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Postby JTieu_1L » Mon Jan 18, 2021 1:25 pm

why is it that with lower pKa the stronger the acid?

Samantha Pedersen 2K
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Re: pKa

Postby Samantha Pedersen 2K » Mon Jan 18, 2021 1:36 pm

I like to think about this question in terms of Ka first. As we know, the expression for Ka is [products] / [reactants]. Stronger acids are ones that almost fully dissociate to form their conjugate bases and H3O+, which increases the equilibrium concentrations of the products and results in a relatively large Ka value. Therefore, higher Ka values indicate stronger acids.

When you see a "p" in front of something in chemistry, it means that you need to take the -log of whatever the "p" is in front of. So to get the pKa, we take the -log of the Ka value. As an example, let's say Acid A has a Ka value of 1.0 x 10^-4 and Acid B has a Ka value of 1.0 x 10^-8. Based on our discussion of Ka values, we know that Acid A is the stronger acid because it has a higher Ka value. However, when we take the -log of the Ka values to get the pKa values, we find that the pKa of Acid A is 4 and the pKa of Acid B is 8. We know that Acid A is the stronger acid, so lower pKa values must indicate stronger acids. I hope this helps!

Madison Wong 3H
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Re: pKa

Postby Madison Wong 3H » Mon Jan 18, 2021 1:52 pm

Since pKA is just the negative log base 10 of Ka, it helps to write out what the equation of Ka is: products over reactants. As stronger acids and bases dissociate more, creating more product, higher Ka values indicate stronger acids and bases. Once you take the negative log base 10, higher Ka values become smaller pKA values.

sophie esherick 3H
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Re: pKa

Postby sophie esherick 3H » Mon Jan 18, 2021 2:01 pm

Ka, as explained in the lecture is the acidity constant of an acid. The value of Ka is given by the expression [products]/[reactants] and is used to show the amount of product formed in relation to the reactant. The larger this value is, the stronger the acid since we know that the larger the value, the more product there is (more H3O+ ions formed). The pKa is just the -log (base 10) of the Ka value. Similar to pH, the more an acid dissociates to form hydronium ions, the lower the pH will be. As the Ka values for different acids increase, their hydronium ion concentration also increases. With higher hydronium ion concentrations, their corresponding pKa values will be lower (higher values of Ka become smaller pKa values).

Olivia Monroy 1A
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Re: pKa

Postby Olivia Monroy 1A » Mon Jan 18, 2021 2:18 pm

Ka= products/reactants, a stronger acid almost 100% dissociates so the concentration of the product (Hydronium/conj acid of a base) is much higher than the reactants, meaning Ka will be a larger value (less negative often). pKa simply means -log base 10 of Ka so larger/less negative Ka values have lower pKa values and smaller Ka values have larger pKa values. 1x10^-2 has pKa=2 vs 1x10^-5 has pKa= 5. Bigger Ka (1x10^-2), smaller pKa (2), the stronger the acid because for a strong acid more products are created than reactants so Ka would be bigger (products/reactants)

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