Lyophilic and Lyophobic Sols

Transcript

0:00Introduction

Hello everyone! Let us start with a small lab puzzle. Picture two beakers. One holds a starch sol, and the other holds a deep red ferric hydroxide sol. We evaporate both to dryness, and then we add water again. The starch sol comes right back, but the red sol never does. So why does one sol come back, and the other not? By the end of this lesson, you will know exactly why. Sols come up often in JEE practical chemistry. So, let's begin.

0:33What is a sol?

So, first things first. We call a colloidal dispersion of a solid in a liquid a sol: tiny solid particles spread all through a liquid. And they really are tiny, between about one nanometre and one thousand nanometres; the n m on screen is the short form for nanometre. Okay? Now, sols come in two kinds. In a lyophilic sol, the particles have a strong affinity for the medium, the liquid around them. So lyophilic means solvent-loving. The other kind is called lyophobic. Now let me ask you this: what would lyophobic mean? Yes, exactly, it means solvent-hating. In a lyophobic sol, the particles have little affinity for the medium. Keep these two words in mind. Clear?

1:17Examples, and how each kind is made

Now let us meet the two families. The lyophilic family is starch, gum, gelatin and egg albumin. Okay? Making these sols is easy: we simply mix them directly with water. The word warm is in brackets because some, like gum, need warm water. Right? Starch needs a little care. First make a paste of starch in cold water, and then pour this paste into boiling water. Now, egg albumin is next. Should we use boiling water for it as well? That would be a mistake, because heat coagulates egg albumin. So egg albumin is simply shaken with cold water. Got it? Now the lyophobic family: ferric hydroxide, aluminium hydroxide, arsenious sulphide, and even gold. In the two hydroxides, the O H group sits in round brackets, taken three times; the small numbers below the symbols are subscripts, and they count atoms. With me? These are made by chemical methods, such as hydrolysis and double decomposition. In a moment, we will make two of them ourselves. Okay?

2:26Stability, electrolytes and reversibility

Now, why is a lyophilic sol so stable? Its particles are solvated: each particle is covered by a layer of solvent, which keeps it apart from the others. That is why a lyophilic sol is stable. Right? Now, a lyophobic particle has no such layer. Instead, its particles carry like charges, and like charges repel, so the particles stay apart. So a lyophobic sol is less stable; only its charge holds it up. Now tell me, what will a small amount of electrolyte do to such a sol? Yes, exactly, even a little electrolyte coagulates a lyophobic sol, which means the particles come together. Okay? But lyophilic sols, with their layer of solvent, need a large amount of electrolyte before they coagulate. Clear? And now, back to our opening puzzle. Lyophilic sols are reversible: dry one, add water, and the sol forms again. Lyophobic sols are irreversible. So the starch sol came back, and the ferric hydroxide sol did not. That solves our puzzle. Got it?

3:30Making ferric hydroxide sol

Now let us make a lyophobic sol ourselves, ferric hydroxide. Add a little ferric chloride solution, F e C l three on screen, drop by drop, to boiling water. Okay? Here is the reaction. Ferric chloride and three molecules of water give ferric hydroxide and three molecules of hydrochloric acid; the arrow means gives. This reaction with water is a hydrolysis. Now, the solution around the particles still has ferric ions. So what charge will they carry? They carry a positive charge. We get a deep red sol, and its particles adsorb F e three plus ions; the raised three plus is the charge on the iron ion. Remember this, because example two needs it. With me?

4:16Making arsenious sulphide sol

Now the second lyophobic sol, arsenious sulphide. We pass hydrogen sulphide, H two S, through a dilute solution of arsenious oxide, the oxide with two arsenic and three oxygen atoms. Okay? Arsenious oxide and three molecules of hydrogen sulphide give arsenious sulphide and three molecules of water. Right? This time we get a yellow sol. Its particles adsorb sulphide ions, so this sol is negatively charged. So, ferric hydroxide is positive, and arsenious sulphide is negative; write that down.

