Order of Reaction

Edited By Team Careers360 | Updated on Jul 02, 2025 04:37 PM IST

Chemical reactions lie at the core of many natural and industrial processes that shape the world around us. The rate at which these reactions proceed is crucial for various scientific and engineering applications, from the rusting of iron to the digestion of food. This rate of reaction is deeply influenced by the "order of reaction." The order of reaction quantifies how the concentration of reactants affects the speed of a chemical reaction. In simpler terms, it is the sum of the powers to which the concentration terms are raised in the rate law expression.

This Story also Contains

Definitions and Explanations
Order of Reaction
Experimental Determination
Types and Examples
Relevance and Applications
Applications in Real-Life Situations
Some Solved Examples
Summary

We will also discuss methods used to determine the order of reaction and why understanding this concept is important both academically and practically. Whether you are a chemistry student or a curious learner, this guide aims to provide comprehensive information on this fundamental concept.

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Definitions and Explanations

The order of reaction is defined as the sum of the exponents of the concentration terms in the rate law expression for a chemical reaction. For a general reaction ${aA} + {bB} \rightarrow{cC} + {dD}$ , the rate law is given by:

$Rate ={k}[{A}]^{x}[{B}]^{y}$

Here, ( x ) and ( y ) are the orders of the reaction with respect to reactants ( A ) and ( B ), respectively, and ( k ) is the rate constant. The overall order of the reaction is ( x + y ).

Experimental Determination

The reaction constant and the order of the reacting species must be determined experimentally and cannot be directly predicted from the balanced chemical equation. The order of such reacting species and other experimental values may be zero, negative, or fractional and all vary with conditions like temperature (T), pressure (P), and reactant concentrations.

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Types and Examples

Elementary Reactions

In a simple or elementary single-step reaction, the rate law can be directly written from the balanced equation. For example, consider the reaction:

$A + B \rightarrow C$

The rate law is:

$Rate ={k}[{A}]^{x}[{B}]^{y}$

where x and y are determined experimentally.

Initial Rate Method

Since this is the rate of reaction at the very beginning, the concentration of reactants is known. By comparing the initial rates at different concentrations, one can determine the correct rate law and order of reaction.

Relevance and Applications

Academic Importance

Understanding the order of reaction is crucial for chemistry students as it forms the basis for more advanced topics in kinetics and reaction mechanisms. It helps predict the behavior of reactions under various conditions.

Applications in Real-Life Situations

Knowing the order of the reaction is important industrially because it can help optimize reaction conditions for maximum yield and efficiency. For example, in the pharmaceutical industry, closely monitoring reaction rates is crucial for proper drug production.

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Some Solved Examples

Example 1
Question: For the reaction $A + 2B \rightarrow C$ , if the rate is given by ${R}=[{A}][{B}]^2$ , what is the order of the reaction?

1) (correct)3

2)6

3)5

4)7

Solution

Order of a Reaction -

The order of a reaction is determined as the sum of the powers of the concentration terms that appear in the experimental rate equation. It is an experimental quantity.

Formula: $\begin{aligned} & a A+b B \rightarrow c C+d D \\ & r=K[A]^a[B]^b \\ & \text { order }=(a+b)\end{aligned}$

$\therefore$ The order has a relation with stoichiometry & is determined experimentally. It can be zero/fraction/negative/positive

So,

For reaction $A+2 B \rightarrow C$

Rate Law $=\mathrm{k}[\mathrm{A}][\mathrm{B}]^2$

Order =1 + 2 = 3

Hence, the answer is the option (1).

Example 2
Question: For a reaction $A + B \rightarrow C + D$ , if the concentration of A is doubled and the rate of reaction doubles, and when the concentration of B is increased by 9 times, the rate gets tripled. What is the order of the reaction?

1)2

2)1

3) (correct)3/2

4)4/3

Solution

Order of a Reaction -
The order of a reaction is determined as the sum of the powers of the concentration terms that appear in the experimental rate equation. It is an experimental quantity.

Rate $=k[A][B]^{1 / 2}$

Order $=1+\frac{1}{2}=\frac{3}{2}$

It can be zero/fraction/negative/positive fractional.

Order = 3/2 =1.5
Hence, the answer is the option (3).

Example 3
Question: What are the possible orders of a reaction?

1)Zero

2)Positive

3)Fractional

4) (correct)All of the above

Solution

The order of the reaction can be zero/fraction/negative/positive/fractional.

