Problem 151 - Introductory Chemistry Atoms First

Question

Suppose a postulated reaction mechanism generates a rate law that does not agree with the experimentally determined rate law. What does this say about the postulated mechanism?

Step-by-Step Solution

Answer

If a postulated reaction mechanism generates a rate law that does not agree with the experimentally determined rate law, it indicates that the proposed mechanism is incorrect or incomplete, possibly occurring through a different mechanism or multiple mechanisms. Researchers should revise the mechanism based on additional data or theories to match the observed rate law, as the discrepancy shows that the proposed mechanism is not accurately representing the reaction process.

Step 1: Understanding the Rate Law

A rate law is a mathematical expression that relates the reaction rate to the concentrations of reactants. It is obtained through experimental observations. A postulated reaction mechanism is a hypothetical step-by-step process outlining the sequence of elementary reactions that occur during a chemical reaction.

Step 2: Comparing Postulated and Experimental Rate Laws

When a postulated reaction mechanism generates a rate law, it should ideally match the experimentally determined rate law. This would potentially validate the proposed mechanism as a correct representation of the reaction.

Step 3: Mismatch between Postulated and Experimental Rate Laws

If a postulated reaction mechanism generates a rate law that does not agree with the experimentally determined rate law, it indicates that the proposed mechanism is incorrect or incomplete. It is also possible that the reaction occurs through an entirely different mechanism or through multiple mechanisms.

Step 4: Revision of Postulated Mechanism

When a mismatch between the postulated rate law and the experimental rate law occurs, researchers should reconsider the proposed reaction mechanism, identifying any missing steps or assumptions. They may need to revise the mechanism based on additional experimental data or new theories to match the observed rate law.

Step 5: Conclusion

In conclusion, a discrepancy between a postulated reaction mechanism's rate law and the experimentally determined rate law means that the proposed mechanism is not an accurate representation of the reaction. It requires further investigation and revision to match the observed rate law and correctly describe the reaction process.

Key Concepts

Rate Law

The rate law is an essential concept in understanding chemical kinetics. It is a mathematical equation that links the speed (or rate) of a chemical reaction to the concentration of the reactants. The general form of a rate law can be written as \( \ ext{rate} = k[A]^m[B]^n \), where \( k \) is the rate constant, \( [A] \) and \( [B] \) are the reactant concentrations, and \( m \) and \( n \) are the order with respect to each reactant. These orders must be determined experimentally and are not always equal to the stoichiometric coefficients of the balanced equation.

The purpose of knowing the rate law is to predict how the concentration of reactants will change over time and how fast the products will form. It provides crucial insights into the reaction's dependence on reactant concentrations. Rate laws derived from experiment allow us to understand the underlying reaction mechanisms.

Elementary Reactions

Elementary reactions are the simplest steps or stages that compose a complex reaction. Each elementary reaction represents a simple molecular event that occurs, like a collision between molecules or the breaking of a bond.

These reactions are characterized by having stoichiometry that matches their reaction orders directly, meaning that the coefficients of the reactants in the balanced equation are the same as the exponents in the rate law. This direct relationship makes elementary reactions distinct from overall reactions in a mechanism because their orders can be deduced directly from their molecularity.

In complex reactions, multiple elementary reactions are combined to form a reaction mechanism. This reaction sequence must align with the experimentally observed rate law to confirm its validity.

Experimental Observations

In chemical kinetics, experimental observations play a crucial role in determining the rate law of a reaction. By conducting experiments, scientists can measure changes in reactant concentrations over time. This data is used to construct a rate law that describes how the rate depends on the concentration of reactants.

• Such observations help confirm or refute proposed reaction mechanisms.
• They provide experimental rate laws that serve as a benchmark for testing mechanisms.
• By comparing observed data with predicted data from different mechanisms, chemists can refine their understanding of reaction processes.

Discrepancies between observed rate laws and those predicted by a mechanism suggest that the mechanism needs to be revised, highlighting the importance of experimental data in the study of reaction kinetics.

Reaction Sequence

The reaction sequence refers to the series of elementary reactions that occur in a step-by-step manner during a chemical reaction. Understanding this sequence is crucial for elucidating the full mechanism of a complex reaction.

• Each step in the sequence must be analyzed to determine how it contributes to the overall reaction and affects the rate law.
• These sequences are proposed based on theoretical knowledge and adjusted with experimental data.
• A correct reaction sequence explains not only the rate law but also other phenomena like intermediate formation and energy changes.

When a proposed sequence leads to an incorrect rate law, it often indicates missing steps or incorrect assumptions. Accurate determination and understanding of the reaction sequence enhance our ability to predict lab outcomes and design more efficient chemical processes.