quiz 3

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: . A flask is charged with 0.065 of in a

total volume of 100.0 . The following data are collected:

Time 0 10 20 30 40

Moles of 0.065 0.051 0.042 0.036 0.031

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:

Correct

Part B



ANSWER:

Correct

Part C



= 1.4!10"2

= 2.3!10"2

ANSWER:

Correct

Part D



ANSWER:

Correct

Part E

Calculate the average rate of disappearance of between 0 and 10 , in units of .


ANSWER:

Answer Requested

Part F



ANSWER:

= 2.9!10"2

= 3.4!10"2

= 2.3!10"4

= 1.5!10"4

Correct

Part G



ANSWER:

Correct

Part H



ANSWER:

All attempts used; correct answer displayed

Part I

Between 10 and 30 , what is the average rate of appearance of in units of ? Assume

that the volume of the solution is constant.


ANSWER:

Answer Requested

= 1.0!10"4

= 8.3!10"5

= 1.2!10"4

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:

Correct

Part B

What is the rate of formation of water vapor?

ANSWER:

Correct

Part C

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

decreasing at the rate of 53 , what is the rate of change of the total pressure of the vessel?

ANSWER:

Correct

Question 3

= 0.23

= 0.46

= 27

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:

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

There is no change.

It doubles.

It triples.

It quadruples.

which simplifies to

Now, solve for .

ANSWER:

CorrectThe rate quadrupled when was doubled. This indicates an order of 2 with respect to because

.

Part B




Compare the values in trial 3 to those in trial 4. Notice that and remain constant whereas is

doubled. What happens to the rate in the transition from trial 3 to trial 4?

ANSWER:





2

There is no change.

It doubles.

It triples.

It quadruples.

2.4!10"4


2.4!10"4


which simplifies to

Now, solve for .

ANSWER:

CorrectThe rate didn't change when was doubled. This indicates an order of 0 with respect to .

Part C




Compare the values in trial 1 to those in trial 2. Notice that and remain constant whereas is

tripled. What happens to the rate in the transition from trial 1 to trial 2?

ANSWER:


0

There is no change.

It doubles.

It triples.

It quadruples.




6!10"5


1.8!10"4


which simplifies to

Now, solve for .

ANSWER:

CorrectThe 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 themultiplication of units explicitly either with a multiplication dot or a dash.

Hint 1. How to approach the problem

Based on your previous answers, . Now, plug in numbers from any one of the trials

and solve for .

1

ANSWER:

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:

Correct

Part B

Will the rate constant change?

ANSWER:

Correct

7.50!10"3

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.

Yes

No

Part C

What is the reaction order for ?


ANSWER:

Correct

Part D

What is the reaction order for ?


ANSWER:

Correct

Part E

What is the overall reaction order?


ANSWER:

Correct

Part F

What are the units of the rate constant?

ANSWER:

0

2

2

Correct

Question 5

Consider the following reaction:

The rate law for this reaction is first order in and first order in . When is and

is 0.050 , the reaction rate at 298 is 0.0432 .

Part A

What is the value of the rate constant?


ANSWER:

Correct

Part B

What are the units of the rate constant?

ANSWER:

= 170

Correct

Part C

What would happen to the rate if the concentration of were tripled?

ANSWER:

Correct

Part D

What would happen to the rate if the concentration of both reactants were tripled?

ANSWER:

Correct

The rate would be multiplied by 27.


The rate would be tripled.


The rate would be tripled.




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:

Experiment Initial Rate of Appearance of

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 from the four data sets.


ANSWER:

Correct

= 1.2!104

Part C

How is the rate of appearance of related to the rate of disappearance of ?

ANSWER:

Correct

Part D

What is the rate of disappearance of when 6.7!10"2 and 0.29 ?


ANSWER:

Answer Requested

Question 7

Some watch dials are coated with a phosphor, like , and a polymer in which some of the atoms have been

replaced by 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 adial be dimmed in a watch that is 45 years old?

Express your answer to two significant figures and include the appropriate units.

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

= 7.8

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:

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 . These time and

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

versus , and versus .

= 76.2 days

( )

( )

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?


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

0 vs.

1 vs.

2 vs.

If the reaction is zeroth order, a graph of versus will be a straight line. If the reaction is first order, a

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

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:

CorrectWhat does a straight line imply about the order of the reaction?

ANSWER:

versus

versus

versus

0

1

2

CorrectNow that we know that the reaction is first order, we know that the units of the rate constant will be inverseseconds, .

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 approachthis 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 is the y intercept. That means that corresponds to ,

corresponds to , corresponds to , and corresponds to .

ANSWER:

Plug values into the first-order integrated rate law and solve for .

Determine the y intercept of the graph of vs. , then change the sign.

Determine the slope of the graph of vs. , then change the sign.

CorrectThe 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 is the y intercept. That means that corresponds to ,

corresponds to , corresponds to , and corresponds to ..

ANSWER:

Answer Requested

ANSWER:

Answer Requested

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

( )

( )

295 0.0125

325 0.0357

= 1.25!10"2

Part C

What is the activation energy of this reaction?

Express your answer to three significant figures and include the appropriate units.


The Arrhenius equation relates the rate constant to temperature as follows:

where is the activation energy, 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:

Correct

Hint 3. Extend the data table

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

( )

( )

( )

295 0.0125 ? ?

