Prelaboratory Questions Name Laboratory Section 1. Consider the following reaction: 2 N2O5 (g) 4 NO2 (g) + O2 (g) -> The initial concentration of N,Os was 0.84 mol/L, and 35 minutes after initiating the reaction, all of the N,O5 has been consumed. a) Calculate the average rate of the reaction over this 35-minute time interval. b) Is it correct to assume that the rate law is Rate = k[N,O5]2 based on the balanced chemical equation? Briefly explain your answer. %3D c) Suppose the reaction rate is determined by simultaneously measuring the rate of decrease of N,O5 concentration (-AIN2O5]/At) and the rate of increase of NO, concentration (A[NO,1/At). VWhich rate will be higher and by what factor? (continued on next page) 137

Prelaboratory Questions Name Laboratory Section 1. Consider the following reaction: 2 N2O5 (g) 4 NO2 (g) + O2 (g) -> The initial concentration of N,Os was 0.84 mol/L, and 35 minutes after initiating the reaction, all of the N,O5 has been consumed. a) Calculate the average rate of the reaction over this 35-minute time interval. b) Is it correct to assume that the rate law is Rate = k[N,O5]2 based on the balanced chemical equation? Briefly explain your answer. %3D c) Suppose the reaction rate is determined by simultaneously measuring the rate of decrease of N,O5 concentration (-AIN2O5]/At) and the rate of increase of NO, concentration (A[NO,1/At). VWhich rate will be higher and by what factor? (continued on next page) 137

Chemistry for Engineering Students

4th Edition

ISBN:9781337398909

Author:Lawrence S. Brown, Tom Holme

Publisher:Lawrence S. Brown, Tom Holme

Chapter11: Chemical Kinetics

Section: Chapter Questions

Problem 11.30PAE: The rate of the decomposition of hydrogen peroxide, H2O2, depends on the concentration of iodide ion...

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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?
The rate of the decomposition of hydrogen peroxide, H2O2, depends on the concentration of iodide ion present. The rate of decomposition was measured at constant temperature and pressure for various concentrations of H2O2and of KI. The data appear below. Determine the order of reaction for each substance, write the rate law, and evaluate the rate constant. Rate [H2OJ [Kll (mL min-’) (mol L ’) (mol L ’) 0.090 0.15 0.033 0.178 0.30 0.033 0.184 0.15 0.066
Consider the following statements: In general, the rate of a chemical reaction increases a bit at first because it takes a while for the reaction to get warmed up. After that, however, the rate of the reaction decreases because its rate is dependent on the concentrations of the reactants, and these are decreasing. Indicate everything that is correct in these statements, and indicate everything that is incorrect. Correct the incorrect statements and explain.
Rank the following in order of increasing reaction rate. (a) Dynamite exploding (b) Iron rusting (c) Paper burning
Give at least two physical properties that might be used to determine the rate of a reaction.
Ozone, O3, in the Earths upper atmosphere decomposes according to the equation 2 O3(g) 3 O2(g) The mechanism of the reaction is thought to proceed through an initial fast, reversible step followed by a slow, second step. Step 1: Fast, reversible O3(g) O2(g) + O(g) Step 2: Slow O3(g) + O(g) 2 O2(g) (a) Which of the steps is rate-determining? (b) Write the rate equation for the rate-determining step
Candle wax is a mixture of hydrocarbons. In the reaction of oxygen with candle w ax in Figure 11.2, the rate of consumption of oxygen decreased with time after the flask was covered, and eventually' the flame went out. From the perspective of the kinetic-molecular theory, describe what is happening in the flask. FIGURE 11.2 When a candle burns in a closed container, the flame will diminish and eventually go out. As the amount of oxygen present decreases, the rate of combustion will also decrease. Eventually, the rate of combustion is no longer sufficient to sustain the flame even though there is still some oxygen present in the vessel.
Iodomethane (CH3I) is a commonly used reagent in organic chemistry. When used properly, this reagent allows chemists to introduce methyl groups in many different useful applications. The chemical does pose a risk as a carcinogen, possibly owing to iodomethanes ability to react with portions of the DNA strand (if they were to come in contact). Consider the following hypothetical initial rates data: [DNA]0 ( mol/L) [CH3I]0 ( mol/L) Initial Rate (mol/Ls) 0.100 0.100 3.20 104 0.100 0.200 6.40 104 0.200 0.200 1.28 103 Which of the following could be a possible mechanism to explain the initial rate data? MechanismIDNA+CH3IDNACH3++IMechanismIICH3ICH3++ISlowDNA+CH3+DNACH3+Fast
Nitrosyl bromide, NOBr, is formed from NO and Br2: 2 NO(g) + Br2(g) 2 NOBr(g) Experiments show that this reaction is second-order in NO and first-order in Br2. (a) Write the rate equation for the reaction. (b) How does the initial reaction rate change if the concentration of Br2 is changed from 0.0022 mol/L to 0.0066 mol/L? (c) What is the change in the initial rate if the concentration of NO is changed from 0.0024 mol/L to 0.0012 mol/L?
A 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.