No, definitely not. You can do the reaction any way you want: isothermally, adiabatically, isochorically, etc. But when we say that we want the reaction enthalpy at 298 K, it just means that the reactants have to start at 298 K and end at 298 K. You just have to include the enthalpy of heating/cooling the reactants and products to 298 K if you start/finish with them at some different temperature. Your question gets at the heart of the difference between state functions and path functions. The products at 298 K is one state. The reactants at 298 K is another state. Because enthalpy is a state function, the enthalpy change is the difference between these two enthalpies -- regardless of what path you take to get there! The freedom this gives you is a huge convenience.
Because when H2O (g) is cooled from 700K to 298 K H2O (l) there is a phase change, is there an additional heat value for the heat of vaporization as water changes from a gas to water? Whereas there is no phase change for CO and CO2 from 700K to 298 K so there is no additional heat associated with a phase change?
Very helpful.......thanks
I'm glad it was useful
Thank you sir 😊😊
You're very welcome
Does enthalpy of reaction being defined at a particular temperature how imply that a reaction is somehow an isothermal process?
No, definitely not. You can do the reaction any way you want: isothermally, adiabatically, isochorically, etc. But when we say that we want the reaction enthalpy at 298 K, it just means that the reactants have to start at 298 K and end at 298 K. You just have to include the enthalpy of heating/cooling the reactants and products to 298 K if you start/finish with them at some different temperature.
Your question gets at the heart of the difference between state functions and path functions. The products at 298 K is one state. The reactants at 298 K is another state. Because enthalpy is a state function, the enthalpy change is the difference between these two enthalpies -- regardless of what path you take to get there! The freedom this gives you is a huge convenience.
@@PhysicalChemistry Thanks for the clarification
Because when H2O (g) is cooled from 700K to 298 K H2O (l) there is a phase change, is there an additional heat value for the heat of vaporization as water changes from a gas to water? Whereas there is no phase change for CO and CO2 from 700K to 298 K so there is no additional heat associated with a phase change?
helpful 😀