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Unformatted text preview: NAME: COLIN FIRTH CHEMISTRY 444, SPRING, 2011 Circle Section Number: 10 11 80 81 Examination 2, April 16, 2011 Answer each question in the space provided; use back of page if extra space is needed. Answer questions so the grader can READILY understand your work; only work on the exam sheet will be considered. Write answers, where appropriate, with reasonable numbers of significant figures. You may use only the "Student Handbook," a calculator, and a straight edge . 1. (15 points) Consider a relaxation experiment on water that measures this dissociation reaction 2 + + . Let the forward rate constant be k f and the reverse rate constant be k r . Also let the equilibrium concentrations of the species be [ 2 ] , [ + ] , and [ ] . (a) Write an equation for the equilibrium constant, K , for the reaction as written in terms of the forward and reverse rate constants. The equilibrium constant is determined when the two rates are equal. Setting these equal and using equilibrium concentrations, one obtains the equation = (b) If the instantaneous water concentration is [ 2 ] in terms of the deviation from equilibrium, , write equations for the instantaneous concentrations of the two ions. By the stoichiometry of the reaction, every time a water molecule disappears each of the ions appears. Since this is measured relative to equilibrium, one gets the following two equations: [ + ] = [ + ] + [ ] = [ ] + (c) Give an expression for the instantaneous disappearance of water in terms of the rate constants, the equilibrium concentrations and the deviation, , from equilibrium. The rate of disappearance of water is [ ] = [ ] [ + ][ ] = [ ] [ + ] + [ ] + (d) What approximation does one usually make to solve the equation in part (c)? The right-hand side of this equation contains terms in . Since is small, these terms are generally neglected in solving the equation. This neglect gives the following equation . [ ] = = [ ] [ + ] [ ] [ + ] + [ ] As an aside, the first two terms on the right-hand side cancel to yield a simple equation for ....
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