Activation Energy and K constant-DAT Destroyer

[Teethyman]

For a half reaction, let's say first order, we know that kt=0.693. There are numerous occasions in which questions ask what these reactions are dependent on. For this first order reaction, I understand why it is dependent on K. However, one should note that the equation for K is K=Ae^(-Ea/RT).

This would mean that K is dependent on temperature and activation energy.

Therefore, when a question asks what is a first order reaction dependent on, why doesn't the answer include activation energy or temperature? Is it simply implied since the answer is that the reaction is dependent on only K?

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[DAT Destroyer]

For a half reaction, let's say first order, we know that kt=0.693. There are numerous occasions in which questions ask what these reactions are dependent on. For this first order reaction, I understand why it is dependent on K. However, one should note that the equation for K is K=Ae^(-Ea/RT).

This would mean that K is dependent on temperature and activation energy.

Therefore, when a question asks what is a first order reaction dependent on, why doesn't the answer include activation energy or temperature? Is it simply implied since the answer is that the reaction is dependent on only K?

A first order reaction , like any reaction depends on factors such as temperature, energy of activation, pressure, and solvent effects among other parameters. Your question is very vague, thus not easy to see what you are driving at. If asked in context of an organic reaction, we often say that a first order reaction is dependent on halide concentration, and yes,,,,,it is assumed that other factors as mentioned above do indeed play a role and can change a rate constant. A catalyst for example also changes the value of a rate constant, k.

Hope this helps.

Dr. Romano

 

[Dr. Dan]

If I understand you correctly, then when a reaction is dependent on k, then there is also an implied dependency on the variables in the Arrhenius equation (Ea and T).

 

[Dr. Dan]

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[Teethyman]

Sorry I was so vague, I believe I am confusing myself more....Taking the DAT this Friday so my mind is going in circles after 3.5 months of studying.

I am referring to a very basic general chemistry problem that basically asks "For a first order reaction the half life for a particular reagent is dependent on?"

The answer says it is only dependent on K, yet there is another answer choice that includes activation energy as well.

I was wondering why one wouldn't say that the reaction was dependent on activation energy and temperature.
I am confused about the Arrhenius equation in general and when to apply it for general chemistry questions in particular.

Thanks!

 

[DAT Destroyer]

Sorry I was so vague, I believe I am confusing myself more....Taking the DAT this Friday so my mind is going in circles after 3.5 months of studying.

I am referring to a very basic general chemistry problem that basically asks "For a first order reaction the half life for a particular reagent is dependent on?"

The answer says it is only dependent on K, yet there is another answer choice that includes activation energy as well.

I was wondering why one wouldn't say that the reaction was dependent on activation energy and temperature.
I am confused about the Arrhenius equation in general and when to apply it for general chemistry questions in particular.

Thanks!

I do indeed see your confusion. The equation kt1/2 = .693 is for NUCLEAR DECAY. One can say that radioactive decay is a first order reaction process. Even though radioactive decay is a first order reaction, where the rate of the reaction depends upon the concentration of one reactant (Rate = k [A}) , it is not affected by factors that alter a typical chemical reactions. In other words, the reaction rate does not depend upon the temperature, pressure, and other physical determinants. However, like a typical rate law equation, radioactive decay rate can be integrated to link the concentration of a reactant with time. For the DAT exam....make sure you know this equation......and can plug in the needed values. More importantly, know how to look at data and find rate laws and rate constants. I have put several of theses questions in the book. The DAT will be cake.

Hope this helps..

Dr. Romano

 

[Teethyman]

Wow, this really clears things up! Thank you so much!

 

[Teethyman]

I do have one more question....about nuclear decay. You can classify a nuclear decay reaction as zero order, first order, or second order etc. As seen in question 105. The zero and second order reactions depend on initial concentration and K, while the first order half life reaction only depends on the K value?

So as you stated the other reaction rates besides nuclear decay can depend on K, concentrations of reactants, as well as Ea or temperature.

Would this be correct?