# Elementary Rate Law: Hydrogen Radical Termination Step

• gfd43tg
In summary, the conversation discusses confusion over writing an elementary rate law for a hydrodealkylation reaction. The question is whether the rate law for the hydrogen radical in the termination step should be written as ##r_{H \bullet} = -2k_{4}C_{H \bullet}^2## or ##r_{H \bullet} = -k_{4}C_{H \bullet}^2##. The former is chosen because k4 is defined with respect to hydrogen, not the hydrogen radical. The conversation also mentions the use of equilibrium constants and the possibility of changing definitions later on if needed.
gfd43tg
Gold Member
Hello, I am having some confusion over elementary rate laws. This is a hydrodealkylation reaction.

The specific reaction rates k1 and k4 are defined w.r.t. H2.

If I want to write the rate law for the hydrogen radical for the termination step, would the elementary rate law be ##r_{H \bullet} = -2k_{4}C_{H \bullet}^2## or ##r_{H \bullet} = -k_{4}C_{H \bullet}^2##. I think it is the former because of the part of k4 being with respect to hydrogen. If it was with respect to the hydrogen radical, then it would be the latter? Thanks

Last edited:
Is one hydrogen radical distinguishable from another?

No, but I don't see how it relates to the rate law

Not only are you being confused, so am I --- I took off the wrong direction with how you came up with "-2k4CH⋅2, and thought you were making distinctions among hydrogen radicals.
Maylis said:
I think it is the former
And rereading the problem statement for the twentieth time, I'll agree, because ...

Maylis said:
The specific reaction rates k1 and k4 are defined w.r.t. H2.
, and this is the source of the confusion, I don't recall ever seeing a rate constant referred to the product. I'm a bit of a dinosaur, and conventions in kinetics may have evolved since I last had any use or interest for the field.

gfd43tg
I wouldn't worry about it. I would just write
Maylis said:
##r_{H \bullet} = -k_{4}C_{H \bullet}^2##.
myself, so as not to have to remember about the 2, and just get on with the problem.
It would be quite reasonable to have an equilibrium constant defined as K = [H.]2/[H2].
If it turns out later the other definition would have been more convenient, I can always change definition.

## 1. What is the "Elementary Rate Law"?

The Elementary Rate Law is a mathematical expression that describes the rate at which a chemical reaction occurs. It is based on the concentration of reactants and the rate constant, and it can be used to predict the rate of a reaction at a given time.

## 2. What is a Hydrogen Radical Termination Step?

A Hydrogen Radical Termination Step is a chemical reaction in which hydrogen radicals combine to form a stable product. This is an important step in many reactions involving hydrogen, as it helps to control the rate of the reaction and prevent runaway reactions.

## 3. How is the rate of a Hydrogen Radical Termination Step determined?

The rate of a Hydrogen Radical Termination Step is determined by the concentration of hydrogen radicals and the rate constant, which is a measure of how quickly the reaction takes place. The rate law for this step is typically second-order, meaning that the rate is directly proportional to the square of the concentration of hydrogen radicals.

## 4. What factors can affect the rate of a Hydrogen Radical Termination Step?

The rate of a Hydrogen Radical Termination Step can be affected by several factors, including temperature, pressure, and the presence of catalysts. Higher temperatures and pressures generally lead to faster reactions, while the addition of a catalyst can significantly increase the rate of the reaction.

## 5. How is the rate constant for a Hydrogen Radical Termination Step determined?

The rate constant for a Hydrogen Radical Termination Step is typically determined experimentally, by measuring the rate of the reaction at different concentrations of reactants and using this data to calculate the rate constant. The activation energy, which is the energy required for the reaction to occur, also plays a role in determining the rate constant.

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