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Here, we begin to think about a specific definition of heat energy known as "enthalpy". It's an important thermodynamic quantity used within chemistry

Internal energy changes

An internal energy change is a state function. It is a function only of the initial state of the system, and the final state.

[Delta U=U_2-U_1]

The first law of thermodynamics says that this change must entirely be accounted for by heat exchanged with the system, and work done by or to the system.

[Delta U=q+w]

At constant volume, there can be no work done because the system cannot expand. Therefore internal energy change is entirely accounted for by heat.

At the other extreme, constant pressure, work can be done, but only the volume change needs to be accounted for.

An intermediate case, where pressure and volume may change is more difficult to account for in a single step.

Defining enthalpy

Knowing the first law of thermodynamics and the definition work:

[Delta U=q+w w=-pDelta V]

We can combine these to define internal energy change as a function of heat and volume change at constant pressure:

[Delta U=q-pDelta V]

And then rearrange this for what it means for heat to transfer in and out of the system at constant pressure.

[q=Delta U+pDelta V]

We call this heat exchange, under constant pressure conditions, enthalpy. It is still just heat exchanged – i.e., collisions at the microscopic level – but it is a specific definition that has a lot of uses within chemistry.

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Introduction to Thermodynamics

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