{"id":5056,"date":"2012-05-09T18:43:20","date_gmt":"2012-05-09T18:43:20","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=5056"},"modified":"2026-05-07T09:00:48","modified_gmt":"2026-05-07T14:00:48","slug":"acid-base-reactions-ka-and-pka","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2012\/05\/09\/acid-base-reactions-ka-and-pka\/","title":{"rendered":"Acid-Base Reactions: Introducing Ka and pKa"},"content":{"rendered":"

The Acidity Constant Ka<\/sub> (And Its Negative Logarithm pKa <\/sub>) Is An Extremely Useful Measure Of Acidity<\/strong><\/p>\n

So last time we went through all the different trends that affect acidity<\/a>. The bottom line is that any factor which stabilizes the conjugate base will result in increased acidity.\u00a0<\/strong><\/p>\n

Now, it’s great to know trends.\u00a0 It’s extremely important, in fact. (See article: Organic Chemistry Study Tips – Learn the Trends<\/a><\/em><\/span>)<\/p>\n

But what do you do when you want to compare the acidity of two molecules that aren’t connected by a trend?<\/p>\n

For instance, resonance increases the acidity of alcohols, but by how much? More than you’d increase acidity by changing OH to SH? What if you added an electron withdrawing group?<\/p>\n

It’s impossible to sort out all the opposing variables this way.\u00a0Trends are only qualitative<\/strong> guides to this topic.<\/p>\n

In order to answer this question we’ll need some hard data, and there’s only one way to get it in organic chemistry – through measurement.<\/strong><\/p>\n

And that’s what the acidity constant Ka<\/sub> (and its negative logarithm, pKa<\/sub>) are for.<\/p>\n

\"-summary-pka<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. The Acidity Constant Ka<\/sub> Represents The Equilibrium Constant For Dissociation Of An Acid Into Its Conjugate Base And A Proton<\/a><\/li>\n
  2. Examples Of Acid Dissociation Constants Ka<\/sub> For A Strong Acid (HI), A Weak Acid (CH3<\/sub>OH), And An Extremely Weak Acid (CH4<\/sub>)<\/a><\/li>\n
  3. The Negative Logarithm Of Ka<\/sub>, “pKa<\/sub>” Is A Far More Convenient Measure Of Acidity Than Ka<\/sub><\/sub><\/a><\/li>\n
  4. The pKa<\/sub> Scale Encompasses Over Sixty Orders Of Magnitude<\/a><\/li>\n
  5. Notes<\/a><\/li>\n
  6. Quiz Yourself!<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. The Acidity Constant Ka<\/sub> Represents The Equilibrium Constant For Dissociation Of An Acid Into Its Conjugate Base And A Proton<\/h2>\n

    Let’s look at hydroiodic acid, \u00a0H\u2013I. In solution (let’s use water) \u00a0H\u2013I will protonate water to give H3<\/sub>O(+) and I(-). The reverse reaction also operates. [I (\u2013) reacts with H3<\/sub>O(+) to give H-I back]. So this reaction is an equilibrium.\u00a0<\/strong><\/p>\n

    \"equilibrium-between-plus-h2o-and-h3o-and-i-favors-creation-of-h3o-plus\"<\/p>\n

    Here’s the useful part. We can measure<\/strong> the equilibrium constant of this reaction, and that will tell us about the relative acidity of H-I. The specifics of how we can measure this are another story, but for our purposes, let me assure you that these types of equilibrium constants can be measured accurately.<\/p>\n

    For H-I, it turns out that the equilibrium constant for this reaction is about 10,000,000,000. That means for every molecule of HI, there are 10 billion molecules of I(-). In other words, H-I donates its proton very readily to give its conjugate base, which is a prime example of a strong acid.<\/p>\n

    \"for-reaction-of-hi-with-water-equilibrium-constant-is-about-1-x-10-to-10th-power-very-rapid-dissociation\"<\/p>\n

    So what? Well, we can do this not just for H-I, but for ANY species with a hydrogen. This equilibrium is the “ruler” by which we can measure any acid – by its equilibrium constant.\u00a0<\/strong>[Note 1<\/a>]<\/p>\n

    We call this equilibrium constant Ka<\/sub> – the “acidity constant”.<\/p>\n

    <\/a>2. Examples Of Acid Dissociation Constants Ka<\/sub> For A Strong Acid (HI), A Weak Acid (CH3<\/sub>OH), And An Extremely Weak Acid (CH4<\/sub>)<\/h2>\n

    Let’s look at 3 examples. Note that these are somewhat oversimplified in that the solvent has been left out.*<\/p>\n

    \"three<\/a><\/p>\n

    Now comparing acidity between numbers with lots of exponents after them is not the most convenient way to do things. So instead, we’ve taken to using a logarithmic scale. These are common – the Richter scale is logarithmic, for instance – an 8.0 magnitude quake is 10 times more powerful than a 7.0 magnitude quake.<\/p>\n

    <\/a>3. The Negative Logarithm Of Ka<\/sub>, “pKa<\/sub>” Is A Far More Convenient Measure Of Acidity Than Ka<\/sub><\/h2>\n

    For acidity, the number we use is called “pKa<\/sub>“. It’s obtained by taking the logarithm<\/strong> of the acidity constant Ka, and we arbitrarily decide to multiply it by negative one [the vast majority of Ka<\/sub> values are less than 1, and we’re generally more comfortable dealing with positive numbers].<\/em><\/span><\/p>\n

    \"origin-of-pka-term-is-negative-logarithm-of-ka-for-hi-pka-is-10-for-ch4-pka-is-50-calculation\"<\/p>\n

    So for H-I, the pKa<\/sub> is \u201310 (representing a very strong acid), that for methanol is 15 (a weak acid), and for methane, it’s 50 (an extremely weak acid).<\/p>\n

    <\/a>4. The pKa<\/sub> Scale Encompasses Over Sixty Orders Of Magnitude<\/h2>\n

    That scale comprises 60 orders of magnitude. That’s a huge number!!!!!<\/p>\n

    To give you an idea of the scale, of pKa<\/sub>, this is the range of the smallest value for length (the Planck length, 10\u201335<\/sup> meters), to the width of the known universe (about 1026<\/sup>\u00a0 meters). (See article: pKa Values Span 60 Orders of Magnitude<\/a><\/em><\/span>)<\/p>\n

    Anyhow, these measurements have been done for thousands of different molecules now. The result is a big table that allows us to compare the acidity of all kinds of different functional groups. Here’s an example of a pKa<\/sub> table from a previous post.<\/p>\n

    \"pka-table-with-strongest-acids-at-top-and-weakest-acids-at-bottom-showing-conjugate-bases\"<\/p>\n

     <\/p>\n

    The pKa<\/sub> table is your friend.<\/strong> In one document, it gives you information on the scope and magnitude of a wide range of chemical behavior – the strongest of the strong acids, and the weakest of the weak acids. And since the stronger the acid, the weaker the conjugate base, it also provides information about basicity.<\/p>\n

    Next Post: <\/strong>How to Use A pKa Table<\/a><\/p>\n

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    <\/a>Notes<\/strong><\/h2>\n

    Related Articles<\/strong><\/p>