Chapter 15: Chemical Equilibrium Notes
Chapter 15: Chemical Equilibrium Notes CHEM 1312
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This 8 page Class Notes was uploaded by Justin Sequerra on Friday February 12, 2016. The Class Notes belongs to CHEM 1312 at University of Texas at Dallas taught by Dr. Sibert in Winter 2016. Since its upload, it has received 190 views. For similar materials see General Chemistry II in Chemistry at University of Texas at Dallas.
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Date Created: 02/12/16
CHAPTER 15: CHEMICAL EQUILIBRIUM GENERAL CHEMISTRY II This general outline is meant as a supplement to the General Chemistry II (1312) course taught at the University of Texas at Dallas and should not be taken as a standalone study guide for the overall curriculum. However, I do hope that this broad summary of the textbook helps you all in becoming successful undergraduate students here at UTD. Justin Sequerra, “Chemistry is the study of matter. But I prefer to see it as the study of change.” – Walter White, Breaking Bad 1 CHEMICAL EQUILIBRIUM 15.1 THE CONCEPT OF EQUILIBRIUM Up until now, we have discussed chemical reactions as if they happen only once and have been completed after all the reactants have turned into the products. However, that is not the case because chemical reactions do not proceed in that manner, since reactions can go BOTH WAYS. As soon as the reactants create enough products, the products start to go into the reverse reaction, whereby the products become the reactants and the reactants become the products. This backandforth transition between reactants and products continues until the system acquires a Chemical or Dynamic Equilibrium. The process hasn’t stopped however, since the reaction is still occurring and the concentrations (of both products and reactants) have reached levels where they remain constant. The rates of the reverse and the forward reactions are equal, where one is producing or consuming as much as the other. (In reference to the figure): The reaction reaches equilibrium when the concentrations remain constant (when slope=0) and NOT at the point where the two concentrations are the same. 2 CHEMICAL EQUILIBRIUM 15.2 THE EQUILIBRIUM CONSTANT Kc is constant at a constant temperature, so if the temperature changes, the Kc changes. (I doubt there will be a question on the test asking you to find Kc at a different temperature than the one given; if they do, answer that you cannot perform the question since they changed temperature, unless it deals with Le Chateliers Principle). Since K remains constant, changing the initial concentrations will have NO EFFECT on it. A large equilibrium constant (Kc>10^2) indicates that the products will be favored at that equilibrium (the concentration of products will be greater than that of the reactants). A small equilibrium constant (Kc<10^2) shows that the reactants are favored at equilibrium (more concentration of reactants than products). Unlike Kc, as the reaction proceeds, Qc will always change since the concentrations of the species are also changing as they try to come to equilibrium, or rather as they try to make Qc=Kc. Figure: This is the general form of the equilibrium expression, however the units given are wrong (mol/L) 3 CHEMICAL EQUILIBRIUM 15.3 EQUILIBRIUM EXPRESSIONS When all of the species in a chemical reaction are not in the same phase, it is called a heterogeneous mixture, as opposed to a homogeneous one. Solids and liquids DO NOT appear in the equilibrium expression for a heterogeneous reaction. We do not include these states because their relative concentrations do not change over the course of the chemical reaction (if we double moles, we also double volume). When chemical equations that represent an equilibrium are reversed, multiplied, combined with other equations, or any combination of the ones mentioned, the corresponding changes must be made to the equilibrium constants. This information is summarized below in the figure Equilibrium expressions that contain only gases can be written as Kc expressions or as Kp expressions. Kp expressions have the same form as Kc expressions but contain partial p rather than molar concentrations. The same goes with Qc and Qk. However, aqueous solutions can only be used in Kc expression and NOT in Kp expressions. Kc and Kp are not usually equal (as stated in the 4 sidebar). They are ONLY the same when the reaction results in no net change in the number of moles of gas. CHEMICAL EQUILIBRIUM 15.5 CALCULATING EQUILIBRIUM CONCENTRATIONS Starting concentrations can be used, along with the equilibrium expression and equilibrium constant, to determine equilibrium concentrations Steps for these kinds of questions: 1. Construct ICE tables (initial [ ], change, and equilibrium [ ]) and fill in any numbers given (even zeros) 2. Calculate Q with the initial concentrations given so you can see which direction the reaction will proceed (compare to K, if Q>K=reactions goes left, and vice versa) 3. Define x as the amount of a particular species (the one getting consumed by the reaction), and then use the stoichiometric coefficients to define in terms of x the amount of change the other species will undergo 4. Add the initial [ ]s and the change in [ ]s to get the equilibrium concentrations. 5. Use the equilibrium constant and concentrations to solve for x (use QUADRATIC EQUATION, but if you find another way that is quicker (perfect square)USE IT) 6. PLUG AND CHUG ALWAYS MAKE SURE YOUR CHEMICAL EQUATIONS ARE BALANCED!!!!!!!! CHECK YOUR ANSWERS (if you have enough time 5 on the test)!!!! You can do this by plugging your calculated equilibrium concentrations into the equilibrium constant expression and compare the equilibrium constant you get (K) with the one given to you in the question. CHEMICAL EQUILIBRIUM 15.6 LE CHATELIER’S PRINCIPLE: FACTORS THAT AFFECT EQUILIBRIUM The stresses that can be applied to a system at equilibrium include the addition or removal of a substance, changes in the volume of the reaction vessel, and changes in the temperature. The summary of how the changes may affect the equilibrium is given in the figure below “Shifting” to the right=more products will be produced “Shifting” to the left=more reactants will be produced Once the reaction once again achieves equilibrium IT WILL HAVE THE SAME K AS BEFORE. The only thing that will change is the equilibrium positions ([ ]s) of the species involved in the reaction. Changing the equilibrium position of a solid or a liquid will NOT affect equilibrium since it is not involved in the reaction By adding an INERT GAS, the pressures will increase, but 6 since the volume remains constant, equilibrium is NOT disturbed CHEMICAL EQUILIBRIUM 7 *PRACTICE PROBLEMS* 1. In an analysis of the following reaction at 100°C, the equilibrium concentrations were found to be [Br2] = 2.3 × 10^−3 M, [Cl2] = 1.2 × 10^−2 M, and [BrCl] = 1.4 × 10^−2 M. Write the equilibrium expression, and calculate the equilibrium constant for this reaction at 100°C. 2. For the reaction, Kc is 2.3 × 10^−2 at 375°C. Calculate Kp for the reaction at this temperature. 3. The following reactions have the indicated equilibrium constants at a particular temperature: Determine the values of the equilibrium constants for the following equations at the same temperature: (a) , (b) , and (c) . 4. Determine the equilibrium partial pressures of H2, I2, and HI if we begin the experiment with 1.75 atm each of H2 and I2 at 430°C. 5. Indicate in which direction the following equilibrium will shift in response to each change: a) Addition of XY 2s), b) Removal of Y (2), c) Decrease in container volume, d) Addition of X(s).
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