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Pressure does not need to be constant, but unless you are doing an experiment in a closeclosed/pressurized (or depressurized) environment, the pressure at which a reaction occurs will be the ambient atmospheric pressure (which is effectively a constant). Moreover, if you are doing a reaction in an open vessel, the energy that you can usefully extract typically comes in the form of the heating of the material, which is what makes the enthalpy $H$ important.

As to why we define the enthalpy $H$, rather than just talking about heat transfer $Q$—the reason is that $H$ is a state function, while $Q$ is not. You can talk about the total enthalpy $U+PV$ of a system, just like you can talk about the total energy. You cannot talk about the "heat" of a system, or the "work" of the system. This makes enthalpy a more powerful tool.

(Your equation for what happens at a variable pressure is not really accurate, in part because it does not adequately distinguish which quantities are state variables—which are determined by the state of the system—and which are the ones, like heat and work, that can only be defined as transfers into or out of the system.)

Pressure does not need to be constant, but unless you are doing an experiment in a close/pressurized (or depressurized) environment, the pressure at which a reaction occurs will be the ambient atmospheric pressure (which is effectively a constant). Moreover, if you are doing a reaction in an open vessel, the energy that you can usefully extract typically comes in the form of the heating of the material, which is what makes the enthalpy $H$ important.

As to why we define the enthalpy $H$, rather than just talking about heat transfer $Q$—the reason is that $H$ is a state function, while $Q$ is not. You can talk about the total enthalpy $U+PV$ of a system, just like you can talk about the total energy. You cannot talk about the "heat" of a system, or the "work" of the system. This makes enthalpy a more powerful tool.

(Your equation for what happens at a variable pressure is not really accurate, in part because it does not adequately distinguish which quantities are state variables—which are determined by the state of the system—and which are the ones, like heat and work, that can only be defined as transfers into or out of the system.)

Pressure does not need to be constant, but unless you are doing an experiment in a closed/pressurized (or depressurized) environment, the pressure at which a reaction occurs will be the ambient atmospheric pressure (which is effectively a constant). Moreover, if you are doing a reaction in an open vessel, the energy that you can usefully extract typically comes in the form of the heating of the material, which is what makes the enthalpy $H$ important.

As to why we define the enthalpy $H$, rather than just talking about heat transfer $Q$—the reason is that $H$ is a state function, while $Q$ is not. You can talk about the total enthalpy $U+PV$ of a system, just like you can talk about the total energy. You cannot talk about the "heat" of a system, or the "work" of the system. This makes enthalpy a more powerful tool.

(Your equation for what happens at a variable pressure is not really accurate, in part because it does not adequately distinguish which quantities are state variables—which are determined by the state of the system—and which are the ones, like heat and work, that can only be defined as transfers into or out of the system.)

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Buzz ♦
Buzz ♦
  • 17.1k
  • 15
  • 49
  • 63

Pressure does not need to be constant, but unless you are doing an experiment in a close/pressurized (or depressurized) environment, the pressure at which a reaction occurs will be the ambient atmospheric pressure (which is effectively a constant). Moreover, if you are doing a reaction in an open vessel, the energy that you can usefully extract typically comes in the form of the heating of the material, which is what makes the enthalpy $H$ important.

As to why we define the enthalpy $H$, rather than just talking about heat transfer $Q$—the reason is that $H$ is a state function, while $Q$ is not. You can talk about the total enthalpy $U+PV$ of a system, just like you can talk about the total energy. You cannot talk about the "heat" of a system, or the "work" of the system. This makes enthalpy a more powerful tool.

(Your equation for what happens at a variable pressure is not really accurate, in part because it does not adequately distinguish which quantities are state variables—which are determined by the state of the system—and which are the ones, like heat and work, that can only be defined as transfers into or out of the system.)