Explain Equilibrium Directions: Complete the following assignment using any resources you have EXCEPT other people. This assignment will be graded as a take-home quiz. 1) A reversible reaction is shown below with its equilibrium constant, Kc, at 25 o C. 2 A(g) + B(g) 2 C(g) Kc > 1 at 25 o C (a) Two moles each of the gases A, B, and C are mixed in a rigid 1 L container and allowed to react. (i) Write an expression for Qc, and determine the value of Qc at the instant that the gases are mixed. (1 point) (ii) The value of Qc is observed to be greater than the value of Kc in this system at some point. Explain what this observation implies about the changes that will occur in the container after that point. (1 point) (iii) Compare the value of Qc determined in part (a)(i) with Kc. Explain what this comparison implies about the relative rates for the forward and reverse reactions, and describe how those rates will change with time. (1 point) (b) The particulate diagram to the right represents an equal number of moles of A and B molecules in the container, separated by a removable barrier. After the barrier is removed, the reaction proceeds to equilibrium, at which point the concentration of A has decreased from 0.40 M, immediately after the barrier was removed, to an equilibrium concentration of 0.10 M.
(i) Draw a particulate diagram, similar to the one above but with the barrier removed, that represents the concentrations of A, B, and C in the container at equilibrium. Explain the reasoning used to determine the concentrations. (1 point) (ii) Use your diagram to determine a value for Kc for this reaction. Justify your answer. (1 point) (c) Assume that the reaction described in part (b) takes about 2 minutes to reach equilibrium. Sketch a concentration vs. time graph showing the changes in the concentrations of A, B, and C over a span of 4 minutes. Use values of 0 to 4 minutes for the x-axis and 0 to 0.4 M for the y-axis of your graph. Explain the relationships among the changes in concentration for each species in the reaction. (2 points)
(d) Another reaction, W + Y 2 Z has Kc = 2.5 x 10-9 at 25 o C, but at a higher temperature, T2, the value of Kc for this reaction is reduced to 8.0 x 10-14. (i) Is this reaction, as written, endothermic or exothermic? Explain your reasoning. (1 point) (ii) If 0.20 mol of W is mixed with 0.10 mol of Y in a 1.0 L container at temperature T2, what will be the concentrations of W, Y, and Z at equilibrium? Justify your response. (2 points) 2) Solid ammonium chloride (NH4Cl) placed in a sealed, evacuated container decomposes until it reaches equilibrium, as shown in the reaction below. NH4Cl(s) NH3(g) + HCl(g) ΔH o = +176 kj/molrxn at 25 o C (a) Write an expression for Qp, and explain how Qp can be used to predict whether a system is at equilibrium. (1 point)
(b) Explain how each of the following changes affects the equilibrium of the reaction, including any changes to the values of Q and K for the reaction. (i) Adding a small amount of NH4Cl(s). (1 point) (ii) Decreasing the volume. (1 point) (iii) Increasing the temperature. (1 point) 3) One of the simplest equilibrium systems occurs when a compound can switch between two isomers, such as with cis-2-pentene and trans-2-pentene. This reaction is shown in the equation to the right. At 127 o C, the isomers are gases. A sample of pure cis- 2-pentene is placed in a rigid, previously evacuated container and allowed to come to equilibrium. The concentrations of the two isomers versus time are shown in the graph to the right. (a) Based on the graph, determine the value of Kc for the reaction, and justify your answer. (1 point)
The reaction is started at a different temperature with another pure sample of cis-2-pentene. The changes in concentrations as the reaction proceeds to equilibrium can be represented by the particulate diagrams below. (b) Explain how the forward and reverse reaction rates change as the system moves from the start to equilibrium. (1 point) (c) Draw a concentration vs. time graph with two curves that represent the changes in the concentrations of cis-2-pentene and trans-2-pentene from the start to equilibrium, as shown in the particulate diagram above, given an initial concentration of 0.10 M cis-2-pentene. Explain your reasoning for each curve. (2 points)