|The First Law of Thermodynamics
|Law of Conservation of Energy: The energy
of the universe is constant; it can be neither created or destroyed, but only
transferred and transformed.
|A state function is any physical property whose value does not depend
on the system's history. Some examples are pressure, volume, and temperature.
For example, a system's temperature at any particular moment does not depend
on what its temperature was the day before, nor does it depend on how
the system reached its current temperature. If the system's temperature
is now 25oC, this is all we have to know about its temperature.
We do not have to say how it got to be that temperature. Also, if the temperature
was to rise to 35oC, the change in temperature, t, is simply
the difference between the final and the initial temperature.
= tfinal - tinitial
|We do not have to know what caused the temperature to change to
calculate this difference. All that we need are the initial and final values.
This independence from the method by which a change occurs is an especially
important property of state functions, and being able to recognize when some
function or property is a state function simplifies many useful calculations.
|Enthalpy is a particularly important state function. The enthalpy
of a system in a given state cannot depend on how the system arrived in
that state. This is useful to know, because when we measure the heat of a
reaction we do not have to worry about how the reaction is occurring,
but only that it is. To determine H, we only have to be sure of our initial
and final states and then measure the total amount of heat absorbed or evolved
as the system changes between these states.