Question about Energy Calculations

[Sphynx]

I have a project that is on a final stage, which is the calculation of energy for a certain reaction. The problem that i accounted for was that which energy is the one that is needed to be counted for a chemical reaction? Bond Dissociation Energy? Enthalpies of formation or any other kind of enthalpies? And how to convert those energies for 1 mole of reaction into Watts that needed to give out for the reaction to go? If you have any ideas please share, thank you.

[insane_alien]

you typically calculate the enthalpy of reaction with the formation enthalpies. this will give you a value in kJ/mol(or equivalent units)

 

to convert this into watts you need to know the rate of reaction.

 

for instance, if you have a reaction enthalpy of -150kJ/mol and a reaction rate of 0.001mol/s then you will have a power output of 150W of heat

 

you can do it with bond energies but this can introduce error on larger more complex molecules or molecules that induce some stress to the bonds by the nature of their structure, cubane would be an example where bond enthalpies would give a misleading result as the bonds are quite stressed so will have a different energy than an unstressed C-C bond.

[Sphynx]

Ok, and how do we account for the stress in the bonds? Is there a factor or factors that we can introduce?

[insane_alien]

i don't know if it's anything as simple as a factor and working it out would likely require an intimate knowledge of quantum mechanics and quite probably a computer.

 

i've certainly never come across such a factor.

 

just use the formation enthalpies, the data is pretty common.

[Sphynx]

Thank you very much for the help. Also, the calculation of the reaction rate requires experimental data of the change in concentration and the time at which the change occured. On the contrary, i have only the initial concentration at hand, and no experimental data, is there a way to calculate the precise reaction rate for a certain reaction only with the theoretical data?

[insane_alien]

http://en.wikipedia.org/wiki/Arrhenius_equation

 

the arrhenius equation will let you calculate a rate coefficient and then with the concentrations an stoichiometric constants you can work out a theoretical reaction rate for any given reaction.

[Sphynx]

Thank you very much.

[louis wu]

Remember that to use Arrhenius correctly you should only consider the chemical species involved in the Rate Determining Step. A little knowledge of the kinetics of your reaction is needed before you use Arrhenius.

[Sphynx]

The problem in my calculation that i accounted, is in finding the order of the reaction, i have the concentrations and volumes of all of the species. The reaction is CO2 + 3H2 --> Methanol + Water. The reaction mechanism of this reaction is a little tricky, since all of 3 hydrogen molecules have to reduce CO2 in their own way and at the same time preferably. Therefore i am not sure about how to find the order of this reaction.

[louis wu]

Sphynx

 

My first point would be that CO₂ and 3H₂ are not all going to meet at once. This 4 molecule collision is statistically virtually impossible. The reaction will proceed in stages involving 2 molecules at a time.

 

CO₂ is actually a fairly stable molecule, it will take a heavy duty reduction system to achieve anything. The system will involve some sort of catalyst; almost certainly a transition metal or rare earth. The catalyst will be involved in the reaction kinetics, it will have an activity coefficient and reaction rates will very likely depend on the surface area of the catalyst. In some circumstances catalysts can be 'poisoned' as the reaction proceeds losing effectiveness.

 

Next point is whether the reaction really finishes at methanol. Is there a reason inherent in the conditions why the methanol cannot be reduced by a 4^th H₂ to produce methane and water? From the data provided this cannot be ruled out.

[Sphynx]

In concern to the points above:

1. Indeed there is a catalyst involved which is a Copper Aluminum complex, and the reaction is going to be carried out under extreme conditions, meaning high temperatures and pressures in addition to the turbulence effects inside a chamber. The poisoning effect of a catalyst is a problem which can be solved at a little later stage.

2. Methanol will be instantly taken out of the chamber, but there is some methane that is going to evolve in addition to the DME (dimethyl ether) which is also going to be produced under those conditions.

 

At this stage, i am having trouble with calculating the reaction order of the reaction of CO2 with hydrogen into methanol, as well as estimating the Activation energy and the pre exponential factor A of the Arrhenius equation. My concern about it: is it possible to calculate or estimate those dimensions on a theoretical basis? Or there is an experimental data that is required?

[louis wu]

Sphynx

 

The usual way to work out the order of reaction for a reactant is with concentration verses time data for that reactant. The traditional method is to have all the reactants but one in large excess so that a single reactant is followed (a reactant in large excess will have essentially constant concentration).

Once the order is worked out for 1 reactant study the next reactant, until the order is known for all reactants. In this case the catalyst will need to be studied as well, probably with a series of experiments in which the amount of catalyst is varied. The order can sometimes be non integer.

 

If you are doing these experiments yourself you should consider the safety implications of working with hydrogen. You would need to write a detailed safety case for working with hydrogen. Extraction to a flame trap is a common requirement for use of molecular hydrogen within a lab. If there is a possibility or likelihood of an explosive atmosphere forming special categories of sparkless equipment would be needed.

 

Finding the order for all ingredients comes before finding the activation energy with Arrhenius. The activation energy you find will only be relevant in the presence of your catalyst.

 

The simplest procedure might be to find a paper written by someone who has studied the reaction.

Have you actually looked in your university or company library?

[Sphynx]

Thank you for the reply. I am have been and currently am looking at some experimental data provided from the libraries that are available to me. Unfortunately, this reaction seems to be very specific, and there is not much information available in the resources, however i will try to find something nonetheless.