The order of a reaction has no relation with stoichiometry & is determined experimentally. It can be zero/fraction/positive.
Hence, the answer is the option (4).

Example 4
Question: For the reaction $A + B \rightarrow C$ , what is the order of the reaction?

1)2

2)1

3)Can not be more than 2

4) (correct)Can not be determined

Solution

Order of a Reaction -
The order of a reaction is determined as the sum of the powers of the concentration terms that appear in the experimental rate equation. It is an experimental quantity.

Formula: $\begin{aligned} & a A+b B \rightarrow c C+d D \\ & r=K[A]^a[B]^b \\ & \text { order }=(a+b)\end{aligned}$

The order has a relation with stoichiometry & is determined experimentally. It can be zero/fraction/negative/positive

So, for the given reaction order can not be determined because experimentally determined stoichiometry of powers of the concentration is not given.

Hence, the answer is the option (4).

Example 5
Question: For the reaction $2{NO} + {Cl}_2 \rightarrow 2 {NOCl}$ , if doubling the concentration of both reactants increases the rate by a factor of 8, but doubling ${Cl}_2$ alone only doubles the rate, what is the order of the reaction with respect to ${NO}$ and ${Cl}_2$ ?

1)1,1

2)1,2

3) (correct)2,1

4)2,2

Solution

Rate $=\mathrm{k}[\mathrm{NO}]^{\mathrm{m}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}}$

Given,

$\begin{aligned} & \mathrm{R}_1=\mathrm{k}[\mathrm{NO}]^{\mathrm{m}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}} \\ & \mathrm{R}_2=\mathrm{k} 2^{\mathrm{m}}[\mathrm{NO}]^{\mathrm{m}} 2^{\mathrm{n}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}}=2^{\mathrm{m}+\mathrm{n}} \mathrm{R}_1 \\ & \mathrm{R}_3=\mathrm{k}[\mathrm{NO}]^{\mathrm{m}} 2^{\mathrm{n}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}}=2^{\mathrm{n}} \mathrm{R}_1\end{aligned}$

According to the experimental data given,

$2^{\mathrm{m}+\mathrm{n}}=8$ and $2^{\mathrm{n}}=2$

Upon solving,

$\mathrm{m}=2, \mathrm{n}=1$

Hence, the answer is the option(3).

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Summary

The order of reaction is a fundamental concept in chemical kinetics that defines how the concentration of reactants influences the rate of a reaction. It is experimentally determined and can vary widely depending on the conditions. This concept is essential for both academic studies and practical applications in various industries. By exploring definitions, types, and methods to determine the order of reaction, this article provides a comprehensive overview that benefits students and professionals alike.

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Frequently Asked Questions (FAQs)

1. Order of reaction - Definition

Taking all reactants into account, the order of reactions is determined by the power dependence of the rate on their concentration. A first-order reaction, for example, is entirely dependent on the concentration of one species in the reaction.

2. In order to determine the reaction order, what are the different methods that can be followed?

Method of initial rates
Integrated Method
Differential Method

3. 3.What is the difference between molecularity and order of reaction?

A reaction is molecular if it requires a collision between atoms, molecules, or ions in a short period of time to take place.

There is evidence that when the order for the reaction is fractional, the reaction is usually a chain reaction or involves some other complicated mechanics. For instance, the pyrolysis of acetaldehyde is an example of fractional reaction order.

4. What is Second-Order Reaction?

In chemistry, a second-order reaction is one that has the number 2 as part of its order.

One can obtain the rate of these reactions by measuring the concentration of one reactant squared or by measuring the concentration of two different reactants separately.

5. Definition of First order reaction definition

One reactant definition concentration is all that is needed to control the rate of reaction in these reactions. Many reactants can be involved in a reaction, but only one reactant will have an effect on the rate of reaction. Reaction order is not affected by the concentration of other reactants.

6. Why might a reaction have a fractional order?

Fractional orders typically arise from complex reaction mechanisms involving multiple steps. They can occur when the rate-determining step involves an intermediate species or when there are competing reactions. Fractional orders cannot be explained by a single elementary reaction step.

7. What's the significance of knowing the order of a reaction?

Knowing the order of reaction helps predict how changes in reactant concentrations will affect the reaction rate. It provides insights into the reaction mechanism, aids in determining rate laws, and is crucial for designing industrial processes and understanding complex chemical systems.

