H is a state function which means one only needs to know the initial and final states to calculate a change in enthalpy; it does not matter how the change occurs.

Heating water is an example of process that is path independent. Let's say in lab two students have to heat a sample of water from 23 °C to 60 °C.

The heat transfered to the water is

q = n s T

Since the process is at constant pressure, and only PV work is being done, q is qp. Thus,

H = n s T

 One student carefully heats 20 g of water from 23 °C to 60°C. Determine the H for this process. H = n s T H = (20g)(4.184 J g-1K-1)(60 - 23)K H = 3096 J Another student puts 20 g of water over a microburner and goes outside for (insert bad habit here)... The sample of water went from 23 °C to 75 °C. Before the student can continue with the experiment the sample of water must be cooled to 60 °C. Determine the H for this process. To heat the water from 23 to 75 °C the amount of heat that when in is H = n s T H = (20g)(4.184 J g-1K-1)(75 - 23)K H = 4351 J But the water had to cool back to 60 °C, and while the water was cooling heat was released. H = n s T H = (20g)(4.184 J g-1K-1)(60 - 75)K H = -1255 J So, H was 4351 J to get to 75 °C but on the way back to 60 °C 1255 J were released. H = 4351 + -1255 = 3096 J

The energy that was actually transfered to the water is determined by comparing the initial and final states. Student 2 transfered extra energy into the water, but it was released by the water to the surroundings when it cooled; so, in the end, students 1 and 2 transfered the same amount of energy to the water.

What we have just demonstrated, by example, is Hess's Law. Hess's Law holds for H of any process.

Typically, chemists use Hess's Law to determine H for reaction for a variety of reasons; the experiment may be difficult, dangerous, or impossible, or we may be making a survey to determine which reactions release the amount of heat we want.