How to find/calculate the activation energy from this constant (in Arrhenius equation) ?

Arrhenius type equations are applicable only for elementary reactions. For a complex reaction mechanism the dependence of observed rate constants on temperature might be a complex function. Therefore, before to apply whatever Arrhenius type equation you must know what you are measuring

Yurii V Geletii Thank you for reply,

Not sure if I got what you mean exactly and I don't know why did you say it but currently I'm trying to simulate detonation of H2, O2, and N2 using the chemical kinetic mechanism that is attached in this post.Here is the reference of the mechanism:

I need to use Fluent (software) to simulate this kind of combustion. Fluent uses the first Arrhenius equation type which is mentioned in the first post while in the table of the mechnism (which is attached in this post), they've provided Tar instead of Er. Then I need to convert all "Tar"s in the table to "Er"s. Hope this explanations be enough clear to know what I'm trying to do. So, What 's your opinion about my technique to convert it? (as I explained in the first post)

Arrhenius equation is available and, of course, suitable only for base element(s) (basic) reactions. For a complex one and for an operational matrix mechanism the inter-dependence of noticed rate consists on the fact that temperature might be a complex evolution and outcome. In this way, before doing anything, for example to apply the Arrhenius type equation one has to find out WHAT he/she is going to analyze / measure / evaluate, such as measuring on the mentioned shapes and formulas one should take into consideration.

Modifications to the Arrhenius equation are intended to reduce the numerical correlation between frequency factor and activation energy. However, this does not apply to the above equations. Replacing E_r/RT with T_r/T only reduces the number of arithmetic operations. In addition, the power term is usually omitted.

Rooholamin Dargahi "I don't know why did you say it." This does not sound polite. What is wrong in my answer?

Yurii V Geletii Yurii, Pardon me but indeed sometimes I get answers from people here which is really "baloney" and bullshit! For instance, take a look here:

See the last two answers. What do you think about them? Isn't my question clear that they've posted such answers? I have many affairs to attend and don't want to get some irrelevnt answers(don't have many time). If you cannot answer to my question, then please remove your answer. Unfortunately none of the answers here were useful, so far.

Regards

Sorin Dobircianu Your answer doesn't help me. Then please remove it.

Mirosław Grzesik Your answer doesn't help me. Then please remove it.

Rooholamin Dargahi Three independent experts gave you the similar answer. None of them helped you probably because you don't understand the basics of chemical kinetics while trying to analyze a pretty complex system. You consider only one simplified reaction mechanism copied from one paper. To my best knowledge the mechanism of H2 combustion by O2 includes more than 150 reactions. The reaction rate constant are documented https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=2ahUKEwjr7vWomurkAhWOiOAKHdtXAL8QFjAGegQIBRAC&url=https%3A%2F%2Fnvlpubs.nist.gov%2Fnistpubs%2FLegacy%2FNSRDS%2Fnbsnsrds67.pdf&usg=AOvVaw0nf1s0PECC2PEZgy5b565T

If you are unhappy with people who tried to help you then why you do not ask your question to the authors of that paper and delete your question from the forum

Yurii V Geletii Thank you for your reply but as I see none of the above answers are what I expected to get. Every answer and contribution is welcome to my question but it MUST be relevent! You think I call everybody expert! All of the above answers are very low level.

Sorry but you made me to report all of the answers here. You can make me happy if you guys remove your answers.

Just for your information:

1- Here is not a forum

2- This is a question NOT a discussion.

Thank you

Rooholamin Dargahi Last remark. All answers are very professional and relevAnt. That's not you who decides who is an expert and who is not.

You can simply calculate theta from your data and report it as it is. (theta would have the unit of temperature then). If you want to relate it to the activation energy which is commonly used in the papers and textbooks, you may consider theta as (Er/R_gas), where Er is the activation energy and R_gas is the gas constant. The unit of Er would then depend on your selection of the unit for R_gas (Er/R_gas should have the unit of Kelvin). Just keep in mind that you have to refer the readers to your equation that you use when you report the values for Er.

P.S. Keep calm buddy. These people are volunteers who are here to help you not to mislead you. Everybody may have a different understanding of a question. Then, it's your responsibility to try to rephrase your question so that it is understandable to all.

Good Luck

Amin Kazemi Thank you, Mohammad Amin. Then what I've said is right.

Re my reaction to fellas in this question

I wanted them(all super geniuses here :) ) to remove their answers (I said it politely. As you see I said "Please" and even excused them). The question is entirely clear. They posted some irrelevant answers here. Alone relevant answer here is yours. Interesting! Yurii has recommended your answer. It made me upset when I saw that someone like Yurii is insisting on his answer just like a child(Is he a professor? if so, then I'll be sooooo disappointed). I've been with Americans and europeans for years. I think Yurii is angry and he's checking the question over and over to shout at us if we make a post that he doesn't like (hope my guess is wrong). I'm cool. I expected the guys over here to be gentlemen and do what I said. Anyway, thank you again for your answer.

Roh,

I'm happy you find your answer. But note that everybody is free to answer as they desire. They don't have to remove their answers if you don't like them. It is up to you to decide which one works for you. Their answers are also valuable but might not point out what you meant. Then simply ignore them. They may be of interest to other people who have a similar question in the future.

Good luck.

The activation energy is clearly defined along with the other variables in the first

png (in the other 2 pngs it is buried in combinations with other variables ).I do not understand what your problem is.

Dikeos Mario Soumpasis The book we used to pass the combustion course was this:

In this book, the Arrhenius equation was mentioned just like the first png. I also checked this book

There is just one form of arrhenius equation in the standard literatures and it's what I showed in the first png. We are not familiar with other two forms. I need to transform Tar in my multi-step mechanism into Er. Thus to make sure what I'm doing is right or you tell me the way to transform it into Er, made this question. That's why I asked "I think I have to multiply thetha or Tar by R. Am I right? R is universal gas constant and here its unit here is J/kgmol-K. I guess it should be 8314. Am I right?"

Any other question?

Rooholamin Dargahi Once I opened up all three attachments to your original post and compared the three forms of the Arrhenius equation, I was able to understand what you are asking for. Here is your confirmation:

T_ar = Theta_j = E_r/R, therefore:

E_r = T_ar*R = Theta_j*R

So, for your Fluent simulation you can calculate E_r from the literature values of T_ar and Theta_j by just multiplying those coefficients by the ideal gas constant.

Douglas Michael Price Thank you, Douglas. I'm getting hopeful when see such clear, decent, and relevant answers! I hope always get such answers in researchgate. Hat off for you, Douglas!

Arrhenius's equation is very useful but is purely empirical. So you can use the form that best fits your results. Statistical mechanics gives a more correct approach to the concept of activation energy and shows that the effect of temperature on kinetic constants is not simple, but in the vast majority of cases, an empirical approach is sufficient.

Similar discussion you can find following the reference https://www.researchgate.net/post/What_is_the_most_accurate_way_of_kinetic_constants_determination#view=5d49a59dd7141b452d2da08e , where you can see my answers.

Dear Roh,

The answer is presented. But, I would recommend using index 'a', instead 'r' and to write Ea or Ta. :-)

you can look at some of my papers where Arrhenius law is used in different ways

Dear Prof. F. Fraga, would you please send me some of your papers concerning this topic.

Thank you.

Anyone Can help to find activation energy ?

There is a problem with the activation energy that, using the Arrhenius equation, we often determine a number that we can only interpret as a measure of the sensitivity of a process rate to temperature change. This is the case of batch adsorption described by empirical functions.