8. How does temperature affect the order of a reaction?

Temperature generally does not affect the order of a reaction. The order is determined by the reaction mechanism, which typically remains constant over a range of temperatures. However, temperature can affect the rate constant and overall reaction rate.

9. How does catalysis affect the order of a reaction?

Catalysts generally do not change the overall order of a reaction. They provide an alternative reaction pathway with lower activation energy, increasing the rate constant. However, in some cases, particularly in heterogeneous catalysis, the apparent order may change due to surface saturation effects.

10. How does the steady-state approximation relate to reaction order?

The steady-state approximation assumes that the concentration of reactive intermediates remains constant during the reaction. This simplifies complex mechanisms and can lead to rate laws with fractional orders, as the intermediates' concentrations are expressed in terms of reactant concentrations.

11. What is the concept of 'pseudo-order' and when is it applicable?

Pseudo-order refers to the apparent simplification of a higher-order reaction to a lower order when one reactant is in large excess. For example, a second-order reaction can appear first-order (pseudo-first-order) if one reactant's concentration remains effectively constant throughout the reaction.

12. What is the significance of 'apparent order' in complex reactions?

Apparent order is the experimentally observed order that may not reflect the true mechanistic complexity. In multi-step or competing reactions, the apparent order can be fractional or change with conditions, providing a simplified description of complex kinetics.

13. What is the concept of 'order with respect to a catalyst' and when might it be non-zero?

While catalysts typically don't appear in the rate law, in some cases, especially in homogeneous catalysis, the rate may depend on catalyst concentration. This can lead to a non-zero order with respect to the catalyst, often indicating a more complex mechanism involving catalyst-substrate interactions.

14. What is the role of rate-determining step in consecutive reactions and its impact on order?

In consecutive reactions, the rate-determining step often governs the overall reaction order. If the first step is rate-determining, the order typically reflects this step. If a later step is rate-determining, the order may be more complex, depending on the kinetics of preceding steps and any equilibria involved.

15. What is the significance of 'fractional life periods' in reaction order studies?

Fractional life periods (time taken for a fraction of reactant to be consumed) provide a way to determine reaction order. The relationship between these periods and initial concentration depends on the order, offering an alternative to half-life analysis for order determination.

16. How can you determine the order of reaction experimentally?

The order of reaction can be determined experimentally using methods such as the method of initial rates, integrated rate laws, or graphical methods. These involve measuring reaction rates at different concentrations and analyzing the data to find how rate depends on concentration.

17. What is the integrated rate law and how does it relate to reaction order?

The integrated rate law is an equation that relates concentration to time for a specific reaction order. It's derived by integrating the differential rate law. The form of the integrated rate law depends on the reaction order, providing a way to determine order by plotting concentration vs. time data.

18. How does reversibility affect the determination of reaction order?

Reversible reactions can complicate order determination because the observed rate is the net result of forward and reverse reactions. As equilibrium is approached, the apparent order may change. Proper analysis requires considering both directions and often leads to more complex rate laws.

19. What is the method of initial rates, and how is it used to determine reaction order?

The method of initial rates involves measuring reaction rates at the beginning of reactions with different initial concentrations. By comparing how the initial rate changes with concentration, you can deduce the order with respect to each reactant. This method avoids complications from reversibility or product effects.

20. What is the significance of the rate-limiting step in determining reaction order?

The rate-limiting step often determines the overall reaction order. It's typically the slowest step in a multi-step mechanism and controls the overall reaction rate. The concentrations of species involved in this step are most likely to appear in the rate law, influencing the observed order.

21. What is the order of reaction in chemical kinetics?

The order of reaction is the sum of the exponents of concentration terms in the rate law equation. It indicates how the reaction rate depends on the concentration of reactants. For example, in a second-order reaction, doubling the concentration of a reactant will quadruple the reaction rate.

22. What's the difference between the rate law and the order of reaction?

The rate law is an equation that expresses how the reaction rate depends on reactant concentrations, while the order of reaction is a numerical value derived from the rate law. The rate law includes the rate constant and concentration terms, whereas the order is the sum of concentration term exponents.

23. How does the order of reaction differ from molecularity?

Order of reaction is an experimentally determined value that can be fractional or zero, while molecularity is always a whole number representing the number of molecules participating in the rate-determining step. Order is based on the rate law, while molecularity is based on the reaction mechanism.

