Dynamic Equilibrium

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. I put ice and water together in a thermos flask, at two hundred and seventy three kelvin and one atmosphere. I come back much later. Same mass of ice, same mass of water. So, has everything inside that flask stopped? Most students say yes. Hold that thought; before this lesson ends, you will see why the answer is no. This idea, dynamic equilibrium, is the base of this whole unit, and in JEE it comes up often. So, let's begin.

0:36Equilibrium: forward rate equals backward rate

So, take a reversible process, one that can go both ways. Look at the left graph: concentration against time. The reactant, in dark blue, falls. The product, in green, rises. And after the dashed line, marked equilibrium reached, both curves go flat. Now tell me: why do they go flat? Good thinking! The answer is in the right graph. The forward rate, in orange, starts high and falls. The backward rate, in purple, starts at zero and rises. Where they meet, it says rates equal. That is equilibrium: in a closed system, where no matter enters or leaves, the forward rate equals the backward rate. Got it? Now, once the rates are equal, the concentration, the pressure, the colour, every property you can observe, stays constant. So here is the big question. Nothing is changing. Has the reaction stopped? No, no, not at all! At the molecular level, both processes continue, at equal rates. Molecules are used up and formed again all the time, so the net change is zero. That is why we say equilibrium is dynamic, not static. Clear?

1:47Proof: the deuterium experiment

Now, let me prove it with a classic experiment. Take an equilibrium mixture of N two, H two and N H three: nitrogen, hydrogen and ammonia. The small numbers are subscripts, okay? Replace some of the H two with D two, deuterium, the heavy isotope of hydrogen. Now think. If the reaction had really stopped, could any deuterium ever get into the ammonia? No, it could not. But look what happens! The overall composition does not change, and yet deuterium soon appears in ammonia, as N H two D, NHD two, and N D three. With me? So the reaction must still be running both ways: ammonia keeps forming, and keeps breaking up. Dynamic, proved. Very good!

2:33Characteristics of equilibrium

Alright, now the characteristics of equilibrium; keep the same graphs on the side. First: it is reached only in a closed system, at constant temperature; the capital T on screen stands for temperature. Okay? Second: it can be approached from either side, starting from the reactants, or starting from the products. Remember this one; our second example is built on it. Right? Third: a catalyst speeds up both directions equally. So equilibrium is reached sooner, but its position does not change. With me? Fourth, and this is a trap. At equilibrium the rates are equal, so the concentrations of reactants and products are equal too, right? Oh no, careful! Constant, not equal. Look at the left graph: after the dashed line, the product line is flat, but it stays well above the reactant line. Both are constant, and here they are not equal. In general, equal concentrations would be only a coincidence. Got it? And fifth: the equilibrium mixture is fixed by the temperature, the volume of the vessel, and the initial amounts. But the ratio capital K, the equilibrium constant you will meet in the next lesson, depends on temperature alone. Clear?

3:46Physical equilibria

Now, physical changes reach equilibrium too. See this symbol: two half arrows, one pointing each way. It means a reversible change at equilibrium. Okay? First, solid and liquid: ice and water at two hundred and seventy three kelvin and one atmosphere, the normal melting point. So, back to our thermos flask. What is equal inside it? Yes, exactly! The rate of melting equals the rate of freezing. Molecules keep passing between ice and water, but the masses of ice and water stay constant. So no, nothing had stopped. Puzzle solved! Second, liquid and vapour. In a closed vessel, the vapour pressure is constant at a given temperature. Right? Third, solid and vapour: the sublimation of iodine, camphor or naphthalene, again in a closed vessel. With me? Fourth, solid and solution: a saturated solution in contact with undissolved solute. Okay? And fifth, gas and solution: C O two gas, the small g in brackets, and C O two dissolved in water, the a q in brackets for aqueous, in a sealed soda bottle. Clear?

4:55Example 1: which of four systems are at equilibrium?

Alright, example one. Which of these are at equilibrium? Option a: water evaporating from an open beaker. Option b: a saturated sugar solution, with undissolved sugar at the bottom. Option c: a sealed bottle of soda water. And option d: a burning candle. Pause the video, and try it. Done? Okay, let's check. Here is a topper's test: ask two questions. Is the system closed? And is the process reversible? Now, option a. Water and its vapour are both there, and liquid is turning into vapour. So it is at equilibrium, right? No, no, careful! The beaker is open, so the system is open, and the vapour escapes. It fails the very first question. Got it? Option b: yes! Dissolution and crystallisation go on at equal rates, just like the solid and solution case we saw. Okay? Option c: yes, again. C O two gas and dissolved C O two, at equilibrium in the sealed bottle. Right? And option d, the burning candle: an irreversible process, in an open system. It fails both questions. With me? So the answer is b and c. Very good!

6:12Example 2: H2 and I2 vs 2 mol HI, same equilibrium mix?

Now example two, a little harder. At a fixed temperature, one mole each of H two and I two, hydrogen and iodine, is heated in a one litre flask. In an identical flask, two moles of H I, hydrogen iodide, is heated. How do the two equilibrium mixtures compare? Pause the video, and try it first. Done? Okay, let's check. The first flask starts with no hydrogen iodide at all. The second starts with nothing but hydrogen iodide. So the two mixtures must end up different, right? No, no, wait! Count the atoms first. Flask one: one mole of H two gives two moles of hydrogen atoms, and one mole of I two gives two moles of iodine atoms. Okay? Flask two: two moles of H I also contain two moles of hydrogen atoms and two moles of iodine atoms. Exactly the same! With me? So both flasks hold the same atoms, in the same one litre volume, at the same temperature. And remember: equilibrium can be approached from either side, from the reactants or from the products. Clear? So the two equilibrium mixtures are identical. Different starting points, the same finish. Perfect!

7:30Quick revision: dynamic, closed, constant not equal

So, quick revision. Equilibrium: forward rate equals backward rate, in a closed system. It is dynamic: both processes go on, while the properties you can observe stay constant. Concentrations are constant, not equal; and a catalyst gets you there sooner, but to the same position. Equilibrium can be reached from either side, and capital K depends on temperature alone. Now go and try the four practice problems on this page. All the best!