Confusion with adding water and ethanol:combustion reaction
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Peach
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Confusion with adding water and ethanol:combustion reaction


I want to start by saying I'm a mechanical engineering student who just covered combustion in my Thermodynamics II course and I want to start a personal project (so chemistry is not something I am completely comfortable with just yet).

I want to simulate adding 25% water by weight to ethanol for a combustion reaction (with liquid O2 as the oxidizer). How does one balance this combustion reaction so that I can find things like the Lower and Higher Heating Value, the pressures, and other values of interest where I believe it's crucial to account for the water added to the ethanol.

So far I have my chemical equation (by mols) as,

0.75(C2H5OH)+0.63986(H2O)+O2 ---> x(CO2)+y(H2O)

I'm not sure if this is the correct way to account for the water added into the ethanol, and I can't seem to figure out how to balance this equation. Could I get some guidance on this topic?

Thanks,
Peach
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ChenBeier
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As everybody know water is not combustable, it does the opposite it extinguish a fire, so if adding water the burning process is more difficult or will not take place.
The chemical reaction is still the same regarding the alcohol

C2H5OH + 3 O2 => 2 CO2 + 3 H2O

If water is already present, then energy is needed to vaporize it. Also it will push the equilibrium to the left side of the reaction.

So you need the burning enthalpy and also the energy values to get liquid water to steam.
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Peach
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ChenBeier wrote:
As everybody know water is not combustable, it does the opposite it extinguish a fire, so if adding water the burning process is more difficult or will not take place.
The chemical reaction is still the same regarding the alcohol

C2H5OH + 3 O2 => 2 CO2 + 3 H2O

If water is already present, then energy is needed to vaporize it. Also it will push the equilibrium to the left side of the reaction.

So you need the burning enthalpy and also the energy values to get liquid water to steam.


So for the enthalpy of the reactants do I calculate the enthalpy of the alcohol and the liquid oxygen and subtract the enthalpy required to get the liquid water to steam? Am I understanding that correctly? And for the products I just calculate the enthalpy of the products as normal?
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ChenBeier
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You calculate the enthalpy for the combustion process of the alcohol and subtract the one to vaporize the extra liquid water.
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