State Functions in Thermochemistry

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  • 0:03 State Function Definition
  • 2:07 Using State Functions
  • 3:05 Important State Functions
  • 5:23 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

This lesson defines state functions and explains why state functions are so useful in thermochemistry and thermodynamics. You'll also see a few examples of common state functions.

State Function Definition

Imagine that you want to climb Mount Snowdon in my home country of Wales. Wales is the little country just left of England. You can get up to the top of the mountain two different ways. You can spend two to four hours hiking to the 3,560 feet peak or you can take the train, which is a nice, leisurely 1-hour ride to the top. Because you can't hike up in a straight line, your hiking distance is much more than the height of the mountain. The train, on the other hand, has a much more direct route to the peak.

Both ways will get you to the top, and in both cases, your altitude from start to finish will change by 3,560 feet. Critically, it didn't matter how you got to the top of the mountain, the change in altitude was the same. This is an example of a state function, which is a property whose value does not depend on the path taken to reach that specific value.

In other words, it didn't matter whether you climbed the mountain by foot or went by train, you still ended up 3,560 feet higher up than when you started. Your final position in both cases was the same. Now, not everything is a state function. Let us explore the way you got up the mountain. If you hiked up, you put in a lot more effort to do the climb.

But if you took the train, you put in very little effort to get up the mountain as the train was doing all the work for you. So the amount of effort, put in by you, was different and depended on which way you went up the mountain. We call this effort work.

Work is a path function and not a state function, because the value changes depending on the pathway you take. The ultimate question in figuring out whether something is a state function is: do I care how I got the final value? If the answer is 'no,' then it is a state function.

Using State Functions

Now you are probably sitting there thinking, why on Earth do I care? Why would I ever need to know about state functions? Mathematicians care a lot about state functions and spend a lot of time writing equations about them.

But fortunately for us, we don't really need to understand the math; we just need to know how we can use state functions. We come across state functions a lot in thermochemistry and thermodynamics. Many of the equations involved in these topics include properties that are state functions. What is really cool about state functions is that they make calculations simple and easy.

All we need to know is the state change - the change in value between the final and starting position. We do not need to know how this change happened. State functions really help us with something called Hess's Law. We won't go into that in detail here since Hess's Law is covered in another lesson.

Important State Functions

Okay. So, now that we've reviewed what state functions are and why they are used, let's explore some properties that are state functions. We'll start with temperature.

Let's say we have a glass of room temperature water measuring 22 degrees Celsius. So, what would happen if I add a large ice cube? The temperature will go down, of course. With the ice cube, our water drops from 22 degrees Celsius to 18 degrees Celsius.

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