24. What's the difference between elementary and overall reaction orders?

Elementary reaction orders are always whole numbers and match the molecularity of the reaction step. Overall reaction orders can be fractional or zero and are determined experimentally for the entire reaction. Complex reactions may have overall orders that don't match any single elementary step.

25. How does the order of reaction affect the units of the rate constant?

The units of the rate constant depend on the overall order of the reaction. For a first-order reaction, k has units of time^-1. For second-order, it's concentration^-1 * time^-1. In general, the units are (concentration^(1-n)) * (time^-1), where n is the overall order.

26. How do competing reactions influence the observed order?

Competing reactions can lead to complex kinetics and fractional orders. The observed order may be a combination of the orders of individual competing pathways. This can result in apparent orders that don't correspond to simple integer values.

27. What is the significance of the rate-determining step in reaction order?

The rate-determining step often governs the overall reaction order. It's typically the slowest step in a multi-step mechanism and acts as a bottleneck for the overall reaction rate. The concentrations of species involved in this step are most likely to appear in the rate law.

28. Can the order of reaction change during the course of a reaction?

Generally, the order of reaction remains constant throughout the reaction. However, in some complex systems or when conditions change significantly (e.g., reactant depletion in autocatalytic reactions), the apparent order might change as the dominant mechanism shifts.

29. What role does stoichiometry play in determining reaction order?

Stoichiometry doesn't directly determine reaction order. The order is based on the rate law, which may not reflect stoichiometric ratios. For example, a reaction with 1:1 stoichiometry could be first-order in one reactant and zero-order in the other, depending on the mechanism.

30. What is the relationship between reaction order and reaction mechanism?

Reaction order provides clues about the mechanism but doesn't definitively determine it. A simple integer order might suggest a single-step mechanism, while fractional orders indicate multi-step processes. However, different mechanisms can sometimes lead to the same observed order.

31. Can a reaction have a zero order? What does this mean?

Yes, a reaction can have a zero order. This means the reaction rate is independent of the concentration of reactants. Zero-order reactions proceed at a constant rate regardless of reactant concentration, often due to a limiting factor like available surface area in heterogeneous catalysis.

32. What is a pseudo-first-order reaction?

A pseudo-first-order reaction is actually a second-order (or higher) reaction that behaves like a first-order reaction because one reactant is in large excess. The concentration of the excess reactant remains essentially constant, simplifying the rate law to appear first-order.

33. Can the order of reaction be negative?

Yes, although rare, negative orders can occur in complex reaction mechanisms. A negative order means that as the concentration of a particular species increases, the reaction rate decreases. This can happen when the species inhibits the reaction or participates in a reverse reaction.

34. How does the half-life of a reaction relate to its order?

The half-life of a reaction depends on its order. For first-order reactions, the half-life is constant and independent of initial concentration. For second-order reactions, the half-life is inversely proportional to initial concentration. Zero-order reactions have half-lives directly proportional to initial concentration.

35. How do consecutive reactions affect the overall order?

In consecutive reactions, the overall order is typically determined by the slowest step (rate-determining step). If the first step is slowest, the overall order matches that step. If a later step is rate-determining, the order may be more complex and depend on the kinetics of preceding steps.

36. How do inhibitors affect the order of reaction?

Inhibitors can change the apparent order of reaction by introducing additional steps in the mechanism. They may lead to more complex rate laws, potentially introducing negative or fractional orders with respect to the inhibitor concentration.

37. Can a reaction be zeroth order in all reactants? What would this imply?

Yes, a reaction can be zeroth order in all reactants. This implies the reaction rate is constant and independent of reactant concentrations. Such situations often occur in heterogeneous catalysis where the reaction is limited by available catalyst surface area rather than reactant concentration.

38. How do chain reactions relate to reaction order?

Chain reactions often exhibit complex kinetics that can lead to fractional or varying orders. The overall order may depend on the relative rates of initiation, propagation, and termination steps. In some cases, the apparent order can change as the reaction progresses.

39. How does solvent choice affect reaction order in solution kinetics?

Solvent choice can affect reaction order by influencing the reaction mechanism. It may stabilize or destabilize intermediates, affect the degree of dissociation of reactants, or change the rate-determining step. In some cases, this can lead to changes in the observed reaction order.

40. What is the difference between differential and integrated rate laws in relation to reaction order?

Differential rate laws express the instantaneous rate of change of concentration with time and directly show the reaction order in the exponents. Integrated rate laws relate concentration to time and take different forms depending on the order. Integrated forms are often used to determine order from experimental data.

