Reaction steps and Activation Energies

[engineer23]

Homework Statement

The overall activation energy E is 60 kcal/mol.
Reaction steps are:
CH3CHO --> .5CH3CO + .5CH3 + .5H2 E1 = 80 kcal/mol
CH3CO --> CH3 + CO E2 = ?
CH3 + CH3CHO --> CH4 + CH3CO E3 = 10 kcal/mol
CH3 + CH3CO --> minor products E4 = 5 kcal/mol

Estimate E2.

Homework Equations

Arrhenius equation k = A exp (-E/RT)

The Attempt at a Solution

Does E1 + E2 + E3 + E4 = E? If so, then E2 = -35 kcal/mol. If this is the case, what does negative activation energy mean (physically)?

Do I need to use the Arrhenius equation and/or d[M]/dt expressions to solve this problem. Since E1 >> E3 and E4, are there any assumptions I can make?

I feel like I'm missing something here.

Thanks!

 

[Stevedye56]

I am almost positive that if you have a negative activation energy that it means that the reaction will take place and that it will not be possible to stop it.

Hope this helps.

Steve

 

[Blueshift5]

it is important to carefully consider the information given and make logical deductions based on established principles and equations. In this case, the given information suggests that the overall activation energy (E) for the reaction is 60 kcal/mol. The reaction is also described in terms of four individual steps, each with their own activation energy (E1, E2, E3, and E4).

Based on the given information, it is reasonable to assume that the activation energies for the individual steps add up to the overall activation energy. Therefore, E1 + E2 + E3 + E4 = E. Using this equation, we can solve for E2 by rearranging the equation to E2 = E - E1 - E3 - E4. Substituting in the given values, we get E2 = 60 kcal/mol - 80 kcal/mol - 10 kcal/mol - 5 kcal/mol = -35 kcal/mol.

A negative activation energy is not physically meaningful, as activation energy is defined as the minimum energy required to initiate a reaction. In this case, a negative activation energy would suggest that the reaction does not require any energy to proceed, which is not possible. This indicates that there may be an error in the given information or that the reaction steps are not representative of the actual reaction.

The Arrhenius equation can be used to calculate the rate constant (k) for a reaction, but it requires the activation energy (E), the pre-exponential factor (A), the temperature (T), and the gas constant (R). Since the given information does not provide all of these values, we cannot use the Arrhenius equation to solve for E2.

In terms of assumptions, we can assume that the overall activation energy is the sum of the individual activation energies, and that the rate constant for each step is proportional to the concentration of the reactants. However, these assumptions may not be accurate if the given information is not representative of the actual reaction.

In conclusion, based on the given information, we can estimate that E2 = -35 kcal/mol, but this value is not physically meaningful. It is important to carefully consider the given information and make logical deductions, but in cases where the information is incomplete or inaccurate, it may not be possible to accurately solve for all parameters.