Unit 5 Topic Questions

NO(g) and O2(g) react to form NO2(g). The rate law of the reaction is rate=k[NO]2[O2]. If the reaction occurs in a single elementary step that is a three-body molecular collision, then which of the following is the equation for the elementary step?

2NO+O2→2NO2

Step 1:H2(g)+ICl(g)→HI(g)+HCl(g)(slow)Step 2:HI(g)+ICl(g)→HCl(g)+I2(g)(fast) Which of the following represents a rate law for the overall reaction that is consistent with the proposed mechanism?

A. Rate=k[H2][ICl]

The hydrolysis of sucrose is represented by the chemical equation above. This reaction is extremely slow in aqueous solution. However, when sucrase is added as shown in the diagram, the rate of the reaction is about 6,000,000 times faster. Based on this information, which of the following best explains the large increase in the rate of hydrolysis that occurs with the addition of sucrase?

A. The reaction proceeds through a different reaction path with a lower activation energy in which the sucrase-sucrose complex is formed as an intermediate.

Step 1: NO2(g)+F2(g)⇄NO2F2(g)(fast) Step 2: NO2F2(g)→NO2F(g)+F(g)(slow) Step 3: F(g)+NO2(g)→NO2F(g)(fast) A proposed mechanism for the chemical reaction 2NO2(g)+F2(g)→2NO2F(g)is shown above. Which of the following rate laws is consistent with this mechanism?

A. rate=k[NO2][F2]

The diagram above illustrates how the reaction N2(g)+3H2(g)→2NH3(g) occurs on the surface of Ru(s). The rate of this reaction is determined by the amount of energy required to break the bond in N2. This bond is weakened when N2 is adsorbed on Ru(s). Based on this information, which of the following provides the best reason for the use of Ru(s) for the synthesis of NH3 ?

B. It provides a reaction path with a lower activation energy.

The energy distribution profile (Curve C) for the Y2 molecules is shown in the graph above for the reaction X+Y2→XY2 when it is done under certain experimental conditions. Line A represents the most probable energy of the Y2 molecules, and Line B represents the activation energy. Which of the following changes should be made to the graph to explain the increase in the rate of the reaction if the only change in experimental conditions is the addition of a catalyst?

B. Line B will move to the left because a larger fraction of the Y2 molecules will have the minimum energy to overcome the activation energy barrier.

Br2(g)+2NO(g)→2NOBr(g) The reaction represented by the equation above has the following proposed mechanism. Step 1: NO(g)+Br2(g)⇌NOBr2(g)(fast equilibrium) Step 2: NOBr2(g)+NO(g)→2NOBr(g)(slow) Based on the information, which of the following is the initial rate law for the reaction?

B. Rate=k[Br2][NO]2

2HBr(g)+O2(g)→H2O2(g)+Br2(g) Based on a kinetics study of the reaction represented by the equation above, the following mechanism for the reaction is proposed. Step 1: HBr(g)+O2(g)→HO2Br(g)(slow) Step 2: HO2Br(g)+HBr(g)→2HOBr(g)(fast) Step 3: 2HOBr(g)→H2O2(g)+Br2(g)(fast) Which of following rate laws is consistent with the proposed mechanism?

B. Rate=k[HBr][O2]

An experiment was conducted to determine the rate law for the reaction A2(g)+B(g)→A2B(g). The table above shows the data collected. Based on the data in the table, which statement is correct?

B. Since the rate law can be expressed as rate=k[A2][B], doubling the concentrations of A2 and B will quadruple the rate of the reaction.

Step 1:H2(g)+ICl(g)→HI(g)+HCl(g)(slow)Step 2:HI(g)+ICl(g)→HCl(g)+I2(g)(fast) The reaction is carried out at constant temperature inside a rigid container. Based on this mechanism, which of the following is the most likely reason for the different rates of step 1 and step 2 ?

B. The amount of energy required for a successful collision between H2 and ICl is greater than the amount of energy required for a successful collision between HIand ICl.