325 0.0357 ? ?

Plot vs. to give a slope of .





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:

Hint 3. Determine the run

What is the value of the "run"?

Express your answer numerically using five significant figures.

ANSWER:

ANSWER:

Hint 5. Putting it all together

We know that the slope is equal to 3354 . We also know that slope is equal to . So,

. Solve for .

Recall that is the constant equal to 8.314 .

ANSWER:

= 1.0494

= -0.00031291

slope = -3354

Answer RequestedIt is important to realize that in the integrated rate law is not the same as in the Arrhenius equation. In

the integrated rate law, 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 theequation 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.


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 of the line formed by connecting thosepoints.

Hint 2. Calculate the rise

Slope is equal to "rise over run." What is the value for the rise?


Hint 1. Formula for the rise

= 27.9

The "rise" is the difference between two points on the y axis. Since for a zero-order reaction the yaxis is concentration, simply subtract the two values:

ANSWER:

Correct

Hint 3. Calculate the run

Slope is equal to "rise over run." What is the value for the run?


Hint 1. Formula for the run

The "run" is the difference between two points on the x axis. Since for a zero-order reaction the xaxis is time, simply subtract the two values:

ANSWER:

Correct

ANSWER:

"8.00!10"2

165

= 4.85!10"4

Correct

Part B

What was the initial reactant concentration for the reaction described in Part A?



Use the integrated rate law for zero-order reactions to solve for . Values for and are given in

Part A, and your answer to Part A was the value of .

ANSWER:

Correct

Part C

The reactant concentration in a first-order reaction was 7.00!10"2 after 40.0 and 8.60!10"3 after 100 .

What is the rate constant for this reaction?


Hint 1. Make a data table

Set up a data table as follows and fill in the missing values.

]

( )

( )

7.00!10"2 ? 40.0

8.60!10"3 100

For example, what value goes in the cell marked with a question mark?

= 0.192

ANSWER:

Answer Requested




The "rise" is the difference between two points on the y axis. Since the y axis for a first-orderreaction is , simply subtract the two values:

ANSWER:

Correct





The "run" is the difference between two points on the x axis. Since the x axis for a first-orderreaction is time, simply subtract the two values:

ANSWER:

-2.66

-2.10

Correct

ANSWER:

Answer Requested

Part D

The reactant concentration in a second-order reaction was 0.870 after 170 and 2.30!10"2 after 880 .

What is the rate constant for this reaction?


Hint 1. Make a data table

Set up a data table as follows and fill in the missing values.

( )

( )

( )

0.870 ? 170

2.30!10"2 880

For example, what value goes in the cell marked with a question mark?

ANSWER:



60.0

= 3.49!10"2

1.15


The "rise" is the difference between two points on the y axis. Since for a second-order reactionthe y axis is , simply subtract the two values:

ANSWER:





The "run" is the difference between two points on the x axis. Since for a second-order reactionthe x axis is time, simply subtract the two values:

ANSWER:

ANSWER:

Answer Requested

Question 11

Part A

42.3

710

= 5.96!10"2

Based on their activation energies and energy changes and assuming that all collision factors are the same,which of the following reactions would be fastest?

ANSWER:

Correct

Part B

Which of the following reactions would be slowest?

ANSWER:

Correct

Question 12

The rate of the reaction

was measured at several temperatures, and the following data were collected:

Temperature

15 0.0521

25 0.101

35 0.184

45 0.332

= 41 , = -27 .

= 32 , = -11 .

= 50 , = 13 .

= 41 , = -27 .

= 50 , = 13 .

= 32 , = -11 .

Part A

Using these data, graph versus .

ANSWER:

Answer Requested

Part B

Calculate the value of by using your graph.

ANSWER:

Correct

Question 13

What is the molecularity of each of the following elementary reactions?

= 47.5

Part A

ANSWER:

Correct

Part B

ANSWER:

Correct

Part C

ANSWER:

Correct

unimolecular

bimolecular

termolecular

unimolecular

bimolecular

termolecular

unimolecular

bimolecular

termolecular

Part D

Write the rate law for the reaction in part A?

ANSWER:

Correct

Part E

Write the rate law for the reaction in part B?

ANSWER:

Correct

Part F

Write the rate law for the reaction in part C?

ANSWER:

Correct

Question 14

Part A

Based on the reaction profile, how many intermediates are formed in the reaction ?

ANSWER:

Correct

Part B

How many transition states are there?

ANSWER:

Correct

2

3

Part C

Which step is the fastest?

ANSWER:

Correct

Part D

Is the reaction exothermic or endothermic?

ANSWER:

Correct

Question 15

The activation energy of an uncatalyzed reaction is 94 . The addition of a catalyst lowers the activation

energy to 55 .

Part A

Assuming that the collision factor remains the same, by what factor will the catalyst increase the rate of thereaction at 25 ?


ANSWER:

exothermic

endothermic

= 6.9!106

Correct

Part B

Assuming that the collision factor remains the same, by what factor will the catalyst increase the rate of thereaction at 140 ?


ANSWER:

Correct

Score Summary:

Your score on this assignment is 75.9%.You received 11.39 out of a possible total of 15 points.

= 8.6!104

quiz 3

Documents

significant figures

trial initial rate

average rate of disappearance

initial reaction rate

rate of formation

rate of consumption

rate of change

maththe reaction rate