Question

The incorrect statement is (a) Rate law is an experimental fact whereas law of mass action is a theoretical proposal. (b) Rate law is always different from the expression of law of mass action. (c) Rate law is more informative than law of mass action for the development of mechanism. (d) Order of a reaction is equal to the sum of powers of concentration terms in the rate law.

Short answer

The incorrect statement is (b) Rate law is always different from the expression of law of mass action.

Step-by-step solution

Identify Incorrect Statement

Examine each statement to determine if it is factually correct. Statement (a) suggests that the rate law is derived from experimental data while the law of mass action is theoretical, which is true. In statement (b), saying 'always' implies an absolute condition which might not hold in every case since rate laws can sometimes coincide with the expression from the law of mass action. Statement (c) is true because the rate law can provide insight into the reaction mechanism by showing how the rate depends on reactant concentrations. Statement (d) is a definition and is typically correct for elementary reactions, so it is often true.

Evaluate Each Statement

Statement (a) is correct since rate laws are indeed based on experimental observations, and the law of mass action is based on a theoretical approach to the behavior of gases which was later applied to reaction rates. Statement (b) is not always true and can be considered a generalization. In some cases, the rate law will align with the mass action expression, especially for elementary reactions where the stoichiometry directly relates to the reaction order. Statement (c) is correct as the rate law can contain information about the mechanism that the law of mass action might not provide. Statement (d) is true for elementary reactions, but not necessarily for complex reactions, as the order of the reaction might not be evident from the reacting species' stoichiometry. However, the exercise is looking for the incorrect statement. Thus, further analysis is required.

Analyze the Tricky Parts

It's worth a second look at statement (b) since it includes the absolute term 'always,' which can often indicate a false statement. Upon closer inspection, the statement can be refuted with examples where the rate law does match the expression of the law of mass action. Therefore, statement (b) is more likely to be the incorrect one compared to the other statements.

Select the Incorrect Statement

Given the analysis, the incorrect statement is (b) since it includes an absolute condition that doesn't hold for every chemical reaction. There are indeed cases where the rate law and the law of mass action can be expressed the same way, particularly for elementary reactions.

Key concepts

Chemical Kinetics

Chemical kinetics is the branch of chemistry that studies the rates of chemical reactions, the factors affecting these rates, and the sequence of steps that constitute the reaction mechanisms. Understanding the time-dependent aspect of chemical transformation is essential to controlling processes ranging from combustion to the metabolism of food in our bodies.

At its core, chemical kinetics delves into how different conditions, such as temperature, pressure, and concentration, affect the speed of reactions. By conducting experiments and interpreting data, chemists deduce rate laws—which are mathematical relationships that link the rate of a reaction to the concentrations of reactants. These rate laws are empirical and specific to each reaction, providing direct insight into the kinetic behavior under given conditions.

When studying chemical kinetics, it's also important to recognize that the rate of a reaction can be affected by the presence of catalysts, which can dramatically increase reaction rates without being consumed in the process. This dynamic field of chemistry sits at the intersection of theoretical study and practical application, enabling chemists to both predict reaction behavior and engineer conditions for desired reaction rates.

Reaction Mechanism

A reaction mechanism is a detailed step-by-step description of how a chemical reaction occurs at the molecular level. It explains which bonds break, which bonds form, and the order of these events. It's like a molecular choreography that dictates the transformation of reactants into products.

The significance of the reaction mechanism extends beyond simple understanding of a process; it allows chemists to predict the outcome of reactions, identify intermediates, and optimize conditions for synthesis. From a kinetic standpoint, the mechanism is a crucial piece of the puzzle. It sometimes involves species that do not appear in the overall balanced equation, like transient intermediates or catalysts.

For instance, a complex reaction may proceed through a series of elementary steps, each with its own rate law. These steps can include formations of unstable intermediates and may involve various molecular entities participating at different stages. The cumulative information from these steps contributes to understanding the overall rate law, as it dictates how the concentrations of reactants influence the rate of reaction.

Reaction Order

Reaction order is a key concept in chemical kinetics that defines the relationship between the concentration of reactants and the rate of the reaction. It provides a quantitative measure of how sensitive the rate of reaction is to the concentration of each reactant. The reaction order can be determined for each reactant individually (partial order) or for the overall reaction (overall order).

For elementary reactions, the reaction order is often the sum of the stoichiometric coefficients of the reactants in the balanced chemical equation. However, for complex reactions, the overall reaction order can be different from the sum of the exponents of the concentration terms in the rate law equation.

Reaction orders are not always integers; they can be fractions or even zero, reflecting the nuanced way reactant concentrations influence the reaction rate. Determining the order of a reaction is typically done through experiments and cannot be predicted solely from the chemical equation of the reaction. This further highlights the distinction between the stoichiometry of the reaction and its kinetics, an important point that often confuses students when first learning about these concepts.