Education Journalist | Study Abroad Lead
In a chemical equation, the rate determining step is considered to be the slowest step which determines the speed of the overall chemical reaction. Chemical reactions take place over a number of elementary steps. Rate determining step determines the rate of equation for a given chemical reaction.
| Table of Content |
Key Terms: Determining step, rate determining step, derivation of rate law, rate of appearance, Elementary reaction, Bio-molecular mechanism
Rate Determining Step
[Click Here for Sample Questions]
The slowest step in a chemical reaction is known as the rate determining step. Chemical reactions do not take place in just one single step, they take place in multiple steps. Therefore, in these multi-step reactions, the rate determining step restrains the overall rate of reaction. These rate laws can be directly derived from the chemical equations (balanced) in case of elementary reactions. However, it might not be the same for the system of multistep reaction. Hence, in these reaction mechanisms the rate of law is deduced subsequently.
Rate Determining Step can be further compared to a neck of funnel through which water flows inside through the width of the neck, not through the speed of pouring the water in it. Likewise, the rate of reaction is understood by the rate determining step.
Also Read:
| Related Articles | ||
|---|---|---|
| Transuranium Element | Atomic Radius | Thermal Conductivity of Copper |
| Systematic Analysis of Cation | Difference between Cations and Anions | Integrated Rate Equations |
Mechanism of Rate Determining Step
[Click Here for Sample Questions]
In chemical kinetics, the rate determining step is mostly used to derive the rate of equation, which is said to be:
r = k [A]x[b]y
Examples
[Click Here for Sample Questions]
Let us consider a chemical reaction as mentioned below:
2NO2 + F2 → 2 NO2 F
In this chemical reaction, the mechanism will take place in two elementary steps:
Step 1: NO2 + F2 → NO2 F + F
Step 2: NO2 + F → NO2 F
Here, the rate of constant for step 1 is k1 and k2 for step 2. So, the rate of determining step will be step 1 in this reaction mechanism.
In the writing pattern of a rate equation, the equation by writing rate is considered equal to the rate constant of the slowest step which determines the concentration of the reactants raised to their order of reaction. So,
rate = k1 [NO2] [F2]
Taking another chemical reaction as,
NO2 (g) + CO (g) → CO2 (g) + NO (g)
The temperature for this experimental rate law of reaction is above 225° C,
rate = k [NO2] [CO]
If we consider the rate law, in first order the reaction will be in respect to NO2 & also to CO.
This step will be consistent in this bio-molecular mechanism and there is possibility of this mechanism to be valid at high temperatures. In temperatures below 225° C, the chemical reaction will be described by a different rate law which is of second order in respect to NO2. Then,
rate = k [NO2]2
As this rate is not consistent with one-step mechanism, but following the two-step mechanism, it is consistent:
NO2 (g) + NO2 (g) → NO3 (g) + NO (g) [Slow]
NO3 (g) + CO (g) → NO2 (g) + CO2 (g) [Fast]
The sum of the two above elementary reactions will give out the overall net reaction. Also, when rate determining step involves rapid reversible reaction, the rate of law in overall reaction will become difficult to derive due to presence of intermediates. Such cases require the reaction to be at equilibrium, so that the rate of reverse & forward processes becomes equal.
We can rearrange the expression to show the intermediates of reactant NO:
k1 [NO2] k-1 = [N2O2]
Derivation of Rate Law
[Click Here for Sample Questions]
If we consider a two-step reaction mechanism of molecular chlorine & nitrogen monoxide as mentioned below,
we can add the two elementary reactions and the overall reaction that we get is:
So the rate derived from this procedure and the rate laws of each elementary reaction are,
In this reaction mechanism, step 2 is the rate determining step. If we consider step 1 at equilibrium, we obtain,
As we substitute the obtained expression in rate law, we get:
Things to Remember
[Click Here for Sample Questions]
- Rate determining step is described as the slowest step in a reaction in the overall chemical reaction.
- Rate of equation is expressed as, rate = k1 [A]2
- Multiple elementary steps take place in one chemical reaction while the rate reaction is determined by one of the elementary reactions in it.
- More specifically, the rate of increase or decrease in concentration of one of the reactants or the products takes place.
- The rate of appearance in a reaction is Rate of appearance of P = Increase in concentration of P/ Time Taken = P/t
- Rate of disappearance of R = Decrease in concentration of R/Time taken = R/t
Also Read:
Sample Questions
Ques. How is the rate of reaction determined? (2 marks)
Ans. The rate or speed of reaction is explained as: In a unit time by the change in the concentration of a product or a reactant. Kinetic reaction shows the rate of change in chemical reaction and also describes the conditions of the reaction by which the rate of the reaction can be altered.
Ques. Define rate reaction. (2 marks)
Ans. In a chemical reaction, the rate of reaction is defined as the speed of the reaction which is proportional with the increase of the product’s concentration per unit time and the decrease in the concentration of reactants per unit time. There can be dramatic changes in the rate reactions. Mostly, decrease in rates takes place with the proceeding of the reactions.
Ques. What is known as the Rate determining step? (2 marks)
Ans. In overall chemical reaction, the slowest step during the completion of reaction is known as the rate determining step. In chemical reactions with multiple reactions, the rate determining step decides the overall speed of the reaction. For example, if in a two step chemical reaction, the first step is the slowest step, then it will be considered as rate determining step.
Ques. What is Rate Law? (2 marks)
Ans. The molar concentration in the reaction in which the reactants participate within a chemical reaction in which the reactants are raised to some power is known as rate law. It is also known as the rate expression. The rate constant in the reactions is denoted by k for some reactions at a specific temperature.
Ques. What is the rate reaction in the chemical equation: NO2 (g) + CO2 (g)? (2 marks)
Ans. The above reaction consists of few elementary reactions:
Step 1: NO2 + NO2 → NO + NO3 ... (where rate constant is k1 is slow)
Step 2: NO3 + CO → NO2 + CO2 ..... (where rate constant k2 is fast)
So the rate = k1 [NO2] [NO2] or rate = k1 [NO2]2
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments