143339386-Quiz-3 - quiz PDF

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Quiz 3 Due: 11:59pm on Friday, October 5, 2012 Note: You will receive no credit for late submissions. To learn more, read your instructor's Grading Policy

Question 1 Consider the following hypothetical aqueous reaction: total volume of 100.0

. A flask is charged with 0.065

of

. The following data are collected: Time

0

Moles of

0.065 0.051 0.042 0.036 0.031

10

20

30

40

Part A Calculate the number of moles of

at 10

, assuming that there are no molecules of

at time zero.

Express your answer using two significant figures. ANSWER: = 1.4!10"2

Correct

Part B Calculate the number of moles of

at 20

, assuming that there are no molecules of

at time zero.

Express your answer using two significant figures. ANSWER: = 2.3!10"2

Correct

Part C Calculate the number of moles of

at 30

, assuming that there are no molecules of

Express your answer using two significant figures.

at time zero.

in a

ANSWER: = 2.9!10"2

Correct

Part D Calculate the number of moles of

at 40

, assuming that there are no molecules of

at time zero.

Express your answer using two significant figures. ANSWER: = 3.4!10"2

Correct

Part E Calculate the average rate of disappearance of

between

0

and

10

, in units of

.

Express your answer using two significant figures. ANSWER: = 2.3!10"4

Answer Requested

Part F Calculate the average rate of disappearance of

between

Express your answer using two significant figures. ANSWER: = 1.5!10"4

10

and

20

, in units of

.

Correct

Part G Calculate the average rate of disappearance of

between

20

and

30

, in units of

.

30

and

40

, in units of

.

Express your answer using two significant figures. ANSWER: = 1.0!10"4

Correct

Part H Calculate the average rate of disappearance of

between

Express your answer using two significant figures. ANSWER: = 8.3!10"5

All attempts used; correct answer displayed

Part I Between

10

and

30

, what is the average rate of appearance of

that the volume of the solution is constant. Express your answer using two significant figures. ANSWER: = 1.2!10"4

Answer Requested

in units of

? Assume

Question 2 Part A Consider the combustion of

. If hydrogen is burning at the rate of 0.46

, what is the rate of consumption of oxygen? ANSWER: = 0.23

Correct

Part B What is the rate of formation of water vapor? ANSWER: = 0.46

Correct

Part C The reaction

is carried out in a closed vessel. If the partial pressure of

decreasing at the rate of 53 ANSWER: = 27

Correct

Question 3

, what is the rate of change of the total pressure of the vessel?

is

For the reaction

, the initial reaction rate was measured for various initial concentrations of

reactants. The following data were collected: Trial

(

)

(

)

(

Initial rate ( )

)

1

0.20

0.20

0.20

6!10"5

2

0.20

0.20

0.60

1.8!10"4

3

0.40

0.20

0.20

2.4!10"4

4

0.40

0.40

0.20

2.4!10"4

Part A What is the reaction order with respect to

?

Express your answer as an integer.

Hint 1. Compare trial 1 with trial 3 Compare the values in trial 1 to those in trial 3. Notice that

and

remain constant whereas

doubles. What happens to the rate in the transition from trial 1 to trial 3? ANSWER: There is no change. It doubles. It triples. It quadruples.

Hint 2. How to set up the math The reaction rate can be determined from the following equation:

where

is the order with respect to

.

For trial 1 this equation is 2.4!10"4 and for trial 3 this equation is 2.4!10"4 Dividing the equation for trial 3 by that for trial 1 gives

which simplifies to

Now, solve for

.

ANSWER: 2

Correct The rate quadrupled when

was doubled. This indicates an order of 2 with respect to

because

.

Part B What is the reaction order with respect to

?

Express your answer as an integer.

Hint 1. Compare trial 3 with trial 4 Compare the values in trial 3 to those in trial 4. Notice that

and

remain constant whereas

doubled. What happens to the rate in the transition from trial 3 to trial 4? ANSWER: There is no change. It doubles. It triples. It quadruples.

Hint 2. How to set up the math The reaction rate can be determined from the following equation:

where

is the order with respect to

For trial 3 this equation is

.

is

2.4!10"4 and for trial 4 this equation is 2.4!10"4 Dividing the equation for trial 4 by that for trial 3 gives

which simplifies to

Now, solve for .

ANSWER: 0

Correct The rate didn't change when

was doubled. This indicates an order of 0 with respect to

.

Part C What is the reaction order with respect to

?

Express your answer as an integer.

