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A topic from the subject of Kinetics in Chemistry.

  • 11 months ago
  • 6 min read
Molecularity of Reactions
Introduction

In chemical reactions, molecularity refers to the number of reactant molecules that participate in the rate-determining step. The molecularity of a reaction helps determine its reaction rate and mechanism.

Basic Concepts

Unimolecular Reactions: One reactant molecule participates in the rate-determining step.

Bimolecular Reactions: Two reactant molecules participate in the rate-determining step.

Termolecular Reactions: Three reactant molecules participate in the rate-determining step. (Rare)

Types of Experiments

Initial Rate Method: Measures the initial rate of a reaction under different reactant concentrations.

Differential Rate Method: Monitors the change in reactant concentrations over time.

Data Analysis

Rate Law: The mathematical equation that relates the rate of the reaction to the reactant concentrations.

Order of Reaction: The exponent of each reactant's concentration in the rate law.

Molecularity: Deduced from the mechanism of the reaction (Note: Molecularity is not directly deduced from the *order* of the reaction. While they are related, the order is experimentally determined, while molecularity is inferred from the proposed mechanism.).

Applications

Determining reaction mechanisms

Predicting reaction rates

Optimizing chemical processes

Conclusion

Molecularity is a crucial concept in understanding the dynamics of chemical reactions. By determining the molecularity of a reaction, chemists can gain insights into its mechanism and reaction kinetics.

Molecularity of Reactions

Definition: Molecularity refers to the number of reactant molecules or atoms that participate in the rate-determining step (the slowest step) of a chemical reaction. It is an important concept in chemical kinetics, providing insights into reaction mechanisms.

Types of Molecularity
  • Unimolecular Reactions (Molecularity = 1): A single reactant molecule or atom undergoes a change to form products. These reactions often involve isomerization (rearrangement of atoms within a molecule) or decomposition (breakdown of a molecule into smaller parts). Examples include the decomposition of N2O5 or the cis-trans isomerization of certain alkenes.
  • Bimolecular Reactions (Molecularity = 2): Two reactant molecules or atoms collide and interact to form products. This is a very common type of reaction. Examples include SN2 reactions and many gas-phase reactions such as the reaction between H2 and I2 to form HI.
  • Termolecular Reactions (Molecularity = 3): Three reactant molecules or atoms collide simultaneously in the rate-determining step. These reactions are relatively rare due to the low probability of three molecules colliding with the correct orientation and energy at the same time. A well-known example (though its mechanism is complex and not strictly termolecular in all interpretations) involves the recombination of three atoms of a radical like 2NO + O2 → 2NO2
  • Higher Molecularity Reactions: Reactions with molecularity greater than three are extremely rare and practically nonexistent due to the incredibly low probability of simultaneous collisions of multiple molecules.
Important Considerations
  • Molecularity is determined from the rate-determining step of the reaction mechanism, not the overall stoichiometry of the balanced equation.
  • Molecularity applies only to elementary reactions (single-step reactions). Complex reactions involving multiple steps are described by their overall reaction order, not molecularity.
  • Molecularity is always a positive integer (1, 2, 3...), unlike the order of a reaction which can be fractional or negative.
Distinction between Molecularity and Order

It's crucial to distinguish molecularity from the order of a reaction. Molecularity describes the number of molecules involved in a single step of a reaction mechanism, while the order of a reaction is an experimentally determined quantity reflecting the overall dependence of the reaction rate on reactant concentrations.

Conclusion: Molecularity is a fundamental concept in chemical kinetics that helps in understanding reaction mechanisms and predicting reaction rates for elementary reactions. It is, however, limited in describing complex reactions where the rate is determined by a series of steps.

Experiment: Determination of Molecularity of Reactions
Materials:
  • Potassium permanganate solution (0.02 M)
  • Oxalic acid solution (0.01 M)
  • Sulfuric acid solution (0.5 M)
  • Stopwatch
  • Test tubes
  • Graduated cylinders
  • Pipettes (for accurate volume measurement)
  • Conical flask or beaker (to mix the solutions)
Procedure:
  1. Experiment 1: Take three clean and dry test tubes and label them as A, B, and C.
  2. Using a pipette, add 10 mL of 0.02 M potassium permanganate solution to test tube A.
  3. Using a pipette, add 10 mL of 0.01 M oxalic acid solution to test tube B.
  4. Using a pipette, add 10 mL of 0.5 M sulfuric acid solution to test tube C.
  5. Pour the contents of test tubes A and B into a clean conical flask. Mix the solutions thoroughly.
  6. Start the stopwatch immediately after mixing.
  7. Observe the reaction (the disappearance of the purple color of potassium permanganate). Record the time it takes for a significant change (e.g., complete decolorization) to occur. Note this observation in a data table.
  8. Repeat steps 2-7 at least three times, and record the time for each trial. Calculate the average time.
  9. Experiment 2: Repeat Experiment 1, but this time vary the volume of 0.01 M oxalic acid solution added to the conical flask (e.g., 5 mL, 10 mL, 15 mL). Keep the volume of potassium permanganate solution constant (10 mL). Ensure the total volume remains consistent by adjusting the amount of distilled water added.
  10. Record the average time for each reaction in a data table. Include the concentration of oxalic acid for each run.
Observations:

Create a data table to record the following observations for both experiments. Example below:

Experiment Trial Volume of Oxalic Acid (mL) Concentration of Oxalic Acid (M) Time for Reaction (s)
1 1 10 0.01 ...
1 2 10 0.01 ...
1 3 10 0.01 ...
2 1 5 0.005 ...
2 2 10 0.01 ...
2 3 15 0.015 ...
Conclusions:

Analyze the data from the table. Plot a graph of reaction rate (1/time) vs. concentration of oxalic acid. The order of the reaction with respect to oxalic acid can be determined from the graph (linear relationship suggests first order, quadratic relationship suggests second order etc.) A similar experiment could be done varying the potassium permanganate concentration to determine the overall order of reaction.

  • Based on your data analysis, determine the order of the reaction with respect to each reactant.
  • State the overall order of the reaction.
  • Discuss the role of sulfuric acid in the reaction. (It acts as a catalyst).
  • Explain the relationship between the order of the reaction and the molecularity of the reaction (note that these are not directly interchangeable concepts. Molecularity refers to the stoichiometry of the *rate determining step* of the reaction mechanism, while order refers to the experimentally determined relationship between rate and concentration).
Significance:

This experiment demonstrates how the rate of a chemical reaction can be used to infer information about its mechanism. By studying the effect of reactant concentrations on reaction rate, we can determine the reaction order and gain insights into the molecularity of the reaction’s rate-determining step. This is a fundamental concept in chemical kinetics.

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