See scenario below:
You are monitoring a certain chemical reaction (browning) of a product stored at different temperatures over a certain period. When applying kinetics to determine the reaction order, sometimes you find that this reaction fits/obeys both reaction orders (zero- and first-order). When determining the activation energy using the Arrhenius equation ( ln k = ln A - Ea/RT), which rate constants (k) do you choose (zero - k0 or first - k1)? Can you apply both or is it always k0?
Dear Lusani
In most of the leaching processes, it is assumed that the main leaching reaction is first order. however, the order of adsorption reactions may be 0, 1, 2 or ... . For more details, take a look at our previous work.
Article Kinetics of leaching: a review
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Lusani Norah Vhangani, Browning of the product during storage is a very complex system of chemical reactions. You can of course describe the degree of browning as a function of time by some simple function, but it is self-deception. From what you are writing you are trying to describe the degree of browning as a function of time with a linear function (zero order) and an exponential function (first order) and you don't see the difference in the fit. This often happens with a small range of changes or measurement errors. Having the values of the coefficients of proportionality ko and k1 vs temperature, you would like to determine the value of the parameter E in the Arrhenius equation. However, this will not be the activation energy, but at most a measure of the sensitivity of the rate of the browning process to temperature changes. You should remember that the term activation energy applies only to elementary reactions, and that is not the browning process. If you have any further questions, please write.
Mirosław Grzesik Thank you for your response. You highlighted that not seeing the difference may be due to two reasons, one being measurement errors and small range of changes. In our case we observed the latter. With reference to not being able to determine EA, the sensitivity of browning process towards temperature changes is also be an important aspect in the project. In this case it will aid in determining the effect of certain variables such as such reactants concentration.
Mirosław Grzesik I think it is not elementary reactions' activation energy that Lusani Norah Vhangani is interested in. I think it is the apparent activation energy NOT the elementary reactions' activation energy. Maybe Lusani Norah Vhangani can clarify if it is misunderstood.
For the other question on which k to use for energy calculation using Arrhenius equation (k0 or k1), the rate-time data fitted into the kinetic models for respective orders (zeroth, first, etc.) is expected to direct which one to use. And the knowledge of the order of the reaction can guide your choice of which one to use among the ks.
Toyese Oyegoke, Of course, browning, presumably food, is not an elementary reaction. There we can deal with the oxidation of various food components, enzymatic reactions and a slow loss of water. Structural and texture changes may also occur. One can, obviously, try to describe the kinetic curves (degree of browning vs. time) with known functions and on this basis infer the order of the process (treated as a single step), but one need to have reliable data. But if the changes in the degree of browning are small, as the author of the question writes, such calculations will be unreliable. And one more thing. In such a situation, the use of the term apparent activation energy does not make sense.
Lusani Norah Vhangani, If the change in the degree of browning (or some other measure) is small, it is difficult to obtain reliable calculation results. Maybe you share the data, it will be easier for us to help.
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