About this video
This video reviews acid strength using equilibrium expressions and the acid dissociation constant Ka. Three weak acids are compared: hydrofluoric acid, acetic acid, and methanol. Because a larger Ka means a stronger acid, hydrofluoric acid is the strongest. The video then converts each Ka to a pKa using the negative log, and shows that a lower pKa means a more acidic acid.
You'll be able to
- write an equilibrium expression for a weak acid reacting with water and identify its conjugate base.
- rank weak acids by comparing their Ka values, where a larger Ka means a stronger acid.
- calculate pKa from Ka and explain why a lower pKa means a more acidic acid.
Key words
- Weak acid
- An acid that only partly donates protons to water, so its Ka is much less than one.
- Ka
- The acid ionization constant, which measures how far a weak acid dissociates in water.
- Conjugate base
- The species left after an acid donates a proton, such as A- from HA.
- pKa
- The negative logarithm of Ka, where a lower value means a stronger acid.
Transcript
- We've already talked about how to write an equilibrium expression, so if we have some generic acid HA that donates a proton to H2O, H2O becomes H3O+ and HA turns into the conjugate base, which is A-. Here's our equilibrium expression and the ionization constant Ka for a weak acid.
We already talked about the fact that it's going to be less than one. Here we have three weak acids: hydrofluoric acid, acetic acid, and methanol. Over here are the Ka values. You can see that hydrofluoric acid has the largest Ka value, so even though they're all considered to be weak acids, 3.5 times 10 to the negative 4 is larger than 1.8 times 10 to the negative 5.
So hydrofluoric acid is stronger than acetic acid, and acetic acid is stronger than methanol. But again, they're all considered to be weak acids, relative to the stronger ones. Let's talk about pKas. The pKa is defined as the negative log of the Ka. If we wanted to find the pKa for methanol, all we have to do is take the Ka and take the negative log of it.
So the pKa is equal to the negative log of 2.9 times 10 to the negative 16. Let's get out the calculator and let's do that. Negative log of 2.9 times 10 to the negative 16. This gives us 15.54 when we round that. So the pKa of methanol is equal to 15.54. We could write in a pKa column right here, and for methanol it's 15.54.
If you did the same calculation for acetic acid, you would get 4.74; and once again, if you did this for hydrofluoric acid, you would get 3.46. So as we go up on our table here, we're increasing in acid strength. Out of our three weak acids, hydrofluoric acid is the strongest, so it has the largest value for Ka, but notice it has the smallest value for the pKa.
The lower the value for pKa, the more acidic your acid. 3.46 is lower than 4.74, and so hydrofluoric acid is more acidic than acetic acid.
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