This makes sense, right? It makes sense that HF is more electronegative than H 2O, NH 3, and CH 4 due to the greater electronegativity of fluorine versus oxygen, nitrogen, and carbon. Here’s the first point: acidity increases as we go across a row in the periodic table. This point causes a lot of confusion due to the presence of two seemingly conflicting trends. While far from impossible, forming the dianion can be difficult due to the buildup of negative charge and the corresponding electronic repulsions that result. Note that once a conjugate base (B-) is negative, a second deprotonation will make the dianion (B 2-). This is, of course, easiest to do when an atom bears a charge of +1 in the first place, and becomes progressively more difficult as the overall charge becomes negative. Removal of a proton, H+, decreases the formal charge on an atom or molecule by one unit. With that out of the way, let’s get started. Put another way, strong acids have weak conjugate bases, and vice versa. Therefore pK a is also a measure of how stable the conjugate base is. According to this, anything which stabilizes the conjugate base will increase the acidity.That’s because pKa is based on the equilibrium: Quick quiz: is pH 1 acidic or basic? pK a is similar to pH in that low (and even negative values) denote strong acids.Conversely, conjugate acids are what you obtain when you add a proton to a compound. For instance, HO – is the conjugate base of water. A conjugate base is what you obtain when you remove a proton (H+) from a compound.Converse: Brønsted base = proton acceptor, Lewis base = electron pair donor. Bronsted acids are proton donors, Lewis acids are electron pair acceptors.Let’s quickly review the basics of acidity and basicity. I’ve written in schoolmarmish tones before about how pK a is one of the most important measures you can learn in organic chemistry, and not knowing some basic pK a values before an exam is a lot like walking up to a poker table without knowing the values of the hands: you’re going to lose your shirt. Today we’ll talk about what’s behind the trends in acidity for different molecules and discuss the most important factors that determine these values.
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