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12 October 2009 Recitation _Equilibrium_

12 October 2009 Recitation _Equilibrium_ - Equilibrium...

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Equilibrium Fundamentals and Concepts Physical Equilibrium The best example of physical equilibrium is vapor pressure as detailed previously. With vapor pressure, at equilibrium, the rate of condensation is equal to the rate of vaporization. The conversion of liquid to gas and gas to liquid does not stop at a given time, but continues with equal conversions occurring; thereby yielding no net change in the number of moles of liquid and gas. Equilibrium is a dynamic process—in other words, the reaction does not stop. Equal rate does not imply equal amounts. For example, at 30 o C, we will have more liquid water than water vapor. Conversely, at 90 o C, we will have more water vapor than liquid water. Equal rate implies that the amount converting between the two phases is constant, thus, the concentrations of the forward and reverse processes do NOT change. What influences equilibrium? Time Some systems develop a rapid equilibrium (as with volatile materials such as hexane which has weak intermolecular attractions); however, equilibrium is gradual for other materials such as water which is not volatile. Temperature If we increase the temperature, the equilibrium will change. In the case of vapor pressure, we will form more gaseous molecules. Time and temperature are the only factors that influence equilibrium. However, temperature is the ONLY factor that will influence the magnitude or direction of equilibrium. Time is only included because some systems reach equilibrium quickly while others are more stagnant. Chemical Equilibria The idea of chemical equilibria is essentially the same as that of physical equilibria, but the equilibrium exists between two chemical reactions and not states of matter. Example: N 2 O 4 (g) 2 NO 2 (g) The following table represents the evolution of an equilibrium process for this reaction: Reactant Initial Time T2 (8 minutes) T3 (20 minutes) T4 (45 minutes) N 2 O 4 11 10 9 9 NO 2 1 3 5 5 **The numbers in the table represent the number of molecules observed at a given time period. There is no net change in the number molecules from T3 to T4. This indicates that chemical equilibrium has been reached at some point between T3 and T4. At the point of chemical equilibrium, there is no change in the number of reactant and product molecules. However, it is important to emphasize that the 5 NO 2 molecules observed at T3 may NOT necessarily
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be the same 5 NO 2 molecules that are observed in T4 because the process is dynamic. The reaction continues at equilibrium and the decomposition of N 2 O 4 continues and the reformation of N 2 O 4 from NO 2 continues; hence, we CANNOT track molecules in equilibrium. At the point of equilibrium, there are no macroscopic changes (i.e. color changes, etc.). Therefore, the reaction does appear to have stopped but on the microscopic level, the reaction is dynamic. For example, the container at the initial temperature is yellow. At T1, the container is a darker yellow, while at T3 and T4 the container is red and there are no further changes observed (no macroscopic changes).
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12 October 2009 Recitation _Equilibrium_ - Equilibrium...

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