Question

In the following question two statements Assertion (A) and Reason (R) are given Mark. a. If \(\mathrm{A}\) and \(\mathrm{R}\) both are correct and \(\mathrm{R}\) is the correct explanation of \(\mathrm{A}\); b. If \(A\) and \(R\) both are correct but \(R\) is not the correct explanation of \(\mathrm{A}\); c. \(\mathrm{A}\) is true but \(\mathrm{R}\) is false; d. \(\mathrm{A}\) is false but \(\mathrm{R}\) is true, e. \(\mathrm{A}\) and \(\mathrm{R}\) both are false. (A): Order of reaction is an experimental property and irrespective of the fact whether the reaction is elementary or complicated, it is the sum of the powers of the concentration terms appearing in the rate law that is, experimentally observed rate law. (R): Order of reaction may change with change in experimental conditions.

Short answer

Option (b): Both A and R are correct, but R is not the correct explanation of A.

Step-by-step solution

Analyze Assertion (A)

The assertion states that the order of a reaction is determined experimentally as the sum of the powers of concentration terms in the rate law, regardless of the reaction complexity. This is true because reaction order depends on experimental rate laws rather than a theoretical reaction mechanism.

Analyze Reason (R)

The reason suggests that the order of a reaction can change with different experimental conditions. This is true, as factors like temperature, catalysts, or other variables can influence the measured rate law and thus change the reaction order.

Evaluate if R explains A

Although reason (R) is true, it does not explain assertion (A). The order of the reaction is explained by the form of the rate law but not by the variability due to changes in experimental conditions directly.

Determine Relationship between A and R

Since both A and R are individually true, but R does not explain A, option b is the correct classification according to the given criteria.

Key concepts

Reaction Order

Have you ever wondered what a reaction's order tells us about a chemical process? Simply put, the reaction order is a way to express how the concentration of reactants affects the rate of a reaction. It's an experimentally determined value. This means scientists perform experiments to find out how the rate varies with changes in concentration.

Reaction order can be zero, first, second, or even fractional, and it gives insight into the relationship between the concentration of reactants and the speed at which products are formed. Here's how to think about it:

• A **zero-order reaction**: The rate is independent of the concentration of the reactant. Even if you double the concentration, the rate remains unchanged.
• A **first-order reaction**: The rate is directly proportional to the concentration of one reactant. Double the concentration, and the rate doubles too.
• A **second-order reaction**: The rate depends on the square of the concentration of a reactant or the combined concentration of two reactants.

This concept of reaction order emphasizes the need for experimentation, as it cannot always be predicted just from the balanced chemical equation.

Rate Law

Rate law is like a recipe that tells us how the rate of a chemical reaction is linked to the concentration of its reactants. Formulating a rate law involves determining which reactant concentrations directly affect how fast the reaction proceeds. This is crucial in understanding and predicting reaction behavior under various conditions.

The rate law is usually expressed as:\[ \text{Rate} = k[A]^m[B]^n \]Here, \(k\) is the rate constant, \([A]\) and \([B]\) are the concentrations of reactants, and \(m\) and \(n\) are the exponents that indicate the reaction orders with respect to each reactant. These exponents are critical because they tell us how sensitive the reaction rate is to changes in concentrations.

Remember:

• Rate laws must be determined experimentally.
• Exponents in rate laws are not necessarily related to the coefficients in the balanced chemical equation.
• Rate laws help in deducing the mechanism of complex reactions.

Understanding rate laws allows chemists to control reaction conditions and predict how changes will affect the reaction rate.

Experimental Conditions

To truly grasp chemical kinetics, understanding experimental conditions is essential. These are the variables that scientists manipulate to investigate how they affect reaction rates and orders. Different experimental conditions can vary the reaction environment, leading to noticeable changes in how fast reactions proceed.

Common experimental factors that influence reaction rates include:

• **Temperature**: An increase in temperature usually increases the reaction rate, often exponentially. This occurs because higher temperatures provide more energy, allowing molecules to collide more frequently and with greater energy.
• **Catalyst presence**: Catalysts speed up reactions without being consumed. They provide an alternative pathway with a lower activation barrier.
• **Concentration of Reactants**: Changing concentrations of reactants can cause significant variation in the rate, directly linking to the reaction order.

These conditions can shift the observed reaction order, making it a dynamic property rather than a fixed one. By systematically altering each variable, chemists can better understand the underlying processes governing the speed and trajectory of chemical reactions.