The rate control step, also known as the speed limiting step, is also referred to as the rate determining step (RDS).The rate control step means that if in a mechanism, the speed of one primitive step is much slower than the other, then this step determines the overallreaction rate。
The importance of recognizing the slowest step in a chemical reaction is to effectively improve the speed of the whole chemical reaction, so as to achieve higher yield.A common metaphor is a narrow pipe - no matter how fast the water flows, it can't change the flow of water.
For actual chemical reactions, not every chemical reaction is a basic reaction, and most reactions are composed of several basic reactions. Among them, the reaction with the slowest reaction rate is the rate control step, which determines the process of the entire chemical reaction.The rate control step is to affect the wholeChemical reaction rateThe water pipe of.
Examples and applications
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With NtwoOfiveThe decomposition reaction of is taken as an example to explain the rate control equation. This reaction is not a basic reaction, and may be carried out according to the following three-step mechanism, of which the first step is carried out twice:
(slow)
To sum up, NtwoOfiveThe decomposition reaction of can be written as:
。
The first step represents a reversible decomposition reaction, and its forward reaction rate constant is KoneThe reverse reaction is Kone’。NO in the second steptwoNo reaction but affect NOthreeDecomposition of.If the second step itself is slower than other steps, the rate is given by the following equation:
The concentration of NO does not appear in the rate equation, because the third step is faster than the second step, and because the first step is also fast and reversible, the concentration of these three components can be correlated with an equilibrium relationship:
Where
Is the equilibrium constant of the first step, which can be eliminated (CNO2）(CNO3）, the rate expression can be written as:
Control steps
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MacrodynamicsThe reaction rate constant in the equation is the result of chemical reaction, diffusion and other factors.
When the reaction rate is accelerated and the mass transfer rate is relatively slow
At this time, the above formula can be simplified as:
；
When the reaction rate becomes slow and the mass transfer rate is relatively fast
At this time, the above formula can be simplified as:
。
When a process is composed of several consecutive steps in series, no matter whether these steps are chemical reaction or diffusion, the slowest step will control the rate of the whole process. This step is called the reaction rate control step of the overall process.If the reaction rate of several steps is close, the rate of the whole process will be determined by several steps.[1]
