Chapter 1: Chemical Equilibrium Part 7 SABIS Grade 11 (Level M) Chemistry

1.4 Film: Equilibrium (Part 1)

Figure 1.10 Iodine crystals.

1. Solid iodine dissolves in an ethanol-water solution to form a saturated solution.

Equilibrium is reached.

2. A closed system contains a fixed amount of material.

3. Two connecting bowls have a barrier between them and fish in one bowl. When the barrier is removed fish migrate in one direction only. Eventually equilibrium is established with: 

a) constant population on each side (constancy) 

b) equal rates of migration (dynamic equilibrium)

4. Two connected glass bulbs: (1) containing bromine gas, (2) containing a vacuum. When the interconnecting stop-clock is opened bromine gas escapes from (1) to (2). Equilibrium is recognized by (a) constant color and (b) constant pressure

5. Iodine dissolved in water actually reacts partially with it: 

3I2 (s)  +  3H2O(l)  ⇌  5I–(aq)    +   IO3–(aq)    +    6H+(aq) 

iodine  +   water    ⇌  iodide ions + iodate ions + hydrogen ions.

a) At equilibrium all reactants and products are present simultaneously.  Water is present: you can see the liquid solution. Iodine is present: you can see its brown color. I–(aq) can be shown to be present by adding lead (II) ions (Pb2+(aq)) which precipitate with the I–(aq) to form a yellow precipitate of lead (II) iodide: 

                                                         Pb2+(aq) + 2I–(aq)  ⇌  PbI2 (s)

b) Equilibrium can be shifted by removing H+(aq). H+ is removed by adding sodium hydroxide, where the hydroxide ions remove the H+ ions by reacting with them to form water: 

                                                              H+(aq) + OH–(aq)  →  H2O(l)

This is what is observed when drops of NaOH solution are added to the beaker.

6. Dynamic nature of equilibrium. To show that at equilibrium opposite reactions take place at equal rates we can do the following: 

a) Add black fish to one bowl. You see that the fish, swimming at random, may go to the other side. Fish may come back from the other side as well. At all times “population equilibrium” is maintained. 

b) Radioactive bromine is added to one of the two bulbs of bromine above. Eventually radioactivity will be detected on the other side as well. 

Since this means that radioactive bromine has moved to the other bulb, and since the pressure and color in the two bulbs remain the same, it follows that an equal amount of non-radioactive bromine moved back to the first bulb.

Thus at equilibrium the opposing reactions continue to take place, at all times, and at equal rates, producing no net change. 

c) Radioactive iodine crystals are added to the saturated iodine solution above. Eventually the solution as well as the crystals become radioactive. This shows that opposing reactions still occur at equilibrium.

Question

Choose the correct answer(s).

At equilibrium,

the forward reaction is complete.

opposite reactions take place at equal rates.

all reactants are consumed.

all reactants and products are present simultaneously.

answer :

all reactants and products are present simultaneously.

opposite reactions take place at equal rates.

Film: Equilibrium (Part 2)

7. Explanation using molecular models: An iodine crystal is a regular array or lattice of molecules and is surrounded by water and ethanol  molecules, which are in random orientation and constant motion. The iodine molecules in the crystal  are constantly vibrating. This kinetic energy, together with the pull of the liquid molecules allows some  of them to escape. At first the only process which takes place, is the I2 leaving the crystal and  becoming part of the solution. As the concentration of dissolved iodine increases, some molecules  find their way back to the crystal. When the concentration of I2 in solution is still low, the rate of dissolving is higher than the rate of crystallization. The rate at which molecules leave the crystal is constant at a fixed temperature. The rate at which molecules return to the crystal depends on the concentration of I2 in solution. As the concentration of dissolved iodine rises the rate of crystallization rises. In time the concentration  rises enough so that rate of crystallization equals rate of dissolving. At this point, there is no further change in macroscopic properties and the I2 concentration is constant. Equilibrium has been reached. There is a dynamic balance between the two opposing reactions, iodine dissolving and crystallizing:                                                                                I2(s)  ⇌  I2(aq) At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. The constant color of iodine does not mean the reaction has ceased.  I2 molecules are continually reacting with water to form I–, IO–3 and H+. I–, IO–3 and H+ are continually reacting to form water and I2 molecules. At equilibrium the two opposing rates become equal. On looking at the radioactive solution it can be seen that I– ions are also radioactive by precipitating lead iodide and washing the precipitate. The PbI2 (s) which is formed is also radioactive. (Radioactive iodine is marked by *):  I2*(s)   ⇌   I2* (aq)  3I*2 +  3H2O   ⇌   5I*– + I*O3– + 6H+  Pb2+ (aq)  + 2I*– (aq)   ⇌   PbI*2 (s)  Generalization:  a) We can recognize equilibrium by observation of constant macroscopic properties in a closed  system:  constant concentration, population, pressure, color, etc. b) We can explain equilibrium in terms of opposing reactions going at equal rates: dynamic equilibrium. What happens at equilibrium when dissolving a crystal of iodine? Choose the best answer. Equilibrium is recognized by

I2 molecules continue to react with water to form I–, IO3– and H+.  The color of the solution becomes constant.  The reaction ceases.  I–, IO3– and H+ continue to react to form water and I2 molecules.

constancy of macroscopic properties.

constancy of macroscopic properties in a closed system at constant temperature.

constancy of microscopic properties in a closed system at constant temperature.

opposite reactions that take place at different rates.

answer :

constancy of macroscopic properties in a closed system at constant temperature. Choose the best answer. Equilibrium is explained by  constancy of microscopic properties in a closed system at constant temperature. opposite reactions that take place at equal rates. constancy of macroscopic properties. constancy of macroscopic properties in a closed system at constant temperature. answer : opposite reactions that take place at equal rates.

Chapter 1: Chemical Equilibrium  Part 8 SABIS Grade 11 (Level M) Chemistry