1. substance or substances under study everything else
system
surroundings
substance or substances under study
everything else
…especially in the vicinity of the substances under study
…what you choose to look at
We’ll always take this perspective!

2. Enthalpy:
The property of a substance that can be used to obtain the heat absorbed or released in a change at constant pressure
Depends only on the present state of a substance…not how it got where it is…
A State Function
There is no natural benchmark for zero enthalpy, therefore it can’t be absolutely measured…
(in other words…a substance’s capacity to exchange heat)
Symbol: H
but it can be measured relatively