Expansion Work in Combustion Reactions: Liquid Water vs. Water Vapor

[ester10]

Expansion Work -- Still confused.

This is more of a general question regarding thermodynamics. If you have a combustion reaction, and it produces either liquid water or water vapor--which case results in higher expansion work? My enthalpy calculations show higher PV work for the reaction with liquid water as the product, but intuitively, I feel like the reaction producing water vapor would do more work because a greater amount of gas is produced.

Thanks in advance for your input!

 

[ester10]

I guess I should specify. The way I calculated the enthalpy of combustion was by subtracting the enthalpies of formation for the reactants from those of the products--a generic way to get the enthalpy change for any reaction. However, since water (gas) has a higher Hf, the overall H of the combustion is also higher (less negative value). This suggests that the combustion of a compound to CO2 and H2O (g) actually releases less energy (i.e., less work can be done).

Please give me your input... Am I not justifying this correctly?

 

[tomcue]

I can provide some insight into this question. The concept of expansion work in thermodynamics refers to the work done by a system as it expands against a constant external pressure. In the case of a combustion reaction, the expansion work is determined by the volume change of the products compared to the reactants.

In general, the reaction that produces more gas as a product will result in higher expansion work. This is because the gas molecules have more freedom to move and exert pressure on the surroundings as compared to liquid molecules. Therefore, in your example, the reaction producing water vapor would indeed result in higher expansion work compared to the one producing liquid water.

However, it is important to note that expansion work is just one aspect of the overall energy balance in a combustion reaction. Other factors such as the heat released, temperature change, and enthalpy change also play a role in determining the overall energy outcome of the reaction. So while the reaction producing water vapor may have higher expansion work, it may also have other factors that contribute to a lower overall energy release.

In conclusion, the reaction producing water vapor would result in higher expansion work due to the increased volume of gas produced. But it is important to consider all aspects of the energy balance in a combustion reaction to fully understand its thermodynamics.