WS 13



Measuring Rates

You will turn in this assignment. It will count as a lab grade.

Experiment: 100 mL of 1 M HCl is added to about 60 g of CaCO3 (marble) chips.

CaCO3(s) + HCl(aq) = CO2(g) + CaCl2(aq) + H2O(l)

1. Balance the equation

2. Name two ways that one might measure the rate of this reaction.

3. How does the rate of disappearance of HCl relate to the rate of formation of CO2?

4. If the reaction is performed in an open container, why can we ignore the reverse reaction?

5. What will happen to the rate as the reaction proceeds? Why?

6. Although CaCO3 is consumed by the reaction, its consumption does not affect the rate of the reaction. Why?

7. Reproduce and complete the following table in Excel. Rather than using your calculator, let Excel do the calculations for you. See me if you don’t know how to do this.

|Time (s)  |Mass (g) |Mass |Moles CO2 |Moles HCl |Moles HCl |

| | |loss (g) |produced |consumed |remaining |

|1 |2.00 |1.00 |3.00 | | |

|2 |2.00 |2.00 |2.00 | | |

|3 |2.00 |3.00 |1.00 | | |

Measuring reaction rates

The consumption of the red dye over time was monitored by measuring the absorbance every 30 seconds using light with a wavelength of 530 nm. The first reaction was monitored for 900 seconds. Reactions 2 and 3 were monitored for only 60 s. The data are shown below. Transfer the data to Excel. For all three reactions, use Beer’s law to calculate [red dye] at each time point. For reaction 1, calculate ln[red dye] and 1/[red dye] for each time point. Use formulas to do the calculations.

Beer’s law is: A = εcl where A is the measured absorbance, ε is the molar absorptivity of the red dye

(M-1-cm-1), c is the concentration of the red dye (M), and l is the path length (cm). For this experiment, ε ’ 48521 M-1-cm-1 and l = 1.00 cm.

Reaction 1:

|Time (s) |Absorbance |[red dye] (M) |ln[red dye] |1/[red dye] (M-1) |

|0 |0.349 | | | |

|30 |0.334 | | | |

|60 |0.318 | | | |

|90 |0.304 | | | |

|120 |0.289 | | | |

|150 |0.281 | | | |

|180 |0.266 | | | |

|210 |0.255 | | | |

|240 |0.242 | | | |

|270 |0.232 | | | |

|300 |0.220 | | | |

|330 |0.210 | | | |

|360 |0.200 | | | |

|390 |0.190 | | | |

|420 |0.182 | | | |

|450 |0.173 | | | |

|480 |0.164 | | | |

|510 |0.157 | | | |

|540 |0.148 | | | |

|570 |0.140 | | | |

|600 |0.134 | | | |

|630 |0.126 | | | |

|660 |0.118 | | | |

|690 |0.113 | | | |

|720 |0.106 | | | |

|750 |0.101 | | | |

|780 |0.095 | | | |

|810 |0.090 | | | |

|840 |0.086 | | | |

|870 |0.081 | | | |

|900 |0.077 | | | |

Reaction 2:

|Time (s) |Absorbance |[red dye] (M) |

|0 |0.324 | |

|30 |0.302 | |

|60 |0.263 | |

Reaction 3:

|Time (s) |Absorbance |[red dye] (M) |

|0 |0.333 | |

|30 |0.286 | |

|60 |0.240 | |

You will use the graphical method to determine the order with respect to the dye (a). You will use the initial rate method to determine the order with respect to OCl¯ (b) and the rate constant (k).

Graphical method (finding a)

In order to use the graphical method, the concentration of only one reactant can change during the reaction. Look at the initial concentrations of the reactants calculated on page 5 and explain why the concentration of the bleach does not change significantly during the reactions.

The rate equation for this reaction is:

Rate = k[red dye]a[OCl¯]b

Since [OCl¯] does not change during this reaction, [OCl¯]b is a constant and can be combined with k to give what is called a “pseudo rate constant” whose symbol is k’.

k’ = k[OCl¯]b

Use the data from reaction 1 and the graphical method to determine the order of the reaction with respect to the red dye. Record the order below:

a =

How would you change the experiment if you wanted to use the graphical method to calculate the order with respect to OCl¯?

Unlike the reaction between calcium carbonate and HCl which had only one aqueous reactant, the magnitude of the slope of your straight line is NOT the rate constant. Instead, it is the pseudo rate constant. What is the value of k’ (include the unit)?

K’ =

Initial Rate Method (finding b and k)

Use the first 60 seconds of each of the three reactions to calculate the initial rate (rateo) of each reaction. Recall that the initial rate is the instantaneous rate at time zero. Although it is impossible to measure this rate, it can be approximated as the average rate after a very short period of time. Calculate the average rate of each reaction for the first 60 seconds and complete the table below. (Copy the initial concentrations from the table above.) Show your work for reaction 1 below the table.

|Rxn # |[dye]o (M) |[OCl−]o (M) |rateo (M-s-1) |

| | | |(in first 60 s) |

|1 | | | |

|2 | | | |

|3 | | | |

Use the initial rate data above to determine the order of the reaction with respect to OCl−. Show your work or explain your logic. Enter the value of b below.

b =

Use the rate equation and the initial rate data to calculate the value of the rate constant (include the unit). Remember that the rate equation is: rate = k[red dye]a[OCl¯]b and you now know the values of a and b. You have 3 different values for [OCl−] so calculate 3 values for k and determine the average. Show your work for one of the calculations and complete the table below.

|Rxn # |k |

| |(include the correct unit) |

|1 | |

|2 | |

|3 | |

|Avg. | |

Another way to calculate the value of the rate constant is to use the pseudo rate constant determined above:

k’ = k[OCl¯]b

You determined k’ for reaction 1 and you determined b from the initial rate data. Use the concentration of OCl¯ in reaction 1 and the equation for k’ to calculate the value of k (include the unit). This should give you the same value for k as you calculated above. Show your work below.

k =

Write the rate equation for this reaction by inserting your experimental values for a, b, and k:

rate =

Directions for report:

1. Answer all of the questions on the worksheet in the spaces provided. Clearly show all of your work for calculations (unless told that no work is required). Since you will use Excel to complete the calculations for the spreadsheet, you don’t have to show your work for those calculations.

2. Print your excel spreadsheets and graphs and attach them to the worksheet. Send me your Excel file as an e-mail attachment. I will check them to make sure that you used Excel to do the calculations in the spreadsheet.

Guidelines for tables made in excel (see me if you need help with any of the following):

1. The column headings should be centered over the data in each column.

2. Column heading should include units when appropriate.

3. Use the correct number of significant figures.

Guidelines for graphs made in excel

1. Graph should have a descriptive title.

2. Label axes with a descriptive title. Give units in parentheses.

3. Make graph big enough to see easily.

4. Delete the legend unless there is more than one curve shown on the graph (this will make your graph larger).

5. Number the scale on each axis but avoid crowding of the numbers and use a minimal number of digits.

6. Adjust the scale on each axis so the curve fills as much of the graph as possible (don’t waste space). You do not necessarily have to start a scale at zero!

Make sure that the spreadsheet is formatted so each table and graph fit on a single page when they are printed. (Students’ printouts often have ½ of a table or graph on one page and the other half on another page!)

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