Crystal

HS-PS1-4

# Reaction Order Study Guide

Order of reaction is the sum of the power of concentration of reactants written in rate law expression, and it can be an integer or fraction.

## INTRODUCTION

Reactions such as rusting take a lot of time to occur, while a neutralization reaction, i.e., when an acid reacts with a base, takes almost no time. So how fast or slow a reaction is, is called the rate of reaction, and the rate of reaction depends on the order. You must be wondering what reaction order is in chemistry? Let’s understand.

### REACTION ORDER

Order of reaction is the sum of the power of concentration of reactants written in rate law expression. Let’s understand in simple words. The rate of a reaction depends or sometimes doesn’t depend on the concentration of reactants.

Reaction rate = is the change in the amount of a reactant or product per unit time.

So, the reaction order is the relationship between the concentrations of species and the rate of a reaction. The order of reaction can be any integer or fraction. It can be found out only experimentally.

### ZERO-ORDER REACTIONS:

``````                                 A → B
``````

Here, the rate is independent of the concentration of the reactant, which is expressed as [A].

``````                             Rate =  k[A]0
``````

The power of [A] is 0, and hence the order of the reaction is zero. This means the rate of reaction won’t change even if we increase or decrease the concentration of the reactant. Source

### FIRST-ORDER REACTIONS:

``````                                A → B
``````

Here, the rate of reaction is proportional to the concentration of A. This means as [A] increases, the rate of reaction increases too and vice versa.

``````                            Rate = k [A]
``````

Since the power of [A] is 1, the order of the reaction is 1. The rate depends linearly on the concentration of reactant here. The rate of reaction is the slope of the graph.

### SECOND-ORDER REACTION:

``````                              A + B → C
``````

Here, the rate is expressed as k [A] [B], which means the rate will be dependent on both of the reactants A and B. If we increase or decrease the concentration of any of these, then the rate will change.

The rate is proportional to [A], and the order for A is 1, and the same is for B. But the overall order is the sum of orders with respect to both the reactants, so it will be 2.

## CONCLUSION:

• Order of reaction is the sum of the power of concentration of reactants written in rate law expression.
• It can be an integer or fraction.

### FAQs:

1. What is the order of a reaction?

Order of reaction is the sum of the power of concentration of reactants written in rate law expression.

2. Can the order of reaction be -½?

Yes, the order of the reaction can be an integer or fraction.

3. For Rate = k [A], what is the order of reaction?

Since the power of concentration of reactant is 1, the reaction is of the first order.

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## SOURCES:

1. Reaction Order: https://www.ck12.org/c/chemistry/reaction-order/lesson/Order-of-Reaction-CHEM/?referrer=concept_details. Accessed 8th March 2022.
2. Reaction Order: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/03%3A_Rate_Laws/3.03%3A_The_Rate_Law/3.3.03%3A_Reaction_Order. Accessed 8th March 2022.