Question

Identify the correct statements: (a) The order of an elementary reaction is equal to its molecularity (b) The order of a reaction can be zero (c) For second order reaction, order of reaction \(=2 \times\) molecularity. (d) The order of inversion of cane sugar is 2 .

Short answer

Correct statements are (a) and (b).

Step-by-step solution

Understanding Reaction Order and Molecularity

The order of a reaction refers to the sum of the exponents of the concentration terms in the rate equation. Molecularity, on the other hand, refers to the number of molecules involved in an elementary reaction step. For elementary reactions, the order is typically equal to the molecularity.

Analyzing Statement (a)

Statement (a) claims that for elementary reactions, the order equals the molecularity. This is true because elementary reactions take place in a single step, and the molecularity gives the direct stoichiometric relationship.

Analyzing Statement (b)

Statement (b) claims the order of a reaction can be zero. This is indeed true, as some reactions have a rate that is independent of the concentration of the reactants, resulting in a zero order reaction.

Analyzing Statement (c)

Statement (c) asserts that for a second-order reaction, the order is twice the molecularity. This is incorrect; the order being twice the molecularity does not hold in general because molecularity is fixed to elementary steps and order is derived from the rate law.

Analyzing Statement (d)

Statement (d) suggests that the order of inversion of cane sugar is 2. In reality, the inversion of cane sugar such as in the hydrolysis of sucrose is typically a first-order reaction with respect to sucrose concentration.

Key concepts

Elementary Reactions

In chemistry, elementary reactions are the simplest type of chemical reactions, taking place in a single step. This means, they are not composed of intermediate steps like complex reactions. In an elementary reaction, every molecule collides and reacts according to a straightforward mechanism.

The significance of elementary reactions lies in their ability to provide immediate insight into how particles interact. Because they occur in one step, the theoretical construct of molecularity naturally applies, showing us how molecular interactions directly translate to reaction rates. This results in the reaction order and molecularity being the same for elementary reactions. Molecularity offers us the number of particles that must collide to bring about the reaction, whereas the reaction order gives the reactive concentration's sum over the involved molecules.

Molecularity

Molecularity is a term that describes the number of molecules that participate in a single, elementary step of a reaction. It is an intrinsic property of elementary reactions. Because it relates only to single-step reactions, molecularity is constant and integer-valued.

When we break it down, molecularity can show us how collisions are necessary for reactions:

• Unimolecular processes involve a single molecule transitioning through a change, often seen in radioactive decay.
• Bimolecular processes need two molecules to collide, typically leading to the most common form of chemical reactions.
• Trimolecular processes, involving three molecules colliding, are rare given the improbability of simultaneous triple collisions under normal conditions.

For any reactions that are not elementary, molecularity is not applicable, as these reactions may go through various multi-step mechanisms.

Zero Order Reactions

Zero order reactions are fascinating because they act independently of the concentration of reactants. In these reactions, the rate remains constant, regardless of alterations in reactant concentrations.

These kinds of reactions often occur in surface reactions or enzyme catalyzed reactions where the rate-limiting step reaches its maximal capacity. Zero order reactions tell us that increasing concentration does not increase reaction rates because the process is not dependent on how much reactant is present.

Mathematically, a zero order reaction is represented by the equation:\[ Rate = k \]where \( k \) is the rate constant. Here, since the concentration terms have no exponent, modifying them does not influence the rate of reaction.

Rate Equation

The rate equation is a cornerstone in understanding how the rate of a chemically complex reaction relates to the concentration of the reactants. Essentially, it is a mathematical expression that ties reaction rate to reactant concentrations.

The form of a rate equation is often shown as:\[ Rate = k [A]^x [B]^y \]In this equation, \( k \) is the rate constant, and \( x \) and \( y \) are the orders of the reaction concerning each reactant, \( A \) and \( B \) respectively.

• The total order of a reaction is the sum of the exponents \( x + y \).
• Orders can be zero, positive, or even fractional, depending on the complex mixtures of steps in a reaction.

Understanding the rate equation allows us to predict how changes in concentration will influence the speed at which a reaction proceeds. It guides chemists in controlling reaction conditions to achieve desired products efficiently.