Chapter 1: Chemical Equilibrium Part 3 SABIS Grade 11 (Level M) Chemistry

1.1.3 Equilibrium In Physical Changes

Solubility of Iodine

Figure 1.4 shows the addition of solid iodine to a mixture of water and alcohol. At first the liquid is colorless but very quickly a reddish color appears near the solid. Stirring the liquid causes swirls of the reddish color to move out—solid iodine is dissolving to become part of the liquid. Changes are evident: the liquid takes on an increasingly deep color and the pieces of solid iodine diminish in size as time passes. Finally, however, the color stops changing (see Figure 1.4). Solid is still present but the pieces of iodine no longer diminish in size. Since we can detect no more evidence of change, we say that the system is at equilibrium.

Vapor pressure

In the previous year, we have noted that if we place liquid water in a flask at 20°C and seal the flask, some water molecules leave the liquid and enter the gas phase. The partial pressure rises as more and more water molecules become part of the gas. Finally, however, the pressure stops rising and the partial pressure of water becomes constant. This partial pressure is the vapor pressure and equilibrium now exists. 

Question

Fill in the blank.

 is the pressure of the vapor present at equilibrium above a certain liquid in a closed flask.

answer :

Vapor pressure 1.1.4 Explaining Equilibrium:  The Dynamic Nature Of Equilibrium Yet, it is reasonable to suppose that water molecules from the liquid are still evaporating,  even at equilibrium. Molecules in the liquid have no way of “knowing” that the partial pressure of the vapor is equal to the vapor pressure. In the gas phase, the randomly moving molecules continue to strike the surface of the liquid and condense. Equilibrium corresponds to a perfect balance between this continuing evaporation and  condensation.Therefore, no net changes can be detected. When the partial pressure of the water equals the vapor pressure, the gas above the liquid  is said to be saturated. The word “saturated” has the same meaning as it did relative to  solubility: the gas phase contains as much water vapor as it can hold at equilibrium. Figure 1.5 shows this schematically. If the partial pressure of the vapor is less than the equilibrium value (as in Figure 1.5A), the rate of evaporation exceeds the rate of condensation until the partial pressure of the vapor equals the equilibrium vapor pressure.  If we inject an excess of vapor into the bottle (as in Figure 1.5C), condensation will proceed faster than evaporation until the excess of vapor has condensed. The equilibrium vapor pressure corresponds to that concentration of water vapor at which  condensation and evaporation occur at exactly the same rate (as in Figure 1.5B). At equilibrium, microscopic processes continue but in a balance that yields no macroscopic changes. Similarly, in the case of iodine dissolving in water, at equilibrium both dissolution of iodine  and precipitation of iodine take place at the same time, at equal rates. This can be proved by adding a few radioactive iodine crystals into the beaker. At first,  no radioactivity is detected in the solution. After some time, the solution becomes radioactive,  implying that some of the radioactive iodine dissolved. The intensity of the color, however remains  the same, which means that an equal amount of iodine must have crystallized out of solution.  At equilibrium, microscopic processes continue but in a balance that yields no macroscopic changes. Question True or false? "At equilibrium, all microscopic processes stop".  True  False answer : False Question In a chemical reaction that has attained a state of equilibrium.  the rate of the forward reaction is equal to the rate of the reverse reaction.  the rate of the forward reaction is greater than the rate of the reverse reaction.  the rate of the forward reaction is smaller than the rate of the reverse reaction.  the amount of reactants is equal to the amount of products. answer : the rate of the forward reaction is equal to the rate of the reverse reaction. Question In a chemical reaction, equilibria is referred to as "dynamic" because  the macroscopic changes are observed and the microscopic processes still exist.  no macroscopic change is observed but the microscopic processes still exist.  the opposite reaction stops occurring.  the opposite reactions still take place at equal rates.  macroscopic changes are observed but the microscopic processes stop. Answer no macroscopic change is observed but the microscopic processes still exist. the opposite reactions still take place at equal rates.

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