41. How do parallel reactions influence the observed reaction order?

Parallel reactions can lead to complex kinetics where the observed order is a combination of the orders of individual pathways. The overall rate is the sum of rates for each pathway, potentially resulting in non-integer orders if the pathways have different individual orders.

42. What is the concept of 'mixed order' reactions?

Mixed order reactions have different orders with respect to different reactants. For example, a reaction might be first-order with respect to one reactant and second-order with respect to another. This often indicates a complex mechanism with multiple steps influencing the rate.

43. How does the pre-equilibrium approximation affect reaction order determination?

The pre-equilibrium approximation assumes a rapid equilibrium before the rate-determining step. This can simplify complex mechanisms and often leads to rate laws with fractional orders, as the concentrations of species in equilibrium are expressed in terms of initial reactant concentrations.

44. How do autocatalytic reactions complicate order determination?

Autocatalytic reactions, where a product catalyzes its own formation, often show changing orders as the reaction progresses. Initially, the rate may appear to follow one order, but as product accumulates, the order can change, leading to sigmoidal concentration-time profiles.

45. How does isotope substitution affect reaction order determination?

Isotope substitution generally doesn't change the reaction order but can affect the rate constant due to kinetic isotope effects. This can be useful in elucidating reaction mechanisms without altering the overall order, providing insights into which bonds are broken in the rate-determining step.

46. How do enzyme-catalyzed reactions relate to reaction order concepts?

Enzyme-catalyzed reactions often follow Michaelis-Menten kinetics, which can show different orders depending on substrate concentration. At low concentrations, they appear first-order in substrate, while at high concentrations, they approach zero-order as the enzyme becomes saturated.

47. How does the steady-state approximation influence the apparent order in complex reactions?

The steady-state approximation assumes that concentrations of reactive intermediates remain constant during the reaction. This can simplify complex mechanisms, often leading to rate laws with fractional orders as intermediate concentrations are expressed in terms of reactant concentrations.

48. How do reaction conditions like pressure affect the order of gas-phase reactions?

In gas-phase reactions, pressure can influence the apparent order by affecting collision frequency and potentially changing the rate-determining step. High pressures might lead to deviations from expected orders due to non-ideal gas behavior or shifts in dominant reaction pathways.

49. What is the relationship between activation energy and reaction order?

Activation energy and reaction order are independent concepts. The order describes how rate depends on concentration, while activation energy relates to the temperature dependence of the rate constant. However, complex reactions with different activation energies for different pathways might show temperature-dependent apparent orders.

50. How does the concept of 'rate-determining step' apply in parallel reaction networks?

In parallel reaction networks, each pathway may have its own rate-determining step. The overall observed order can be a complex function of the orders of individual pathways. The dominant pathway (with the fastest rate) often has the most influence on the apparent overall order.

51. What is the significance of 'induction periods' in relation to reaction order?

Induction periods, where there's a delay before the main reaction proceeds, can complicate order determination. During this period, the apparent order may differ from the main reaction phase, often due to the build-up of an intermediate or catalyst. Analysis typically focuses on the post-induction period for accurate order determination.

52. How do solvent cage effects influence reaction order in solution kinetics?

Solvent cage effects can influence reaction order by affecting the probability of reactive species escaping the solvent cage before reacting. This can lead to apparent orders that differ from those expected based on simple collision theory, particularly in reactions involving radical or ion pair intermediates.

53. What is the concept of 'mixed order' in reversible reactions?

In reversible reactions, the observed order can be a complex function of forward and reverse reaction orders. As the system approaches equilibrium, the apparent order may change. Analysis of such systems often requires considering both directions simultaneously, leading to more complex rate laws.

54. How does the presence of a heterogeneous catalyst affect reaction order determination?

Heterogeneous catalysis can lead to apparent orders that differ from homogeneous kinetics. Surface saturation effects can cause transitions from first-order to zero-order kinetics as reactant concentration increases. The observed order may also depend on mass transfer limitations and available catalyst surface area.

55. What is the relationship between reaction order and the concept of 'rate-controlling step' in complex mechanisms?

The rate-controlling step, which may differ from the rate-determining step, is the step that most influences the overall rate. In complex mechanisms, this step often governs the observed order. Changes in conditions that affect the rate-controlling step can lead to changes in apparent order, providing insights into the reaction mechanism.