CHEM 162: Chapter 16 page 1 of 27 Chapter 16: Chemical Equilibrium End-of-Chapter Problems: 16.2-16.11, 16.13-16.14, 16.17-16.98, 16.119-16.122 Example: Ice melting is a dynamic process: H2O(s) H2O(l) Ex.1 This occurs at what temperatures under normal atmospheric conditions? _________ Ex. 2 If a glass at room temperature is filled with ice cubes then water is added, can the reverse process occur? Explain. Ex. 3 Enough ice cubes are added to half fill each of three plastic jars then enough water is added to cover the ice cubes. Each jar is capped to be airtight. The first jar is placed in a refrigerator set at 38°F, the second in a freezer set at 0°F, and the third in a refrigerator set at 32°F. Explain what you expect to find in each jar after 24 hours. 1. The jar kept at 38°F 2. The jar kept at 0°F 3. The jar kept at 32°F Reactants are not always converted to products in a chemical reaction. – When carrying out stoichiometry problems (e.g. “Calculate the mass of hydrogen gas produced when ….”), we have assumed that reactions always proceed to completion. In reality this is not always the case. – Depending on the reaction and the conditions, 1. In some reactions, all the reactants are converted to products.  The reaction proceeds essentially to completion.  The final composition consists mainly of products. 2. In some reactions, very little of the reactants are converted to products.  The reaction occurs only to a slight extent.  The final composition consists mainly of reactants. 3. In some reactions, some of the reactants are converted to products.  The reaction stops short of completion.  The final composition consists of appreciable amounts of reactants and products.CHEM 162: Chapter 16 page 2 of 27 Case #1: a. Complete the following: HCl(aq) + NaOH(aq)  b. When 10.00 mL of 1.00M hydrochloric acid are added to a flask containing 10.50 mL of 1.00M sodium hydroxide, The limiting reactant=____________, and the reactant in excess=_____________. c. Using phenolphthalein, how can you show that this reaction proceeds to completion—i.e., only products and the reactant in excess are present after the reagents mix? Case #2: About 95% of dry eggshells consist of calcium carbonate, which decomposes as follows, CaCO3(s)  CaO(s) + CO2(g) Given how quickly eggshells decompose at room temperature, to what extent does this reaction occur at room temperature? Case #3: Consider the following reaction between the hexaaquacobalt(II) ion and chloride ion to form the tetrachlorocobalt(II) ion: [Co(H2O)6]2+(aq) + 4 Cl(aq)  CoCl42(aq) + 6 H2O(l) pink blue At room temperature the equilibrium mixture is purple. To what extent does the reaction occur? Explain.  In both the ice-water mixture and the examples above, some or all of the reactants react to form products, and when enough products form, the reverse reaction occurs. These are examples of reversible reactions—i.e., both the forward and reverse reactions take place. Because the ice-water mixture and the reactions in Cases #2 and #3 above do not always go to completion, they are more correctly represented using a double-arrow ( ): H2O(l) H2O(s) CaCO3(s) CaO(s) + CO2(g) [Co(H2O)6]2+(aq) + 4 Cl(aq) CoCl42(aq) + 6 H2O(l)CHEM 162: Chapter 16 page 3 of 27 16.1 THE DYNAMICS OF CHEMICAL EQUILIBRIUM Traffic Analogy: Consider two island cities connected by two one-way bridges: – Cars are not allowed on Island #2 after 11pm, and the bridges are closed from 11pm to 6am.  When the bridges open at 6am, all of the cars are on Island #1. Ex. 1: Provide two explanations for the number of cars on each island not changing. Ex. 2 One day traffic on the two bridges is equal by noon, so as soon as a car gets off the bridge at Island #2, another car on Island #2 goes onto the bridge to Island #1. Question 1: Is the number of cars on Island #1 changing? Yes No Question 2: Is the number of cars on Island #2 changing? Yes No Question 3: If the number of cars on each island is not changing, does that mean traffic has stopped? Yes No Question 4: Do the number of cars on each island have to Yes No be equal for them to not be changing? Thus, in this example the islands have achieved a state of equilibrium, – The rate of traffic to Island #2 = the rate of traffic to Island #1. – The # of cars on each island are not changing with time, but they need not be equal to one another. Any chemical reaction in a closed vessel will eventually achieve chemical equilibrium—a state in the concentrations of all reactants and products remain constant with time.  At equilibrium, the rates of the forward and reverse reactions are equal. Island #1Island #2CHEM 162: Chapter 16 page 4 of 27 Example: Sulfuryl chloride decomposes as follows: SO2Cl2(g) SO2(g) + Cl2(g) The figures above show closed systems of SO2Cl2, SO2, and Cl2 at 375K. - Initially, only SO2Cl2 molecules are present. - When heated, the SO2Cl2 decomposes, and all three molecules are present. - Given enough time, the system achieves equilibrium. Ex. 1: Using the figures above, indicate the number of SO2, Cl2, and SO2Cl2 molecules at equilibrium at 375K. _______ SO2Cl2 molecules _______ SO2 molecules _______ Cl2 molecules Ex. 2 : Indicate the number of SO2, Cl2, and SO2Cl2 molecules present 15 minutes after the equilibrium is initially achieved at the same temperature. _______ SO2Cl2 molecules _______ SO2 molecules _______ Cl2 molecules Ex. 3 : If 9 SO2 molecules and 9 Cl2 molecules are placed in an empty container like those above, the container is closed, and the system once again achieves equilibrium at 375K, indicate the number of SO2, Cl2, and SO2Cl2 molecules present at equilibrium. _______ SO2Cl2 molecules _______ SO2 molecules _______ Cl2 molecules Ex. 4 : In another experiment 9 SO2Cl2 are placed in an empty container like those above the container is closed, and the system once again achieves equilibrium at 375K, indicate the number of SO2, Cl2, and SO2Cl2 molecules present at equilibrium. _______ SO2Cl2 molecules _______ SO2 molecules _______ Cl2