Reaction order and rate laws lab answers

Reaction Order and Rate Laws

Michele Hopkins

General Chemistry II/ CHEM-182-DL1

Professor Amal Bassa

June 27, 2014

Introduction

This lab experiment studies the effects of dilution for a reaction, in order obtain the order of the reactants,

plus acquire the reaction’s rate law. This lab as well teaches the environmental factors that affect reaction rates by identifying zero, first, as well as second order.

Procedure

Part I:

Set up the data tables by calculating the initial concentrations after diluting with water

Part II: Varying the Concentration of 1.0 M HCl

1. I placed the 24-well plate at the top of a white piece of paper and drew three equally-thick,

black X signs beneath each of the lower three left wells. Then I moved the plate to where the

lower three left wells are above the X’s.

2. I than added distilled water 6 drops to well #D-2 and 8 drops to well #D-3( no water is added to well #D-1)

3. From the HCl bottle I carefully added 12 drops to well #1, 6 drops to well #2, and 4 drops to well #3

4. Into the top three left wells of the 24-well plate (A-1, A-2, A-3) I added 8 drops of Na2S2O3, sodium thiosulfate.

5. I labeled an empty pipet Na2S2O3 where I pinch the sides together.

6. I slightly tipped the 24-well plate forward so that the drops of Na2S2O3 puddle together at the bottom of the rim.

7. Inserted the pipet’s into the puddle and released the pressure and sucked all of the Na2S2O3 into the pipet.

8. I next grabbed the stopwatch and began timing.

9. Than I took the pipet of Na2S2O3 in my right hand and squeezed all of the contents from the bulb into

well #1 (immediately begin timing with stopwatch).

10. I observed well #1 while a reaction occured. (no longer see the X sign through the bottom of the well)

11. This was reapted for well #2 and well #3

Part III: Varying the Concentration of 0.30 M Na2S2O3

Next I placed the 24-well plate at the top of a white piece of paper and drew three black X

signs beneath each of the lower three left wells. I then moved the plate to where the lower three

left wells are above the Xs. On my paper I labeled the well numbers 1, 2, and 3 underneath each of them.

1.From the distilled water bottle I carefully added 6 drops to well #2; b. 8 drops to well #3c (no water was

added to well #1)

2. From the Na2S2O3 bottle, I carefully added 12 drops to well #1; 6 drops to well #2; and 4 drops to well #3

3. Into each of the top three left wells of the 24-well plate I added 8 drops of HCl.

4. I than labeled the second empty pipet HCl.

5. Slightly tipped the 24-well plate forward so that the drops of Na2S2O3 puddle together along the bottom

rim.

6. I inserted the pipet’s into the puddle and release the pressure and sucked all of the HCl into the pipet.

7. I next grabbed the stopwatch and began timing.

8. I placed the pipet of HCl in my right hand and squeezed all of the contents from the bulb into well

#1 and begin timing with the stopwatch.

9. I observed well #1 while a reaction occured. (no longer see the X sign through the bottom of the well)

10. I repeated for well #2 and well #3

11. Immediately wash with liquid soap and thoroughly dry your well plate with paper towels and

cotton swabs to prevent precipitates from clinging to the bottom of the wells.

12. Lastly a second set of data was performed by repeating all of the above steps.

Reaction Order and Rate Laws

Peter Jeschofnig, Ph.D. Version 42-0195-00-01

Lab Report Assistant

This document is not meant to be a substitute for a formal laboratory report. The Lab Report Assistant is simply a summary of the experiment’s questions, diagrams if needed, and data tables that should be addressed in a formal lab report. The intent is to facilitate students’ writing of lab reports by providing this information in an editable file which can be sent to an instructor.

Observations & Questions

A. Calculate the initial and final concentrations as needed to complete Tables 1 and 2.

B. Calculate the average reaction time for each reaction by adding the times for the two trials and dividing by 2.

Data Table 1: Varying the Concentration of 1.0 M HCL

---- C o n c e n t r a t i o n s ----

#

drops

#

drops

# drops

Initial

Initial

Final

Final

Reaction Time (sec)

Reaction

Well

#

HCl

Water

Na2S2O3

HCl

Na2S2O3

HCl

Na2S2O3

Trial 1

Trial 2

Average

Rate (sec-1)

1

12

0

8

1M

0.3M

16.06s

16.72s

16.39s

2

6

6

8

0.5M

0.3M

49.16s

27.66s

38.41

3

4

8

8

0.33M

0.3M

50.10s

32.26s

41.18

Experiment

Reaction Order and Rate Laws

65

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Data Table 2: Varying the Concentration of 0.3 M Na2S2O3

---- C o n c e n t r a t i o n s ---

#

drops

#

drops

# drops

Initial

Initial

Final

Final

Reaction Time (sec)

Reaction

Well

#

HCl

water

Na2S2O3

HCl

Na2S2O3

HCl

Na2S2O3

Trial 1

Trial 2

Average

Rate (sec-1)

1

8

0

12

1M

0.3M

16.06s

16.72s

16.39s

2

8

6

6

1M

0.15

49.16s

27.66s

38.41

3

8

8

4

1M

0.1

50.10s

32.26s

41.18

C. Calculate the reaction rate by taking the inverse of the average reaction time, i.e., 1 divided by the average reaction time.

1. Use table 1 to determine the reaction order for HCl.

2. Use table 2 to determine the reaction order for Na2S2O3.

Remember, you want to see what happens to the reaction rate when you double the concentration of one reactant while the second reactant remains unchanged. In Part 1, we varied the concentration of HCl while we kept the concentration of Na2S2O3 the same. In Part 2 we varied the concentration of Na2S2O3 while keeping the concentration of HCl the same.

These are experimental data and results will be different from some of the nice, even numbers you saw on textbook problems. For example, in this experiment you may double the concentration of a reactant but the reaction rate may increase anywhere from 1.7 times to 2.4 times. This still means an approximate doubling of the reaction rate. On the other hand, if you double a reactant concentration and the reaction rate increases by 0.7 to 1.3 times that probably means that the reaction rate multiplier is one (1).

D. Write the rate law for the reaction.

E. Using the rate law, the rate, and the appropriate concentration(s) from one (or more) of your experiments calculate k.

F. What are the potential errors in this experiment?

 Conclusion

In this lab, it would appear that the concentration of HCl produced minor effect on the reaction rate. The concentration of the Na2 S2 O3 on the other hand had a substantial part in the reaction rate. The main variation

was amongst wells #1 and #2 where the reaction rate was divided while the concentration of Na2 S2 O3 was

divided. In well # 3 (prior to adding HCI) it diminished with the smallest concentration of Na2 S2 O3 was present

in the original solution. Therefore for this specific reaction, the rate is reliant on the concentration of Na2 S2 O3

and not HCl.

The errors that occurred in this lab was contamination involving both parts of the experiment, on the other

hand I was able to use the other empty wells in the 24-platet. An additional error was the correct volume that

was pipetted from one well to the other well. Lastly, was the stopwatch starting and stopping at the exact

moment.

Reference

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