4:47Protecting the sol, and the Hardy–Schulze rule

Clear? Both reactions leave an electrolyte behind: hydrochloric acid, H C l, in the first, and excess hydrogen sulphide in the second. Remember what a little electrolyte does to a lyophobic sol. So why must we get rid of it? Yes, exactly, it would coagulate the sol we just made. So it is removed by dialysis or by boiling. And for the same reason, all the apparatus is cleaned, and only distilled water is used. Got it? Now, the rule for which ion does the coagulating is the Hardy Schulze rule. The ion with a charge opposite to that of the sol causes coagulation. So a positive sol is coagulated by negative ions, and a negative sol by positive ions. Okay? And the coagulating power of that ion rises steeply with its valency, that is, with the size of its charge.

5:39The two kinds of sol, side by side

Right? Now let us put it all together in one picture. In the middle is a starch particle, the lyophilic kind. The dashed circle of H two O around it, the small two being a subscript, is its layer of solvent. That layer keeps the sol stable, and that is why it needs much electrolyte to coagulate, and why it is reversible. Okay? On the left is a ferric hydroxide particle, the lyophobic kind. The plus signs right around it are the adsorbed F e three plus ions you see in the label below, so the particles carry like charges and repel each other. The minus signs further out are the oppositely charged ions in the liquid. And the label reminds you: a little electrolyte coagulates it, and it is irreversible. With me? And the box on the right sums up the preparations. For the lyophilic sols, we warm or shake the substance with water. For the lyophobic ones, there are our two reactions, with the word boiling written over the first arrow, and then dialysis to remove the electrolyte.

6:38Example 1: classify five sols as lyophilic or lyophobic

Clear? Alright, example one, an easy one. Classify as lyophilic or lyophobic: starch sol, ferric hydroxide sol, gum sol, arsenious sulphide sol, and egg albumin sol. Pause the video and try it first. Done? Okay, let's check. Here is a quick trick: ask how each sol is made. Starch, gum and egg albumin are made by direct mixing with water, so all three are lyophilic. Okay? Ferric hydroxide and arsenious sulphide are made by chemical methods, exactly as we saw today, so both are lyophobic. Very good if you got all five! Clear?

7:21Example 2: coagulating power of KCl, K2SO4, ferrocyanide

Now example two, a little harder. Arrange potassium chloride, potassium sulphate and potassium ferrocyanide in order of their power to coagulate ferric hydroxide sol. In the ferrocyanide, four potassium ions go with one complex ion in square brackets: iron with six cyanide groups, C N, in round brackets. Pause the video and try it first. Now, many students look at the potassium first. All three salts give the same potassium ion. So all three should be equally good, right? That is exactly the trap, because the ferric hydroxide sol is positive. By the Hardy Schulze rule, the potassium ion carries the same charge as the sol, so it is not the one that coagulates; the anion is. Okay? Now the anions. Chloride, C l minus, has valency one. Sulphate, S O four, with a charge of two minus, has valency two. And ferrocyanide, with the raised four minus outside its square brackets, has valency four. With me? Coagulating power rises steeply with valency, so ferrocyanide comes first. The order is potassium ferrocyanide, greater than potassium sulphate, greater than potassium chloride; the sign on screen means greater than. Very good if you got that! So always check the charge of the sol first. Clear?

8:45Quick revision: lyophilic vs lyophobic, Hardy-Schulze rule

So, quick revision. Lyophilic sols are made by direct mixing; they are stable, need a lot of electrolyte to coagulate, and are reversible. Lyophobic sols are made by chemical methods; held up only by their charge, they coagulate with a little electrolyte, and they are irreversible. And by the Hardy Schulze rule, the ion opposite in charge to the sol coagulates it, and its power rises steeply with its valency. Now go and try the four practice problems on this page. All the best!