Hint 1. Compare trial 1 with trial 2 Compare the values in trial 1 to those in trial 2. Notice that

and

tripled. What happens to the rate in the transition from trial 1 to trial 2? ANSWER: There is no change. It doubles. It triples. It quadruples.

Hint 2. How to set up the math

remain constant whereas

is

The reaction rate can be determined from the following equation:

where

is the order with respect to

.

For trial 1 this equation is 6!10"5 and for trial 2 this equation is 1.8!10"4 Dividing the equation for trial 2 by that for trial 1 gives

which simplifies to

Now, solve for .

ANSWER: 1

Correct The rate tripled when

was tripled. This indicates an order of 1 with respect to

because

.

Part D What is the value of the rate constant

for this reaction?

Express your answer to three significant figures and include the appropriate units. Indicate the multiplication of units explicitly either with a multiplication dot or a dash.

Hint 1. How to approach the problem Based on your previous answers, and solve for .

. Now, plug in numbers from any one of the trials

ANSWER: 7.50!10"3

All attempts used; correct answer displayed

Question 4 A reaction

obeys the following rate law:

Part A If

is doubled, how will the rate change?

ANSWER: The rate of the reaction would be halved. The rate of the reaction would be multiplied by 4. The rate of the reaction would be doubled. The rate would remain the same.

Correct

Part B Will the rate constant change? ANSWER: Yes No

Correct

.

Part C What is the reaction order for

?

Express your answer as an integer. ANSWER: 0

Correct

Part D What is the reaction order for

?

Express your answer as an integer. ANSWER: 2

Correct

Part E What is the overall reaction order? Express your answer as an integer. ANSWER: 2

Correct

Part F What are the units of the rate constant? ANSWER:

Correct

Question 5 Consider the following reaction: The rate law for this reaction is first order in is 0.050

, the reaction rate at 298

and first order in is 0.0432

Part A What is the value of the rate constant? Express your answer using two significant figures. ANSWER: = 170

Correct

Part B What are the units of the rate constant? ANSWER:

.

. When

is

and

Correct

Part C What would happen to the rate if the concentration of

were tripled?

ANSWER: The rate would be multiplied by 27. The rate would be multiplied by 9. The rate would be tripled. The rate would remain the same.

Correct

Part D What would happen to the rate if the concentration of both reactants were tripled? ANSWER: The rate would be tripled. The rate would be multiplied by 9. The rate would be multiplied by 27. The rate would remain the same.

Correct

Question 6 Consider the gas-phase reaction between nitric oxide and bromine at 273 . The following data for the initial rate of appearance of

were obtained: Initial Rate of Appearance of

Experiment 1

0.10

0.20

24

2

0.25

0.20

150

3

0.10

0.50

60

4

0.35

0.50

735

Part A Determine the rate law. ANSWER:

Correct

Part B Calculate the average value of the rate constant for the appearance of Express your answer using two significant figures. ANSWER: = 1.2!104

Correct

from the four data sets.

Part C How is the rate of appearance of

related to the rate of disappearance of

?

ANSWER: the rate of appearance of

is half the rate of disappearance of

the rate of disappearance of

is half the rate of appearance of

the rate of disappearance of

is equal to the rate of appearance of

Correct

Part D What is the rate of disappearance of

6.7!10"2

when

and

0.29

?

Express your answer using two significant figures. ANSWER: = 7.8

Answer Requested

Question 7 Some watch dials are coated with a phosphor, like replaced by

, and a polymer in which some of the

atoms have been

atoms, tritium. The phosphor emits light when struck by the beta particle from the tritium decay,

causing the dials to glow in the dark. The half-life of tritium is 12.3 yr.

Part A If the light given off is assumed to be directly proportional to the amount of tritium, by how many percents will a dial be dimmed in a watch that is 45 years old? Express your answer to two significant figures and include the appropriate units. ANSWER: 92

Answer Requested

Question 8 Part A How much time is required for a 5.35

sample of

to decay to 0.800

if it has a half-life of 27.8 days?

ANSWER: = 76.2 days

Correct

Question 9 Learning Goal: To learn how graphs can be used to answer kinetics questions. A student collected time ( ) and concentration (

) data at 295

for the reaction

concentration data are shown in the table to the right. The student then plotted graphs of versus

, and

versus

.

. These time and versus

,

( )

(

)

0.00

0.500

0.693

2.00

20.0

0.389

0.944

2.57

40.0

0.303

1.19

3.30

60.0

0.236

1.44

4.24

80.0

0.184

1.69

5.43

Part A What is the order of this reaction?

