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.
State functions
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.
                      Δt = 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.