Go ahead and look at the first and easiest one and that's the element effect. The element effect determines how loosely or strongly a particular element bonds with hydrogen. We can use these effects to compare different protonated elements to each other. For example, a perfect example of using the element effect would be having a nitrogen attached to an H and a sulfur attached to the H. Do we know the pKa of the nitrogen? Actually, yes. I taught you that the pKa of NH3 should be around 38. What about SH2? I actually didn't teach you that one. So then in this case, if I were to ask you which one is the most, the strongest acid, you would have really no clue on how to tell me because I never gave you the pKa of SH2. So that's why we have to use these factors.
It turns out that the element effect is going to consist of two trends, and the first one is electronegativity. Electronegativity just says that the stronger the electronegativity, the more willing the molecule will be to accept a lone pair as the conjugate. Here's the electronegativity trend, let's say HF, which is an H attached to the most electronegative atom, and then we have also CH4 over here. After each of these molecules gives up a proton, H+, it's going to turn into F- for the fluorine. This is the conjugate base. Now let's look at the conjugate base for the carbon, which would look like this, CH3-. Which of these conjugate bases is the most stable? It would be the fluorine since it is the most electronegative, so it's more comfortable having electrons on it compared to carbon. Does that first trend make sense so far?
Now let's look at our periodic table here. We were comparing nitrogen and sulfur. Just with the electronegativity trend, the sulfur would be the better acid, as even though you don't know the pKa of the sulfur, you know that the sulfur is more electronegative.
However, there's another effect that we need to know which is the size trend. What the size trend says is that the bigger or the squishier the atom is, the more willing it will be to accept a lone pair. Imagine F- versus a much bigger atom, I-. Because iodine is much bigger and more diffused, it can distribute the electrons over a larger space, making it a more stable conjugate base. Therefore, the stronger acid is HI, not HF. Hopefully, that makes sense to you guys in terms of the element effect. Just keep in mind that the element effect only has to do with hydrogens that are directly attached to different atoms. Now, take a break and try to solve this one and predict which one is going to be more acidic.
