Question

We introduced enthalpy to work with reactions that were carried out at constant external pressure. But then I came across this formula (for ideal gas): ∆H=∆U + p∆V where p∆V=∆n(RT). But doesn't the p in the second formula represent the external pressure? So how are we using pv=nRT which is only valid if the pressure we are dealing with is the internal pressure of the gas?

Answer 1

Enthalpy is a property used to describe the thermodynamics of chemical and physical processes. The equation ΔH = ΔU + PΔV represents the enthalpy change for an ideal gas. In this equation, the pressure (P) represents the external pressure for the system, not the internal pressure of the gas.

Step-by-step explanation:

Enthalpy (H) is a property used by chemists to describe the thermodynamics of chemical and physical processes. It is defined as the sum of a system's internal energy (U) and the product of its pressure (P) and volume (V). The enthalpy change (ΔH) for a chemical change that occurs at constant pressure is equal to the heat gained or lost by the system at constant pressure.

The equation ΔH = ΔU + PΔV represents the enthalpy change for an ideal gas. The term PΔV can be further simplified as Δn(RT), where Δn is the change in the number of moles of gas, R is the ideal gas constant, and T is the temperature in kelvin.

It is important to note that in this equation, the pressure (P) represents the external pressure for the system, not the internal pressure of the gas.