Polyprotic Acids and the Strength of Acids

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Hydrogen sulphide in water holds a tiny amount of sulphide ion. Add hydrochloric acid, zero point one molar, and the sulphide ion concentration falls a million times. One million times, just from adding an acid? How? Chemists use this trick in group two analysis. Today we find out how, and then what makes one acid stronger than another. Both come up often in JEE, so let's begin.

0:33Polyprotic acids give their protons in steps

So, a polyprotic acid has more than one H plus to give: hydrogen sulphide, H two S, carbonic acid, H two C O three, and phosphoric acid, H three P O four. The small numbers are subscripts; the plus up top is a positive charge. Okay? They give their protons one at a time. For a general acid, H two A, step one gives H plus and H A minus; the double half arrow means an equilibrium. Its constant is K a one, the a one written small, as a subscript. With me? Step two: H A minus gives one more H plus, and A two minus, charge minus two. Its constant is K a two. Right? Now the key fact: K a one is much, much greater than K a two, which is much greater than K a three, if there is a third step; that is the double greater than sign. So, where does almost all the H plus come from? The first step, yes, exactly! The second adds too little to matter for the p H, okay? So, careful: do not add a second H plus for every molecule. For the p H, use just K a one. Clear?

1:38The acid alone in water: two shortcuts

Now, H two A alone in water. Step one makes H plus and H A minus one for one, so both are about the square root of K a one times c, the acid's concentration. Square brackets mean concentration; the wavy equals sign, approximately. This needs alpha, the degree of ionisation, to be about five percent or less, the wavy less than sign. Okay? So, how much A two minus? Write K a two: H plus times A two minus, over H A minus. Right? Now, H plus on top and H A minus below are almost equal. So what happens? They cancel, yes, exactly! So A two minus is about K a two itself, no c anywhere. But only for the acid alone in water. Got it?

2:24Add a strong acid: the common ion

Now add a strong acid, H C l. It ionises fully, flooding the solution with H plus, the common ion. Now H C l sets the H plus, not our weak acid. With me? Rearrange K a one: H A minus equals K a one times H two A, over H plus. Okay? Multiply K a one and K a two: A two minus equals K a one times K a two times H two A, over H plus squared. Now think. If H plus goes up, what happens to A two minus? It falls fast, because H plus is squared! Double the H plus, and A two minus becomes one fourth. That is how H C l controls the sulphide ion, S two minus, in group two analysis; the roman two means group two. The secret of our puzzle! Right?

3:13What makes an acid strong? The bond to hydrogen

Now, what makes an acid strong? Take hydrogen fluoride, chloride, bromide and iodide. Fluorine is the most electronegative element. So hydrogen fluoride must be the strongest acid, right? No, no, careful! It is the weakest. Down a group, the H X bond, the dash between hydrogen and the halogen X, gets longer and weaker, so the proton comes off more easily. So, H F, less than H C l, less than H B r, less than H I, okay? Same in oxygen's group: water, less than H two S, less than H two S e, less than H two T e. Right? But across a period, X gets more electronegative, and the anion left behind is more stable. So methane, less than ammonia, less than water, less than hydrogen fluoride, now the strongest! Clear? So remember: down a group, bond strength wins; across a period, electronegativity wins. With me?

4:10Oxyacids, chloroacetic acids and phosphorus acids

Now, oxyacids. More oxygen atoms on the central atom give a stronger acid: H C l O, less than H C l O two, then three, then four. Okay? And a more electronegative central atom helps too: HOI, less than HOB r, less than HOC l. Chlorine pulls electron density from the O H bond, stabilising the anion. Right? Same idea in carboxylic acids. Each chlorine on acetic acid withdraws electrons, the inductive effect, and stabilises the carboxylate ion. So acetic acid, C H three COOH, is the weakest; then one chlorine, two chlorines, three chlorines: stronger and stronger. Got it? Now, basicity: how many H plus one molecule can give. For phosphorus oxyacids, it is the number of POH groups. Phosphorous acid, H three P O three, has three hydrogens. So it is tribasic, right? Oh no, careful! One hydrogen sits on phosphorus, and P H hydrogens are not acidic. So the basicities are: H three P O four, three; H three P O three, only two; H three P O two, one. Clear?

5:19Example 1: pH, HS- and S2- in 0.10 M H2S

Now, Example one. Zero point one zero molar hydrogen sulphide; capital M means molar. K a one is one point zero times ten to the minus seven, and K a two is one point two times ten to the minus thirteen. Find the p H, and the H S minus and S two minus concentrations. Pause the video, and try it first. Done? Okay, let's check. H plus is about the square root of K a one times c. One point zero times ten to the minus seven, times zero point one zero, is ten to the minus eight. Its square root: one point zero times ten to the minus four molar. Okay? So the p H is four point zero. Check: alpha is ten to the minus four over zero point one zero, just zero point one percent. Shortcut safe. Right? H S minus comes one for one with H plus: also one point zero times ten to the minus four molar. And S two minus? Remember our shortcut? Yes, exactly! About K a two: one point two times ten to the minus thirteen molar. Keep it for the next one.

6:28Example 2: S2- in H2S when 0.10 M HCl is present

Now, Example two, our puzzle. Saturated hydrogen sulphide, zero point one zero molar, same constants, but also zero point one zero molar in H C l. Find the S two minus concentration. Pause and try it. Hint: who sets the H plus now? Done? Okay, let's check. So S two minus is just K a two again, one point two times ten to the minus thirteen, right? No, no, wait! That shortcut is for the acid alone. Here H C l sets the H plus at zero point one zero molar, and hydrogen sulphide hardly ionises, so it stays at zero point one zero molar. Okay? So, use the full formula, K a one times K a two times H two S, over H plus squared. K a one times K a two is one point two times ten to the minus twenty; times zero point one zero, over zero point one zero squared. Right? Work it out: one point two times ten to the minus nineteen molar. Ten to the minus thirteen has become ten to the minus nineteen, which is a million times less, just from adding H C l! Puzzle solved!

7:38Quick revision: polyprotic acids and acid strength order

So, quick revision. Polyprotic acids: the H plus comes from step one, and alone, A two minus is about K a two. With a strong acid, use K a one times K a two times H two A, over H plus squared. Down a group, bond strength decides; across a period, electronegativity. Phosphorus oxyacids: basicity is the number of POH groups. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!