Reaction Rates (College Board AP® Chemistry)

Na₂C₃₇H₃₄N₂S₃O₉ + OCl⁻  →    products

            blue                                  colorless

Blue food coloring can be oxidized by household bleach (which contains OCl⁻ ) to form colorless products, as represented by the equation above. A student used a spectrophotometer set at a wavelength of 635 nm to study the absorbance of the food coloring over time during the bleaching process. In the study, bleach is present in large excess so that the concentration of OCl⁻ is essentially constant throughout the reaction. The student used data from the study to generate the graphs below. 

Based on the graphs above, what is the order of the reaction with respect to the blue food coloring?

The reaction is known to be first order with respect to bleach. In a second experiment, the student prepares solutions of food coloring and bleach with concentrations that differ from those used in the first experiment. When the solutions are combined, the student observes that the reaction mixture reaches an absorbance near zero too rapidly. In order to correct the problem, the student proposes the following three possible modifications to the experiment.

• Increasing the temperature

• Increasing the concentration of the food coloring

• Increasing the concentration of the bleach

Circle the one proposed modification above that could correct the problem, and explain how that modification increases the time for the reaction mixture to reach an absorbance near zero.

In another experiment, a student wishes to study the oxidation of red food coloring with bleach. How would  the student need to modify the original experimental procedure to determine the order of the reaction with respect to the red food coloring?

The half-life (t_½) of the catalyzed isomerization of cis-2-butene gas to produce trans-2-butene gas , represented above, was measured under various conditions, as shown in the table below.

 The reaction is first order. Explain how the data in the table are consistent with a first-order reaction.

Calculate the rate constant, k, for the reaction at 350 K . Include appropriate units with your answer.

Is the initial rate of the reaction in trial 1 greater than, less than, or equal to the initial rate in trial 2? Justify your answer.

The half-life of the reaction in trial 4 is less than the half-life in trial 1. Explain why, in terms of activation energy.

 2C₄H₆(g)   →  C₈H₁₂(g)

At high temperatures the compound C₄H₆(1,3-butadiene) reacts according to the equation The rate of the reaction was studied at 625 K in a rigid reaction vessel. Two different trials, each with a different starting concentration , were carried out. The data were plotted in three different ways, as shown below.

For trial 1, calculate the initial pressure, in atm, in the vessel at 625 K. Assume that initially all the gas present in the vessel is C₄H₆.

Use the data plotted in the graphs to determine the order of the reaction with respect to C₄H₆ .

The initial rate of the reaction in trial 1 is 0.0010 mol/(L·s). Calculate the rate constant, k , for the reaction at 625 K.

The complete photoelectron spectrum of an element is represented above.

Identify the element.

Calculate the value of the rate constant, k , for the radioactive decay. Include units with your answer.

If 64 atoms of the radioactive isotope are originally present in a sample, what is the expected amount of time that will pass until only one atom of the isotope remains? Show how you arrived at your answer.

The following equation represents the decomposition of N₂O₅, for which the rate law is rate = k[N₂O₅]

2 N₂O₅ (g) → 4 NO₂ (g) + O₂ (g) 

A sample of pure N₂O₅ (g) is placed in an evacuated container and allowed to decompose at a constant temperature of 300 K. The concentration of N₂O₅ (g) in the container is measured over a period of time, and the measurements are recorded in the following table.

 Determine the value of the rate constant, k , for the reaction. Include units in your answer.

The following mechanism is proposed for the decomposition of N₂O₅ (g).

Step 1: N₂O₅ (g) → NO₂ (g) + NO₃ (g)

Step 2: NO₂ (g) + NO₃ (g) → NO₂ (g) + NO (g) + O₂ (g)

Step 3: N₂O₅ (g) + NO (g) → 3 NO₂ (g)

Identify which step of the proposed mechanism (1, 2, or 3) is the rate-determining step. Justify your answer in terms of the rate law given.

If this experiment was repeated at the same temperature but with twice the initial concentration of N₂O₅, would the value of k increase, decrease, or remain the same? Explain your reasoning.