Question

A zero-order reaction is one (a) in which reactants do not react. (b) in which one of the reactants is in large excess. (c) whose rate does not change with time. (d) whose rate increases with time.

Short answer

The correct answer to the characteristic of a zero-order reaction is (c) whose rate does not change with time.

Step-by-step solution

Understanding Zero-Order Reactions

A zero-order reaction is a type of chemical reaction where the rate is independent of the concentration of the reactants. This means that the reactants can be present in any concentration, and the reaction rate will remain constant until one of the reactants is depleted.

Analyzing the Options

Option (a) suggests reactants do not react, which is incorrect as the reaction does take place in a zero-order reaction. Option (b) is not necessarily characteristic of zero-order kinetics; it can be true for any reaction order. Option (c) correctly describes a zero-order reaction, where the reaction rate remains constant over time. Lastly, option (d) is incorrect as the rate does not increase with time in a zero-order reaction; it is independent of the concentration of reactants.

Choosing the Correct Answer

Based on the definition of a zero-order reaction, where the rate remains constant over time and is independent of the concentration of reactants, the correct answer is option (c).

Key concepts

Chemical Reaction Rate

The rate of a chemical reaction is a measure of how quickly the reactants are turned into products. In simpler terms, it's how fast a chemical reaction proceeds. Factors that can affect this rate include the concentration of reactants, temperature, presence of a catalyst, and surface area. When the concentration of reactants is higher, there are more collisions between particles, which generally leads to an increased reaction rate.

However, in the particular scenario of a zero-order reaction, this typical dependency on concentration does not apply. While most reactions speed up with increased concentration of reactants, a zero-order reaction maintains a constant rate regardless of such changes. This is a unique characteristic that sets zero-order kinetics apart from other reaction orders.

Reaction Kinetics

Reaction kinetics is the study of the rates at which chemical processes occur and the factors that affect these rates. It deals with understanding the steps, or mechanisms, by which a reaction progresses from reactants to products and measuring the speed of each of these steps. Kinetics provides insights into the energetic and molecular dynamics of reactions.

Rate Laws

One of the fundamental aspects of reaction kinetics is the rate law, an equation that relates the reaction rate to the concentrations of reactants. For most reactions, the rate increases when the concentration of one or more reactants increases. However, for a zero-order reaction, the rate does not depend on the concentration of reactants, which is what sets it apart in kinetic studies.

Chemical Reaction Concentration

The concentration of a reactant in a chemical reaction is typically expressed as the amount of substance per unit volume, such as moles per liter (Molarity). In most chemical reactions, the concentration of reactants is crucial because it directly influences the reaction rate through the frequency of particle collisions.

Concentration Effects

In reactions of orders other than zero, increasing the concentration generally results in more frequent collisions and, therefore, a higher rate of reaction. Conversely, decreasing the concentration leads to fewer collisions and a slower reaction. Understanding the role of concentration in reaction kinetics is essential for controlling and optimizing chemical processes in various industries, such as pharmaceuticals, manufacturing, and environmental engineering.

Zero-Order Kinetics

Zero-order kinetics refers to a situation where the rate of reaction is constant and uninfluenced by the concentration of the reactants. In practice, this means that as long as there is some amount of reactant present, the reaction rate stays the same. The reaction proceeds at a linear rate, with a constant amount of product being formed per unit time until one of the reactants is completely consumed.

Graphical Representation

A plot of reactant concentration versus time for a zero-order reaction will show a straight line with a negative slope, indicating the constant rate of reaction. This type of kinetics is less common than first or second-order kinetics but can occur under certain conditions, such as when a reactant is present in large excess or when the reaction is catalyzed by a surface and the surface becomes fully covered by the reactants.