Hint 1. How to approach the problem Recall that the equation for a straight line takes the form

, where

is the slope and

is the

y intercept. Also consider the following integrated rate law equations: Order

Integrated rate law

Straight line graph vs.

0

vs.

1

vs.

2

If the reaction is zeroth order, a graph of graph of

versus

versus

will be a straight line. If the reaction is first order, a

will be a straight line. If the reaction is second order, a graph of

will be a straight line.

Hint 2. Which graph shows a straight line? Look at each graph shown in the introduction. Which one shows a straight line? ANSWER: versus versus versus

Correct What does a straight line imply about the order of the reaction?

ANSWER: 0 1 2

versus

Correct Now that we know that the reaction is first order, we know that the units of the rate constant will be inverse seconds, .

Part B What is the value of the rate constant for this reaction? Express your answer to three significant figures and include the appropriate units.

Hint 1. Determine how to approach the problem There are two possible ways to solve this problem.Which of the following is not a valid way to approach this problem?

Hint 1. Equation for a line Notice that the integrated rate law

resembles the equation for a straight line

where

is the slope and

corresponds to

,

is the y intercept. That means that

corresponds to

, and

corresponds to ,

corresponds to .

ANSWER: Plug values into the first-order integrated rate law and solve for . Determine the y intercept of the graph of Determine the slope of the graph of

vs. , then change the sign. vs. , then change the sign.

Correct The two valid methods for solving this problem are: Plug values into the first-order integrated rate law and solve for . Determine the slope of the graph of

vs. , then change the sign.

The remaining hints will address the latter method.

Hint 2. How to find the slope of a line The slope of a line is defined as "rise over run" by

Hint 3. Identify the graph components Which components in the graph of

versus

correspond to slope, y intercept, x values, and y

values? Drag each item to the appropriate bin.

Hint 1. Equation for a line Notice that the integrated rate law

resembles the equation for a straight line

where

is the slope and

corresponds to

ANSWER:

,

is the y intercept. That means that

corresponds to

, and

corresponds to ,

corresponds to ..

Answer Requested

ANSWER: = 1.25!10"2

Answer Requested

Next, the student ran the same reaction at a different temperature and measured a different reaction rate, with the following results:

(

)

(

)

295

0.0125

325

0.0357

Part C What is the activation energy of this reaction? Express your answer to three significant figures and include the appropriate units.

Hint 1. How to approach the problem The Arrhenius equation relates the rate constant

where

to temperature

is the activation energy,

as follows:

is the gas constant, and

is a constant

called the frequency factor. Notice that this equation also resembles the equation for a straight line,

. So, by plotting the

appropriate values, you can use what you know about a straight line to find the activation energy.

Hint 2. Determine what to plot What relationship will give you points that will plot to a straight line? ANSWER: Plot

vs.

to give a slope of

.

Plot

vs.

to give a slope of

Plot

vs.

to give a slope of

Plot

vs.

Plot

to give a slope of

vs.

to give a slope of

. . . .

Correct

Hint 3. Extend the data table To plot

(

)

on the y axis and

(

)

(

)

295

0.0125

?

?

325

0.0357

?

?

on the x axis, you will need to fill in the rest of the following table:

Hint 4. Determine the slope What is the slope of a plot of

versus

?

Express your answer as an integer and include the appropriate units.

Hint 1. How to find the slope of a line The slope of a line is defined as "rise over run" by the following equation:

Hint 2. Determine the rise What is the value of the "rise"? Express your answer numerically using five significant figures. ANSWER: = 1.0494

Hint 3. Determine the run What is the value of the "run"? Express your answer numerically using five significant figures. ANSWER: = -0.00031291

ANSWER: slope = -3354

Hint 5. Putting it all together We know that the slope is equal to . Solve for Recall that

ANSWER:

3354

. We also know that slope is equal to

.

is the constant equal to 8.314

.

. So,

= 27.9

Answer Requested It is important to realize that the integrated rate law,

in the integrated rate law is not the same as

in the Arrhenius equation. In

refers to the concentration of reactant. In the Arrhenius equation,

is a

constant called the frequency factor.

Question 10 The integrated rate laws for zero-, first-, and second-order reaction may be arranged such that they resemble the equation for a straight line, . Order

Integrated Rate Law

Graph

Slope

0 1 2

Part A The reactant concentration in a zero-order reaction was 0.100

after 190 and 2.00!10"2

after 355 . What

is the rate constant for this reaction? Express your answer with the appropriate units.

Hint 1. How to approach the problem The graph of concentration versus time yields a straight line with a slope of

. Therefore if you treat

the given values as point coordinates, you can calculate the slope ...