Acid Strength: the Ideas

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Ethyne is just carbon and hydrogen, so nobody calls it an acid, right? Yet sodamide, a base, pulls a proton off ethyne completely, while sodium hydroxide gives no reaction at all. Why does one base work, and the other fail? One small subtraction will settle it. This comes up often in JEE, so let's begin.

0:25Measuring acid strength: Ka and pKa

So, how do we measure acid strength? Any acid, H A, gives up a proton, H plus, leaving A minus, its conjugate base; the small signs written high are charges, and the two half arrows mean equilibrium. Its equilibrium constant is K a, where the small letter written low stands for acid. K a equals H plus times A minus, over H A, where square brackets mean concentration. Okay? Next, p K a equals minus log of K a, with log to the base ten. So the small p just means minus log. With me? Now tell me this. The stronger the acid, the larger its K a. So is its p K a larger, or smaller? It is smaller, yes, exactly! Because of that minus sign, a larger K a means a smaller p K a. And one unit of p K a is a factor of ten in strength. Clear?

1:25The master rule: look at the conjugate base

Now, why is one acid stronger than another? When H A gives away its proton, what is left behind? The conjugate base, A minus, yes! So the master rule is: the more stable the conjugate base, the stronger the acid, and the double arrow means implies. Spreading out the negative charge stabilises it, and we will take four ways of doing that, one by one. Okay? Number one, the atom carrying the charge. Across a period, acidity rises with electronegativity, from methane, C H four, through ammonia and water, up to H F, the most acidic. The small numbers written low are subscripts, and each less than sign means less acidic than. Right? Now go down the halogen group. Fluorine is the most electronegative, so H F should be the strongest acid of its group too, shouldn't it? Oh no, careful, it is the other way round! Down a group, acidity rises with size: the larger atom spreads the charge over a bigger volume, so H I is the strongest. Likewise, RSH beats ROH, where R is an alkyl group and S is sulphur. Got it?

2:38Hybridisation, inductive effect and resonance

Number two, hybridisation: more s character holds the electron pair better. So ethane, with s p three carbons, is the least acidic, then ethene, s p two, and ethyne, s p, is the most acidic. The small numbers written high are superscripts. Okay? Number three, the inductive effect. Minus I groups, with I for inductive, pull electrons away and stabilise the anion, so the acid gets stronger. Plus I groups destabilise it. With me? And number four, resonance. In the carboxylate ion, RCOO minus, two equivalent structures share the charge between two oxygens. The phenoxide ion, C six H five O minus, spreads it into the benzene ring, and the alkoxide, R O minus, has no resonance at all. Here each greater than sign means more stable than. Clear?

3:36The order of acidity

Now let us see it all on one ladder, the course's figure: a p K a scale from zero to fifty. The arrow pointing left means stronger acid; the one pointing right, stronger conjugate base. Picric acid, at zero point four, is the strongest here. Okay? So here is the order of acidity, strongest first: sulphonic acids, then carboxylic acids, then carbonic acid, then phenol, then water. Each greater than sign means more acidic than. With me? Then alcohols, then ethyne, where the three lines are a triple bond, then ammonia, then ethene, with its double bond, and last, methane. See, ethyne, a hydrocarbon, beats ammonia; that is the s character at work. Right?

4:26Three useful tests

This order gives us three useful tests. The first uses sodium hydrogen carbonate: only an acid stronger than carbonic acid liberates carbon dioxide from it. Okay? So tell me, phenol is an acid. Does it give carbon dioxide with sodium hydrogen carbonate? No, it does not, and this trap catches many students! Phenol is weaker than carbonic acid. Carboxylic and sulphonic acids, two, four dinitrophenol and picric acid all do. Clear? The second test uses aqueous sodium hydroxide. Phenols and carboxylic acids react with it and dissolve as their sodium salts, but alcohols are too weak as acids to react. Right? And the third test is sodium metal: alcohols, phenols, acids and terminal alkynes all liberate hydrogen gas with it. Got it?

5:17Which way does an acid–base reaction go?

Now the last idea, which settles our puzzle. When an acid H A meets a base B minus, the proton moves, giving A minus and a new acid, H B, on the right. The equilibrium lies on the side of the weaker acid and weaker base. Okay? And here is the shortcut: K equals ten to the power, p K a of H B, minus p K a of H A, where the brackets show which acid each p K a belongs to. With me? Now think. If H A is the stronger acid, its p K a is the smaller one. So is this power positive, or negative? It is positive, very good! So K is large, and the reaction goes forward whenever H A is the stronger acid of the two. Clear?

6:04Example 1: pKa 3.75 vs 4.76, which acid is stronger

Alright, example one. Acid X has a p K a of three point seven five, and acid Y has four point seven six. Which is stronger, and by about what factor? Pause the video and try it first. Done? Okay, let's check. Four point seven six is the bigger number, so acid Y must be the stronger one, right? Oh no, careful, remember the minus sign! The smaller p K a means the stronger acid, so acid X is stronger. Okay? Now the factor. Each unit is a factor of ten, so we raise ten to the difference. Four point seven six minus three point seven five is one point zero one, and ten to the power one point zero one is about ten; the wavy equals sign means approximately. So acid X is about ten times stronger.

6:58Example 2: ethyne with NaNH2 and NaOH, find K

Now example two, our puzzle from the start. Ethyne, with a p K a of twenty five, is treated with sodamide, N a N H two, and with sodium hydroxide. Ammonia has a p K a of thirty eight, and water, fifteen point seven. Find the equilibrium constant in each case. Pause the video and try it. Done? Okay, let's check part A first. The amide ion, N H two minus, takes a proton from ethyne, giving the acetylide ion, H C triple bond C minus, and ammonia. So K equals ten to the power, thirty eight minus twenty five, which is ten to the power thirteen. That is huge, so the reaction is complete. Right? Now for part B, the base is hydroxide, O H minus, and the acid formed would be water. Before you calculate, will this one work? It will not, and here is why. K equals ten to the power, fifteen point seven minus twenty five, which is ten to the power minus nine point three. That is tiny, so there is no reaction; water is a much stronger acid than ethyne. So our puzzle is solved with one subtraction. See how useful p K a is?

8:19Quick revision: pKa, conjugate base, reaction direction

So, quick revision. The stronger acid has the larger K a, the smaller p K a, and one unit is a factor of ten. The more stable the conjugate base, the stronger the acid: check the atom, hybridisation, the inductive effect and resonance. And the equilibrium lies on the weaker acid side, so subtract the p K a values. Now try the practice problems on this page. All the best!