First Order Reactions

Introduction

How fast a reaction proceeds have been termed the rate of reactions. The rate of reactions can be determined relying on the concentration of reactants. The rate of a reaction is directly proportional to the formation of the product and is also inversely proportional to the concentration of reactant that is decreasing. Many chemical reactions proceed at a faster rate. For example, the combustion reaction that takes place on the cellulose is a faster reaction that happens within seconds. While some chemical reactions have. For example, the rusting of iron is a very slow process, and it takes a lot of time for it to happen. By determining the change in concentration, the rate of reactions can be easily found.

What is the Order of Reaction?

The connection between the rate of a particular reaction and the concentration of reactant is the order of the reaction. The order of a reaction can be distinguished from the rate equation as it is the sum of exponents present in the concentration values. So, the order of a reaction is the exponents to which the reactant species are raised and it depends on the rate of that particular reaction. The order is achieved from the rate law. For example, in a reaction,

$$\mathrm{aA+bB\longrightarrow P}$$

The rate equation is,

$$\mathrm{rate=k[A]^x [B]^y}$$

The order of the reactant A from the equation is x from the concentration of A, [A]. For the reactant B, the order from the equation is y from the concentration of B, [B]. And the term K is rate coefficient or rate constant. Thus, the net order of this reaction can be calculated by adding these two orders together. So, n=x+y is the order of a particular reaction.

Characteristics

There are some characteristics of the order of the reaction. The order of the reaction is not always an integer value. Even from the equations, we get the idea that it truly relies on the concentration of a reactant to form a product. But it is not always following this characteristic. This means that some reactions do not even rely on the concentration of reactant present in the reaction. So based on this there are some characteristics of the order of reactions. The order of reaction can also be zero, that is Zero order reaction in which the rate of a reaction is unrelated to the concentration of chemical species involved in the reaction.

Is also significant to know that the rate of reaction may also be negatively affected by the concentration of reactant. That is the rate of reaction is inversely equal to the concentration of reactant. And so, in that case, the order becomes a negative value.

And the order of reaction can be a positive value in which the rate directly depends on the concentration. The increase in the concentration of a reactant increases the rate of reaction.

And there are some cases in which the rate of a reaction can be a non-integer value which means that it will not be a positive or a negative value. Non-integer values are present in the case of several complex reactions.

How to Find Order of Reaction?

For every chemical reaction, the idea of the rate of reaction is very important. The order of reaction deals with the idea that how a reactant will influence the rate of a particular reaction. And by the knowledge of the order of reaction, we can focus more on the reactant that will be going to affect the rate of the chemical reaction. So, it is will not be obtained by simply looking into the chemical equation. It requires another method for finding the order of the reaction.

Following Methods Can be Used for Determination of Order of Reaction −

The order of reactions can be easily obtained by the following methods.

• From the rate equation − By using the rate equation of chemical reactions we can find out the order of the reaction. For example, the order of the following rate is,

  Rate = k[A]², and the order is two.

• By doubling reactants − Doubling the concentration of reactants present chemical equations and observing its effect can help to find out the order of reactions. The experimental method is always used for the determination of order.

• Differential method − The method is also called the initial rate method. As it involves the determination of the rate of reaction by altering one of the reactant concentrations while the rest of the reactant is kept constant.

• Graphical method − In this method, the chemical reaction that involves only one reactant is determined method. If A is the reactant involved, a graph is plotted in which logA vs time provides a straight line, it will be a first-order reaction. While the graph 1/A vs time is a straight line it will be second order.

• Integral method − The use of an integral form of rate law is used the determination the order of the reaction.

Difference Between Molecularity and Order of Reaction

Molecularity and order of reaction are two important terms that deal with chemical reactions. The difference between these two is tabulated below.

Molecularity	Order
It deals with the red determining step in which the number of molecules takes part.	The rate of reaction and concentration of reactance is related to the order of the reaction.
The value is always a whole number and cannot be negative.	The value of an order of chemical reactions can be zero, integers, non-integers, negative, etc.
From the reaction mechanism, the value of molecularity can be easily obtained.	Several methods and some experiments and methods shall be adopted for the order determination.
The value of a molecularity does not vary with any external circumstances such as temperature and pressure.	The order relies on fact circumstances such as temperature and pressure.

Zero Order Reaction

Zeroth order or Zero-order reactions are chemical reactions that don't rely on the concentration of chemical species involved in the reaction. The exponents of the reactant in the rate equation are zero.

$$\mathrm{Rate=K[reactant]^0=K}$$

For such reactions, the graph is plotted against concentration and time. The graph is,

An example of a reaction that involves a zero-order reaction is the decomposition of N₂ O, Dinitrogen monoxide at a temperature of 200°C to 400°C. The reaction is,

$$\mathrm{2N_2 O(g)+ Pt\rightarrow 2N_2 (g)+O_2 (g)}$$

First Order Reaction

The rate of reaction which counts on only one of the reactants is a first-order reaction. For such a reaction if the concentration of reactant is doubled the rate of the reaction will also get doubled. Hydrolysis of cisplatin, the drug used for the treatment of cancer is an example of a first-order reaction. The graph of such reaction is,

The rate equation for such reaction is,

$$\mathrm{rate=k[A]}$$

Second Order Reaction

The reaction in which the rate relies on the square of the concentration of reactant is a second-order reaction. For reactions,

$$\mathrm{2 A \rightarrow products}$$

The rate equation for the reaction will be,

$$\mathrm{rate = k[A]^2}$$

An example of such a reaction is the decomposition of Nitrogen dioxide,

$$\mathrm{2 NO_2 \rightarrow 2 NO + O_2}$$

The graph of such reactions will be,

Pseudo First Order Reaction

They are second-order reactions but their behaviour of them is like first-order reactions. The rate of such chemical reactions depends on two reactants but the change in concentration of the other reactant will be very high and cannot even determine its appearance. The hydrolysis of ethyl acetate is an example of a pseudo-first-order reaction.

$$\mathrm{CH_3 COOC_2 H_5 + H_2 O \rightarrow CH_3 COOH + C_2 H_5 OH}$$

Conclusion

Order of reaction is associated with the rate of chemical equations. The knowledge of rate is very important for the proceeding of chemical reactions. The order of a chemical reaction is the sum of exponents present in the concentration values. There is a particular characteristic for the order of reaction in which it can be negative, non-integer, and positive values. They are classified into different types depending on the order of reaction; they are zero order, first order, second order, pseudo-order, etc. Several methods can be adopted for the determination of the order of the reaction. Molecularity is also a term similarity order of reaction that is associated with chemical reactions. But are entirely different terms.

FAQs

1. Give an example of a reaction that has a third order?

The reaction Fe + Br₂ → FeBr₂ is an example of a third-order reaction.

2. What is the half-life of reactions?

The time required for the completion of half of the initial concentration of reactant is half the life of reactions.

3. What is the unit of 1^st order reaction?

s^-1 is the unit of rate constant for the first-order reaction.

4. What is the unit of 2^nd order reaction?

Mol^-1 s^-1 is the unit of the rate constant of second order reaction is

5. What are units of nth order?

(molL^-1 )^1-n s^-1 is the unit of the rate constant of nth order reaction.