s_a_lewis/.

"If a compound has a lone pair that does not guarantee that it's a base. Base and acid are relative, not absolute. A molecule is acid or base in relation to another molecule. for example, HCN is a stronger acid than water. So when you put HCN in water, H+ is donated to water forming Hydronium ion. As you can see, lone pair of nitrogen did not pull off the hydrogen from H2O."

The sapling question says "In an aqueous" so we just assume we are putting HCN in water.
Hope this helps!


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