How do you solve for enthalpy of vaporization? I know it's liquid to gas, but I can't figure out how to solve an actual problem like C2H4O2. Could you please explain?
Certainly! The enthalpy of vaporization (ΔHvap) is the amount of heat energy required to convert one mole of a substance from its liquid phase to its gaseous phase at a constant temperature and pressure. To solve for the enthalpy of vaporization, you typically need experimental data or values from reference sources.
However, I can guide you through the process of finding the enthalpy of vaporization using a thermodynamic data table or standard references. Let's take the example of acetic acid (C2H4O2) and its enthalpy of vaporization.
1. Start by finding the standard enthalpy of formation (ΔHf) values for the reactants and products involved. You can find these values in tables or thermodynamic databases. For acetic acid, the standard enthalpy of formation is -485.70 kJ/mol.
2. Next, find the standard enthalpy of combustion (ΔHc) for the substance. The standard enthalpy of combustion for acetic acid is -871.35 kJ/mol.
3. Now, find the standard enthalpy of formation and combustion for the products of the reaction. In this case, the products are CO2 (carbon dioxide) and H2O (water). The standard enthalpy of formation for CO2 is -393.51 kJ/mol, and for H2O, it is -241.82 kJ/mol. The standard enthalpy of combustion for CO2 is -393.51 kJ/mol, and for H2O, it is -285.83 kJ/mol.
4. Use the given data and apply the Hess's Law of constant heat summation. According to Hess's Law, the change in enthalpy for a reaction is independent of the pathway taken.
The reaction for the complete combustion of acetic acid can be represented as follows:
C2H4O2(l) + O2(g) → CO2(g) + H2O(l)
Since the reaction involves a liquid-to-gas phase change, the enthalpy change for the formation of H2O(l) from H2O(g) is the enthalpy of vaporization (ΔHvap).
5. Calculate the overall enthalpy change for the reaction by summing the enthalpies of formation and combustion of the reactants and products:
Overall ΔH = [ΔHf(CO2) + ΔHf(H2O)] - [ΔHf(C2H4O2) + ΔHc(C2H4O2)]
= [(-393.51 kJ/mol) + (-241.82 kJ/mol)] - [(-485.70 kJ/mol) + (-871.35 kJ/mol)]
= -635.33 kJ/mol
Since the enthalpy of vaporization (ΔHvap) is equal to the enthalpy change for the reaction, the calculated value of the overall ΔH (-635.33 kJ/mol) would be the enthalpy of vaporization for acetic acid.
In summary, to solve for the enthalpy of vaporization, you need to use thermodynamic data, such as standard enthalpies of formation and combustion, and apply Hess's Law to calculate the overall enthalpy change for the reaction.