Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is incorrect about order of reaction? (a) it is calculated experimentally (b) it is sum of powers of concentration in rate law expression (c) the order of reaction cannot be fractional (d) there is not necessarily a connection between order and stoichiometry of a reaction.

Step-by-Step Solution

Answer

Option (c) is incorrect because reaction orders can indeed be fractional.

Step 1: Understand the Concepts

The order of reaction is determined experimentally, based on how the rate changes with concentration. It is the sum of the powers of the concentration terms in the rate law expression. This may not always match the stoichiometry of the balanced chemical equation.

Step 2: Evaluate Option (a)

Option (a) states that the order of reaction is calculated experimentally, which is correct. The order is determined based on experimental data rather than theoretical predictions.

Step 3: Evaluate Option (b)

Option (b) indicates that the order of reaction is the sum of powers of concentration in the rate law expression. This is also correct as it aligns with the definition of how order is determined.

Step 4: Evaluate Option (c)

Option (c) asserts that the order cannot be fractional. This is incorrect, because orders of reaction can indeed be fractional, whole, or even zero, depending on how they fit the observed data.

Step 5: Evaluate Option (d)

Option (d) suggests there isn't necessarily a connection between order and stoichiometry. This is correct, as stoichiometry does not always directly dictate the order of reaction because the latter is experimentally determined.

Key Concepts

Rate Law Expression

A rate law expression gives us insight into how the rate of a chemical reaction depends on the concentration of reactants. Generally, the expression takes the form: \( ext{{Rate}} = k[A]^m[B]^n \), where \( k \) is the rate constant, and \( [A] \) and \( [B] \) are the concentrations of the reactants. The exponents \( m \) and \( n \) represent the order of the reaction with respect to each reactant. The overall order of the reaction is the sum of these individual orders: \( m + n \).

• Only experimentally determined values can truly define these orders.
• The rate constant \( k \) is unique to every reaction and changes with temperature.

Understanding the rate law expression is crucial because it allows chemists to predict how changes in concentration affect the reaction rate.

Experimental Determination

The order of reaction is fundamentally defined through experimentation. Unlike coefficients in a balanced equation, which are based on stoichiometry and simply reflect the molar ratios, the order of reaction is determined by observing how different concentration levels impact the rate of the reaction.

• Common methods to determine reaction order include the method of initial rates and the integrated rate laws approach.
• Laboratory experiments are essential, and they involve measuring how quickly products form or reactants disappear under various concentrations.

By analyzing the experimental data, chemists can derive a precise rate equation for the reaction.

Stoichiometry and Reaction Order

Though stoichiometry and reaction order may seem related, they often differ significantly. Stoichiometry tells us the ratio in which reactants combine in a reaction. Reaction order, on the other hand, is determined by observing the kinetic data.

• The stoichiometric coefficients from a balanced equation do not always correlate with the order of the reaction.
• The reaction order can depend on complex mechanisms that may involve intermediate steps not directly visible in the stoichiometric equation.

This makes comprehending the kinetic behavior essential to understanding a chemical reaction beyond its surface-level description.

Fractional Order Reactions

Fractional order reactions are an intriguing aspect of kinetics that defy the simplicity of whole number orders commonly taught in basic chemistry. A fractional order means that the concentration of a reactant affects the rate of reaction in a non-integer proportion.

• For example, if a reaction is 1.5 order with respect to substance \( A \), the rate would vary with \([A]^{1.5}\).
• Such anomalies often arise from complex sequences of multiple reaction steps or surface-catalyzed reactions.

Fractional orders underscore the necessity of experimental verification in kinetics, exploring paths not easily explained by a simplistic view of chemical reactions.