The particle models shown above represent a proposed two-step mechanism for the destruction of ozone (O3) in the upper atmosphere. Based on the models, which of the following represents a species that acts as a catalyst for the reaction?

C. Cl

The rate constant (k) for the decay of the radioactive isotope I-131 is 3.6×10−3 hours−1. The slope of which of the following graphs is correct for the decay and could be used to confirm the value of k ?

C. In[I-131] vs. time

The gas-phase reaction A2(g)+B2(g)→2 AB(g) is assumed to occur in a single step. Two experiments were done at the same temperature inside rigid containers. The initial partial pressures of A2and B2 used in experiment 1 were twice the initial pressures used in experiment 2. Which statement provides the best comparison of the initial rate of formation of AB in experiments 1 and 2 ?

C. The initial rate of formation of AB is faster in experiment 1 than in experiment 2 because at a higher pressure the collisions between A2 and B2 molecules would have been more frequent, increasing the probability of a successful collision.

The particle models shown above represent a proposed two-step mechanism for the destruction of ozone (O3) in the upper atmosphere. Based on the proposed mechanism, what is the balanced chemical equation for the overall reaction?

D. 2O3(g) -> 3O2

The initial rates of the reaction represented by the equation shown above were measured for different initial concentrations of NO(g) and Cl2(g). Based on the data given in the table above, which of the following is the rate law expression for the reaction, and why?

D. Rate=k[NO]2[Cl2], because the initial rate quadrupled when [NO] was doubled and doubled when [Cl2] was doubled.

The oxidation of NO(g) producing NO2(g) is represented by the chemical equation shown above. The initial concentrations of NO and O2 are given in the table above. The changes in the concentration of NO(g) as a function of time are shown in the following graph. Which graph best represents the changes in concentration of O2(g), and why?

Graph 1, because the rate of O2 consumption is half the rate at which NOis consumed; two molecules of NO react for each molecule of O2 that reacts.

2HO2(g)→H2O2(g)+O2(g) The reaction represented by the chemical equation shown above occurs in Earth's atmosphere. In an experiment, [HO2] was monitored over time and the data plotted as shown in the following graph.

Rate=k[HO2]2

Step 1: ?(slow) Step 2: NO3(g)+CO(g)→NO2(g)+CO2(g)(fast) Overall: NO2(g)+CO(g)→NO(g)+CO2(g) A two-step reaction mechanism is proposed for a gas-phase reaction, as represented above. Which of the following correctly identifies both the chemical equation for step 1 and the rate law for the overall reaction?

The chemical equation for step 1 is 2NO2(g)→NO(g)+NO3(g), and the rate law is rate=[NO2]2.

The particle models shown above represent a proposed two-step mechanism for the destruction of ozone (O3) in the upper atmosphere. Based on the proposed mechanism, which of the following best describes the concentration of the species represented above(ClO) as the reaction occurs?

The concentration is very low for the duration of the reaction.

Step 1:H2(g)+ICl(g)→HI(g)+HCl(g)(slow)Step 2:HI(g)+ICl(g)→HCl(g)+I2(g)(fast) Which of the following represents the overall chemical equation for the reaction and the rate law for elementary step 2 ?

The overall reaction is H2(g)+2ICl(g)→2HCl(g)+I2(g); the rate law for step 2 is rate=k[HI][ICl].

S2O82−(aq)+3I−(aq)→2SO42−(aq)+I3−(aq) In aqueous solution, the reaction represented by the balanced equation shown above has the experimentally determined rate law: rate=k[S2O82−][I−]. If the concentration of [S2O82−] is doubled while keeping [I−] constant, which of the following experimental results is predicted based on the rate law, and why?

The rate of reaction will double, because the rate is directly proportional to [S2O82−].

H2O2→H2O+12O2 Hydrogen peroxide decomposes to produce water and oxygen according to the equation above. An experimentally determined graph for the first-order decomposition of hydrogen peroxide is provided below.

k=−(slope of plot)