Problem 96 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is correct? (1) order of a reaction can be known from experimental results and not from the stoichiometry of a reaction. (2) molecularity a reaction refers to (i) each of the elementary steps in (an overall mechanism of) a complex reaction or (ii) a single step reaction. (3) overall molecularity of a reaction may be determined in a manner similar to overall order of reaction. (4) overall order of a reaction \(\mathrm{A}^{\mathrm{m}}+\mathrm{B}^{n} \longrightarrow \mathrm{AB}_{\mathrm{x}}\) is \(\mathrm{m}+\mathrm{n} .\) Select the correct answer using the following codes: (a) 2 and 3 (b) 1,3 and 4 (c) 2,3 and 4 (d) 1,2 and 3

Step-by-Step Solution

Answer

The correct answer is (d) 1 and 2.

Step 1: Understanding Option 1

Option 1 states that the order of a reaction can be known from experimental results and not from the stoichiometry of a reaction. This is true as the order of a reaction is determined experimentally and not necessarily related to the stoichiometric coefficients in the balanced chemical equation. Thus, this statement is correct.

Step 2: Understanding Option 2

Option 2 is about the molecularity of a reaction which refers to (i) each of the elementary steps in an overall mechanism of a complex reaction or (ii) a single step reaction. Molecularity is related to a single elementary step of a reaction and is defined as the number of molecules involved in that step. Hence, this statement is correct; molecularity does not apply to the overall reaction like reaction order.

Step 3: Analyzing Option 3

Option 3 states that overall molecularity of a reaction may be determined similarly to the overall order of reaction. This is incorrect because molecularity applies to individual elementary steps with a fixed number of molecules, whereas overall reaction order is determined experimentally and applies to the entire reaction.

Step 4: Evaluating Option 4

Option 4 claims that the overall order of a reaction \(\mathrm{A}^{\mathrm{m}}+\mathrm{B}^{n} \longrightarrow \mathrm{AB}_{\mathrm{x}}\) is \(\mathrm{m}+\mathrm{n}\). This is incorrect because reaction order is not necessarily the sum of stoichiometric coefficients; rather, it is determined experimentally.

Step 5: Identify Correct Combination

From the analysis, options 1 and 2 are correct. Therefore, by finding which selection includes both correct options without incorrect ones, we notice option (d) is not possible due to incorrect elements. The correct answer including only valid assertions is option (b), containing options 1 and some other correct element.

Key Concepts

Chemical Kinetics

Chemical kinetics is the branch of chemistry that deals with understanding the rates of chemical reactions. It focuses on how different conditions affect the speed of reactions and provides insights into the steps involved in a reaction mechanism.

• Kinetics helps identify how quickly a reaction reaches completion.
• It distinguishes between reactants and products over time.
• The reaction rate can be affected by several factors, including temperature, concentration of reactants, and the presence of catalysts.

To determine the order of a reaction, chemists rely on experimental data. Reaction order is an empirical expression that indicates how the concentration of the reactants affects the rate of reaction. Unlike stoichiometric coefficients, which are easily deduced from a balanced chemical equation, reaction orders can be a whole number, fraction, or even zero. They must be determined through experiments and cannot be simply inferred from the equation.

Molecularity of Reaction

Molecularity refers to the number of reacting species (atoms, ions, or molecules) that come together to react in an elementary step of a reaction mechanism. This concept exclusively pertains to elementary reactions, which make up the overall complex reaction mechanisms.

• Unimolecular: Involves a single molecule, such as the decomposition of hydrogen peroxide.
• Bimolecular: Involves two molecules, like when two molecules of hydrogen gas react with one molecule of oxygen gas to form water.
• Termolecular: Involves three molecules, which is less common due to the low probability of three molecules colliding simultaneously.

It's important to note that molecularity is not the same as reaction order. Whereas reaction order is determined empirically and applies to the overall reaction, molecularity is a theoretical concept applying only to elementary steps and is always a whole number.

Stoichiometry

Stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. It helps in calculating the amounts of substances consumed and produced, ensuring that chemical equations are balanced according to the law of conservation of mass.

• Stoichiometry enables the calculation of reactants needed to produce a desired amount of product.
• It involves using the mole concept to relate masses, volumes, and numbers of particles.
• Coherent calculations rely on a balanced chemical equation, where the stoichiometric coefficients indicate the proportions of substances.

While stoichiometry provides the necessary framework for balancing chemical equations, it does not offer insights into reaction rates or mechanisms. It does, however, provide a foundation for studying kinetics as it identifies the initial concentrations or conditions needed to design experiments that measure reaction rates.