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Kinetics

Perform a sample experiment for determining the order of a reaction.

Chemical kinetics is the study of chemical reactions with respect to reaction rates. This is often expressed as a concentration unit per time, such as mol/(L·s). According to Beer's law, there is a direct relationship between a solution's concentration and the absorbance of light that passes through it. This means that absorbance per second can be used as a unit for reaction rate, as in the following procedure.

Rate laws describe the relationship between concentration and reaction rate. A rate law for a reaction that has two reactants will be expressed in the following format:

Rate = k[A]n[B]m

where [A] and [B] are the respective concentrations of the reacting species A and B. The rate constant k is a proportionality constant and has derived units. The exponents n and m are the order for the preceding reactant in that specific reaction. The order of a chemical species describes the way in which that reactant affects the reaction rate. For example:

  • If the order of a reactant is zero (n = 0), that reactant mathematically drops out of the expression and therefore has no effect on the reaction rate. When the concentration or absorbance for a reactant is plotted against time and the resulting graph is linear (as shown below), the reaction is zero order with respect that reactant. The slope of this line will be equal to -k, where k is the rate constant.

    Zero order reactant graph

  • If the reactant is first order (n = 1), the reactant has a direct relationship with the reaction rate. For example, if the amount of a first order reactant is doubled, the reaction rate also doubles. When the natural logarithm of concentration or absorbance for a specific reactant is plotted against time and the resulting graph is linear (as shown below), the reaction is first order with respect to that reactant. The slope of this line will be equal to -k.

    First order reactant graph

  • When the reactant is second order (n = 2), the reactant has an exponential relationship with the reaction rate. For example, if the amount of the reactant is doubled, the reaction rate becomes four times faster. When the inverse of concentration or absorption for a specific reactant is plotted against time and the resulting graph is linear (as shown below), the reaction is second order with respect to that reactant. The slope of this line will be equal to k.

    Second order reactant graph

Required materials

  • Spectrometer
  • Cuvettes and Caps (SE-8739)
  • 3× 50-mL glass beakers
  • 2× 10-mL graduated cylinders
  • 100-mL or 250-mL volumetric flask
  • 500-mL volumetric flask
  • 3× 1-L volumetric flasks
  • 8 mL of 5.0×10-5 M crystal violet
  • 2.0 mL of 0.2 M sodium hydroxide (NaOH)
  • Distilled water
  • 2.0 mL of 3.1×10-3 M phenolphthalein
  • 500 mL of 95% ethyl alcohol

Prepare solutions

2.5×10-3 M crystal violet stock solution:

Wear gloves to avoid staining your hands. Dissolve 0.1 g of solid crystal violet in about 200 mL of distilled water in a 1-L volumetric flask, then fill the flask to the line with distilled water. (Alternatively, starting from a 1% solution of crystal violet, place 50 mL of 1% solution in a 1-L volumetric flask and fill to the line with distilled water.)

5.0×10-5 M crystal violet (1 L):

Measure 20.0 mL of the 2.5×10-3 M stock solution into a 1-L volumetric flask, then fill to the line with distilled water.

0.2 M NaOH (500 mL):

Place 4.0 g of solid NaOH in about 100 mL of distilled water in a 500-mL volumetric flask. Stir it well and let it sit (dissolving NaOH in water produces a significant amount of heat). When the solid has completely dissolved, fill the flask to the line with distilled water.

3.1×10-3 M phenolphthalein (500 mL):

Fill a 1-L volumetric flask 1/3 full of 95% ethanol. Add 0.485 g of sodium hydroxide to the flask and swirl to dissolve. Fill the flask to the line with 95% ethanol.

Connect and calibrate the spectrometer

Before the experiment, your chosen spectrometer needs to be properly calibrated to both the absence of light and a reference solution. To do this:

  1. Prepare a reference solution by filling a cuvette ¾ full with distilled water. Handle this cuvette only by the lined sides and wipe the smooth sides clean with a lint-free cloth.
  2. Start Chemvue, then connect the spectrometer to the program. The Spectrometry Module should automatically open to the solution spectra page .
  3. Cover the spectrometer's sample chamber to block out ambient light, then select Calibrate Dark at the bottom of the screen. The spectrometer will turn off light sources to perform a calibration. The icon will change to a new appearance when the calibration is complete.
  4. Place the cuvette containing the reference solution into the spectrometer so that one of the clear sides is facing towards the white light source icon.
  5. Select Calibrate Reference to begin the reference calibration. The icon will change to a new appearance when the calibration is complete.

Select an analysis wavelength

  1. Place 4 mL of the 5.0×10-5 M crystal violet solution into a cuvette. Always handle this cuvette by the lined sides, and wipe away any fingerprints using a lint-free wipe.
  2. Place this cuvette into the spectrometer's sample chamber, then select Start to begin analyzing the solution.
  3. Select Stop when you are satisfied with the data displayed.
  4. Select Scale to Fit to ensure that the full curve is on-screen and clearly visible.

  5. Click the Select Wavelength tool, then drag the coordinate data target across the curve to locate a wavelength to analyze. This will usually be a high point on the curve.

    Note

    If your curve plateaus near the top of the graph, the absorbance in that area is too large to be used for analysis and another wavelength should be selected.

  6. Once you have found a desirable wavelength, select the data target, then click the floating Select Wavelength button to set the currently selected wavelength as the analysis wavelength.

Collect data

  1. Switch to the concentration change over time page using the navigation bar.
  2. Using a 50-mL beaker, place 2.0 mL of 5.0×10-5 M crystal violet into a graduated cylinder.
  3. Using a different 50-mL beaker, place 2.0 mL of 0.2 M sodium hydroxide (NaOH) into a second graduated cylinder.
  4. Pour the contents of both graduated cylinders into a third 50-mL beaker. Quickly fill a cuvette ¾ full with the mixed solution, insert the cuvette into the spectrometer, and select Start .
  5. Select Scale to Fit to rescale the graph display as data is collected.
  6. Allow the spectrometer to collect data for at least 90 seconds, then select Stop to end data collection.

Analyze data

  1. Select Toggle Live Scan Display to remove the live scan display from the graph.
  2. Select Linear Fit to create a best fit line and display the equation for the line.
  3. Use the Select QuickCalc button on the y-axis to cycle between Absorbance, ln(Absorbance), and 1/Absorbance until you find a linear trend. Use the results to determine the order of the reaction with respect to 5.0×10-5 M crystal violet.

Going further

Repeat the experimental procedure to determine the order for the reaction of phenolphthalein and sodium hydroxide (NaOH).