The color change accompanying the reaction of phenolphthalein with strong base is illustrated below. The change in concentration of the dye can be followed by spectrophotometry (Section 4.9), and some data collected by that approach are given below. The initial concentrations were [phenolphthalein] = 0.0050 mol/L and [OH‒] = 0.61 mol/L. (Data are taken from review materials for kinetics at chemed.chem.purdue.edu.) (For more details on this reaction see L Nicholson, Journal of Chemical Education, Vol. 66, p. 725, 1989.)
(a) Plot the data above as [phenolphthalein] versus time, and determine the average rate from t = 0 to t = 15 seconds and from t = 100 seconds to t = 125 seconds. Does the rate change? If so, why?
(b) Use a graphical method to determine the order of the reaction with respect to phenolphthalein. Write the rate law, and determine the rate constant.
(c) What is the half-life for the reaction?
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Chapter 14 Solutions
Chemistry & Chemical Reactivity
- The label on a bottle of 3% (by volume) hydrogen peroxide, H2O2, purchased at a grocery store, states that the solution should be stored in a cool, dark place. H2O2decomposes slowly over time, and the rate of decomposition increases with an increase in temperature and in the presence of light. However, the rate of decomposition increases dramatically if a small amount of powdered MnO- is added to the solution. The decomposition products are H2O and O2. MnO2 is not consumed in the reaction. Write the equation for the decomposition of H2O2. What role does MnO2 play? In the chemistry lab, a student substituted a chunk of MnO2 for the powdered compound. The reaction rate was not appreciably increased. WTiat is one possible explanation for this observation? Is MnO2 part of the stoichiometry of the decomposition of H2O2?arrow_forward11.41 For a drug to be effective in treating an illness, its levels in the bloodstream must be maintained for a period of time. One way to measure the level of a drug in the body is to measure its rate of appearance in the urine. The rate of excretion of penicillin is first order, with a half-life of about 30 min. If a person receives an injection of 25 mg of penicillin at t = 0, how much penicillin remains in the body after 3 hours?arrow_forwardMost reactions occur by a series of steps. The energy profile for a certain reaction that proceeds by a two-step mechanism is On the energy profile, indicate a. the positions of reactants and products. b. the activation energy for the overall reaction. c. E for the reaction. d. Which point on the plot represents the energy of the intermediate in the two-step reaction? e. Which step in the mechanism for this reaction is rate determining, the first or the second step? Explain.arrow_forward
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- Methyl acetate, CH3COOCH3, reacts in basic solution to give acetate ion, CH3COO, and methanol, CH3OH. CH3COOCH3(aq)+OH(aq)CH3COO(aq)+CH3OH(aq) The overall order of the reaction was determined by starting with methyl acetate and hydroxide ion at the same concentrations, so [CH3COOCH3] = [OH] = x. Then Rate=k[CH3COOCH3]m[OH]n=kxm+n Determine the overall order and the value of the rate constant by plotting the following data assuming first- and then second-order kinetics. Time (min) [CH3COOCH3] (mol/L) 0.00 0.01000 3.00 0.00740 4.00 0.00683 5.00 0.00634 10.00 0.00463 20.00 0.00304 30.00 0.00224arrow_forwardFor the reaction of nitrogen monoxide, NO, with chlorine, Cl2, 2NO(g)+Cl2(g)2NOCl(g) the observed rate law is Rate=k[NO]2[Cl2] What is the reaction order with respect to nitrogen monoxide and with respect to Cl2? What is the overall order?arrow_forwardA study of the rate of the reaction represented as 2AB gave the following data: Time (s) 0.0 5.0 10.0 15.0 20.0 25.0 35.0 [A](M) 1.00 0.775 0.625 0.465 0.350 0.205 0.230 (a) Determine the average rate of disappearance of A between 0.0 s and 10.0 s, and between 10.0 s and 20.0 s. (b) Estimate the instantaneous rate of disappearance of A at 15.0 s from a graph of time versus [A]. What are theunits of this rate? (c) Use the rates found in parts (a) and (b) to determine the average rate of formation of B between 0.00 s and 10.0 s, and the instantaneous rate of formation of B at 15.0 s.arrow_forward
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