Question

For a zero-order reaction, the plot of concentration vs time is linear with (a) +ve slope and zero intercept (b) -ve slope and zero intercept (c) tve slope and non-zero intercept (d) -ve slope and non-zero intercept

Short answer

(d) -ve slope and non-zero intercept.

Step-by-step solution

Understand the Reaction Order

Zero-order reactions have a constant rate that is independent of the concentration of the reactant. This means the rate at which the reaction occurs is not affected by how much of the reactant is present.

Recognize the Linear Relationship

For a zero-order reaction, the concentration of the reactant decreases linearly over time because the rate is constant. This implies that a graph of concentration vs time will be a straight line.

Determine the Slope

Since the concentration decreases over time, the line will have a negative slope. A straight line with a negative slope will run downwards from left to right on the graph.

Analyze the Intercept

A zero-order reaction typically begins with an initial concentration at time zero. Thus, when the time is zero, the concentration is equal to the initial concentration, providing a non-zero intercept on the concentration axis.

Identify the Correct Option

Given the characteristics of the graph with a negative slope and a non-zero intercept, the correct answer corresponds to option (d).

Key concepts

Reaction Kinetics

Reaction kinetics is the branch of chemistry that explores the rates of chemical reactions and the factors that influence them. In simple terms, it tells us how fast or slow a reaction occurs. Kinetics provides insight into the steps involved in a reaction, known as the reaction mechanism. Understanding this can help chemists control reaction conditions for desired outcomes.

There are different types of reaction orders based on how the concentration of reactants affects the rate:

• Zero-order reactions: The rate is constant and does not depend on the concentration of reactants. This means that even if the concentration changes, the reaction rate remains the same.
• First-order reactions: The rate is directly proportional to the concentration of one reactant. If you double the concentration, the reaction rate doubles.
• Second-order reactions: The rate depends on the concentration of one reactant squared or two reactants multiplied together.

Reaction kinetics is essential for predicting how a reaction progresses over time and determining conditions that either speed up or slow down the process.

Concentration vs Time Graph

A concentration vs time graph is a visual representation of how the concentration of a reactant changes over time during a chemical reaction. Understanding this graph is crucial for identifying the order of a reaction.

In the case of a zero-order reaction, the graph is particularly straightforward:

• It appears as a straight line.
• The slope of the line is negative, indicating that the concentration of the reactant decreases over time.
• This negative slope reflects the constant reaction rate of zero-order reactions, as the reactant is consumed at a steady pace.

The intercept on the concentration (y) axis is usually the initial concentration, showing where the reaction starts when time equals zero. This gives it a non-zero intercept, as the reaction typically begins with a definite amount of reactant.

Such graphs are handy as they allow chemists to quickly assess reaction progress and verify reaction order parameters.

Reaction Rate

The reaction rate quantifies how fast or slow a chemical reaction proceeds. It can vary significantly depending on several factors, such as concentration, temperature, and presence of a catalyst.

For zero-order reactions, the rate remains constant throughout the reaction, regardless of concentration. This is unique, as most reactions speed up or slow down as reactants get used up or produced.

• In a zero-order reaction, the rate expression is denoted as: \[\text{Rate} = k\]where \( k \) is the rate constant.
• This implies that if you were to measure the rate, it would not change even if you change the concentration of the reactant.

This stability in reaction rate is why zero-order reactions show a linear decrease in concentration over time, represented by the negative slope in their concentration vs time graph. Reaction rates are vital not just for academic study but for real-world applications where controlling the speed of chemical processes is critical.