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. The means 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 as the following:

Rate = k[A]ⁿ[B]^m

The concentrations of the reacting species are shown as [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 that reactant affects the reaction rate. For example, if the order is zero (n = 0), that reactant mathematically drops out of the expression and therefore has no effect on the reaction rate. When the reactant is first order (n = 1), the reactant has a direct relationship with the reaction rate. For example, if a first order reactant is doubled, the reaction rate also doubles. When the reactant is second order (n = 2), the reactant has an exponential relationship with the reaction rate. In this case, if the reactant is doubled, the rate becomes four times faster.

Figure 1: Zero Order. When the concentration or absorbance for a specific reactant is plotted against time and the resulting graph is linear, the reaction is zero order with respect to that reactant. The slope of the line is the negative value of the rate constant k.

Figure 2: First Order. When ln(concentration) or ln(absorbance) for a specific reactant is plotted against time and the resulting graph is linear, the reaction is first order with respect to that reactant. The slope of the line is the negative value of the rate constant k.

Figure 3: Second Order. When 1/(concentration) or 1/(absorbance) for a specific reactant is plotted against time and the resulting graph is linear, the reaction is second order with respect to that reactant. The slope of the line is the rate constant k.

Gather materials

• Spectrometer
• Beakers (3), glass, 50-mL
• Graduated cylinder (2), 10-mL
• Volumetric flask, 100-mL or 250-mL
• 5.0×10^-5 M Crystal violet, 8 mL
• 0.2 M Sodium hydroxide (NaOH), 2.0 mL
• Distilled water
• 3.1×10^-3 M Phenolphthalein, 2.0 mL
• 95% Ethyl alcohol, 500 mL

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

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 1000-mL 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.

Calibrate the spectrometer

Before the experiment, the spectrometer needs to be properly calibrated to both the absence of light and a reference solution. To do this, follow the instructions under Two point calibration.

Select an analysis wavelength

1. Place 4 mL of the most concentrated solution to be analyzed into a cuvette. Always handle the cuvette by the lined sides. Wipe off any fingerprints using a lint-free wipe. Place the cuvette into the spectrometer as you did in the calibration.
2. Click Record at the bottom left of the screen to start analyzing the solution. Click Stop when you are satisfied with the result.
3. Click to rescale your data.

4. Use the coordinate tool on the screen to locate a wavelength to analyze on the curve. This is usually a high point on the curve.

   1. Drag the coordinate tool box to the curve until it snaps in place.

      Tip

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

   2. Once you have found a desirable wavelength, click to the left of the selected wavelength value.

Collect data

1. Select the Time page from the top of the screen.
2. Use a 50-mL beaker to place 2.0 mL of 5.0×10^-5 M crystal violet into a graduated cylinder.
3. Use a different 50-mL beaker to place 2.0 mL 0.2 M sodium hydroxide (NaOH) into a graduated cylinder.
4. Pour the contents of both graduated cylinders into a third 50-mL beaker. Put that solution into a cuvette, quickly insert the cuvette into the spectrometer, and click Record .
5. Click to rescale your data.
6. Allow the spectrometer to collect data for at least 90 seconds, then click Stop .

Analyze data

1. Click to remove the Live Scan Display from the graph.
2. Click to create a best fit line and display the equation for the line.
3. Click on the y-axis to cycle between Absorbance, ln(Absorbance), and 1/Absorbance to find a linear trend. Use the results to determine the order of the reaction.

Going further

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