Question

Explain why rate laws generally cannot be written from balanced equations. Under what circumstance is the rate law related directly to the balanced equation for a reaction?

Short answer

In summary, rate laws cannot be directly derived from balanced equations, as they depend on reaction mechanisms and rate-determining steps, which may not correlate with stoichiometric coefficients. However, when a reaction occurs in a single elementary step, the rate law is directly related to the balanced equation.

Step-by-step solution

Introduction to Stoichiometry and Kinetics

Stoichiometry is the study of the quantitative relationships between reactants and products in a chemical reaction, as represented by a balanced equation. Balanced equations give the proportion of reactants and products that participate in the reaction, but they do not provide information about the reaction speed or how it occurs on the molecular level. Kinetics is the study of reaction rates and the factors that affect these rates. The rate law of a chemical reaction describes the relationship between the rate of the reaction and the concentrations of reactants. It usually takes the form: rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are the concentrations of the reactants, and m and n are the reaction orders.

Reaction Orders

The reaction orders (m and n) are determined experimentally, as they depend on the reaction mechanism, which is the detailed sequence of elementary steps that a reaction undergoes. The reaction orders are not always equal to the stoichiometric coefficients in the balanced equation. This is because the stoichiometric coefficients describe the overall reaction, whereas the reaction orders are determined by the elementary steps which may or may not involve all reactants involved in the balanced equation.

Rate-determining step

In a multi-step reaction, the rate-determining step is the slowest elementary step in the reaction mechanism, which dictates the overall rate of the reaction. The rate law is determined by the rate-determining step and may involve reactants that aren't necessarily involved in other steps. This is another reason why a rate law generally cannot be derived directly from a balanced equation because the balanced equation represents the overall reaction, not the rate-determining step.

Cases where Rate Law is Related to Balanced Equation

There is one circumstance in which the rate law can be directly related to the balanced equation: when the overall reaction occurs in a single step (elementary reaction) with no intermediate steps. In this case, the rate-determining step is the overall reaction, and the reaction orders directly correspond to the stoichiometric coefficients of the reactants in the balanced equation. This is relatively rare for complex reactions but can be observed in simpler reactions. In conclusion, rate laws generally cannot be written directly from balanced equations because they are determined by the reaction mechanism and rate-determining step, which may not directly correspond to the stoichiometric coefficients in the balanced equation. However, if the overall reaction occurs in a single elementary step, the rate law can be directly related to the balanced equation.