Zero Order Reaction Rate Law and Key Examples
Zero order reaction is essential in chemistry and helps students understand various practical and theoretical applications related to this topic. Mastering this concept builds a strong base for learning about chemical kinetics, surface catalysis, and reaction mechanisms.
What is Zero Order Reaction in Chemistry?
A zero order reaction refers to a chemical reaction whose rate remains constant and does not depend on the concentration of reactants. This concept appears in chapters related to chemical kinetics, rate law, and surface catalysis, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
Zero order reactions do not have a specific molecular formula because the term describes a type of reaction rate behavior, not a particular compound. Such reactions can involve different substances, such as ammonia (NH3) in catalytic decomposition or hydrogen and chlorine in photochemical processes.
Preparation and Synthesis Methods
Zero order reactions are usually observed under special conditions. For example, the decomposition of NH3 on a platinum or molybdenum catalyst shows zero order behavior when the catalyst surface is saturated. Industrial photochemical reactions like the breakdown of HI with light are also classic examples.
Physical Properties of Zero Order Reactions
Physical properties like concentration and time are key to understanding zero order reactions. Their rate remains unchanged as the reactant concentration decreases. Graphically, the concentration-time plot is a straight line with a negative slope, showing steady reactant depletion.
Chemical Properties and Reactions
In a zero order reaction, the rate law is written as:
rate = k
Here, the rate constant (k) has units of mol L-1 s-1. The integrated rate law is:
[A] = [A]0 - kt
This means the concentration decreases steadily over time, regardless of how much reactant is present.
Frequent Related Errors
- Assuming the rate always increases with more reactant.
- Confusing zero order reactions with first or second order ones.
- Not using the correct units for the zero order rate constant.
- Forgetting that zero order is possible only under specific conditions, like surface saturation or enzyme limitation.
Uses of Zero Order Reaction in Real Life
Zero order reactions are widely observed in real life. Enzyme-catalyzed reactions like drug breakdown in the human body follow zero order when enzymes are saturated.
Industrial production of hydrogen chloride via photochemical reaction and ammonia decomposition on metal catalysts are classic uses.
Relation with Other Chemistry Concepts
Zero order reactions are closely related to topics such as First Order Reaction and Second Order Reaction. Understanding zero, first, and second order reactions helps students build a strong bridge between chemical kinetics and industrial process optimization.
Step-by-Step Reaction Example
- Consider the catalytic decomposition of ammonia (NH3) on platinum:
2NH3(g) → N2(g) + 3H2(g) - Under high NH3 concentration, all catalyst sites are saturated.
The rate becomes independent of [NH3]. - Write the rate law:
rate = k [NH3]0 = k - Integrated rate law:
[NH3] = [NH3]0 - kt - To find the half-life:
Set [NH3] = [NH3]0/2 and solve - t1/2 = [NH3]0 / (2k)
Lab or Experimental Tips
To identify a zero order reaction, check if the concentration vs. time graph is a straight line and the rate remains constant even if reactant is added. Vedantu educators often use this tip to help students spot zero order kinetics during live doubt sessions.
Try This Yourself
- Plot a graph of concentration vs. time using [A] = [A]0 - kt and identify the slope.
- Name two industrial reactions that show zero order behavior.
- Derive the half-life equation for a zero order reaction.
- Compare the units of k for zero, first, and second order reactions.
Final Wrap-Up
We explored zero order reaction—its core definition, rate equations, practical and industrial relevance, and graph interpretations. Build on your knowledge with more examples and deeper explanations by exploring chemistry notes and live classes on Vedantu.
Want to learn more about reaction rates and their impact? Check out Rate Law and Chemical Kinetics for related concepts.
FAQs on Zero Order Reaction Explained for Students
1. What is a zero order reaction in chemistry?
A zero order reaction is a chemical reaction in which the rate remains constant and is independent of the concentration of the reactant. This means that as the reaction progresses, the rate does not change regardless of how much reactant is present. Zero order kinetics are typical when the reaction rate is limited by factors such as surface saturation of a catalyst or light intensity in photochemical processes.
2. Give an example of a zero order reaction.
A classic example of a zero order reaction is the decomposition of ammonia on a hot platinum surface. Other key examples include:
- Photochemical decomposition of hydrogen iodide (HI) under high light intensity
- Catalytic decomposition of nitrous oxide (N2O) on a hot platinum or gold surface
- Enzyme-catalyzed reactions at high substrate concentration (saturation kinetics)
3. How is the rate law of a zero order reaction written?
The rate law for a zero order reaction is written as: rate = k, where k is the zero order rate constant. This shows that the rate does not depend on the concentration of reactant.
4. What is the unit of the rate constant for a zero order reaction?
The unit of the zero order rate constant (k) is mol L-1 s-1. This is different from first and second order reactions, whose rate constant units depend on concentration powers in the rate law.
5. How does the concentration vs. time graph look for zero order reactions?
The concentration vs. time graph for a zero order reaction is a straight line with negative slope. As time increases, the reactant concentration decreases linearly until it reaches zero. The graph equation is: [A] = [A]0 - kt.
6. Why don’t most reactions show zero order kinetics under normal conditions?
Most reactions are not zero order because their rates typically depend on reactant concentration. Zero order kinetics only arise under special conditions, such as:
- Surface or catalyst saturation limits the reaction rate
- Enzyme-catalyzed reactions where all enzyme sites are occupied (saturation)
- Photochemical reactions with constant light intensity
7. How does zero order kinetics relate to enzyme-catalyzed reactions (Michaelis-Menten)?
In enzyme-catalyzed reactions, zero order kinetics occur when substrate concentration greatly exceeds the enzyme concentration. At this point, every enzyme is saturated and working at maximum capacity, so increasing the substrate further does not increase the rate.
8. Can a reaction change its order from zero under different conditions?
Yes, the order of a reaction can shift between zero, first, or second order depending on physical or chemical conditions. For example, if a catalyst surface is no longer saturated or substrate concentration drops, a previously zero order reaction may become first order.
9. What is the half-life formula for a zero order reaction?
The half-life of a zero order reaction is given by: t1/2 = [A]0 / (2k), where [A]0 is the initial concentration and k is the zero order rate constant. Unlike first order reactions, the half-life depends on the starting concentration.
10. What are practical applications of zero order reactions?
Zero order kinetics are important in:
- Catalytic reactors where reaction rate is controlled by catalyst saturation
- Drug delivery systems designed for constant release rates
- Industrial manufacturing involving surface-limited processes and photochemical transformations
11. What physical or chemical circumstances cause a reaction to become zero order?
A reaction can show zero order kinetics when its rate is limited by external factors rather than reactant concentration. Common conditions include:
- Saturated catalyst or enzyme surfaces
- Constant and excess light intensity in photochemical reactions
- System constraints keeping the rate fixed regardless of reactant amount
12. What is the difference between zero order and first order reactions?
In a zero order reaction, the rate is independent of reactant concentration, while in a first order reaction, the rate is directly proportional to the reactant concentration. Key differences include:
- Zero order rate law: rate = k
- First order rate law: rate = k[A]
- Graph of zero order: straight line, concentration falls linearly
- Graph of first order: exponential decay curve
