Hello Everyone! Thanks for stopping by today to learn about state functions with me.
Why you should understand state functions:
Understanding what a state function is, is very important not only to general chemistry but also physics. In general, understanding that a variable is a state function makes the math easier to use and solve for the answer.
Key Ideas
- Defining a state function
- Examples of state functions
- Why aren't heat and work state functions
Defining a State Function
State functions are variables in chemistry or physics which are path-independent. A path-independent variable is a measurement that doesn't depend on the path. A path-independent variable only depends on the starting value and the final value.
The image to the left depicts an example of both a path independent variable (or a state function) and a path-dependent variable.
On the left side of the image, is a picture of my car. On my drive home from college, my car starts in the same location. My car always ends up in the same location at my parent's house. There are several different roads I can take to drive home. Each path has a different distance.
However, regardless of the path I take, my car uses 10 gallons of gas. In this example, the amount of gas my car uses is completely dependent on where my car starts and where it ends. This means the amount of gas or energy required is a state function in this example. The distance traveled is not a state function because each path has a different length regardless of if I start and end at the same location.
Keep in mind that this example is just that, an example. In the next section of this post, I've created a list of the most common state functions and non-state functions.
Examples of State Functions
Here I've created a list of the most common variables that are state functions and the two most common that aren't. I would not recommend memorizing these variables. I personally don't believe memorizing information is a worthwhile use of anyone's time. Rather, I think that recognizing information and being able to apply it is the best use of any student's time. It demonstrates a higher level of understanding and development of critical reasoning skills.
***Notice that State functions are represented with a capital letter while non-state functions are represented by a lower case letter. This is a huge clue that a variable is a state function. If you are taking a test or quiz and don't know for sure if a variable is a state function or not, check to see if it has a capital letter designation.
What does a state function mean, mathematically? The change in a state function like Entropy or Enthalpy can be measured by simply subtracting the final minus the initial value. The change in any variable is represented by a triangle or the greek symbol delta.
Why heat and work aren't state functions
Looking at my list above, you can see that there are two specific variables that are NOT state functions. They are heat and work.
Let's start with work. Below you'll see the equation for work. Work is described as a force multiplied by a distance. The work required to move an object over a distance changes depending on how far the distance is. Here's an example to clarify the idea.
Imagine I'm trying to move my couch across my living room floor. The work required to move my couch depends on how far I want to move it and how much force it takes me.
If I push my couch in circles around my living room before settling on a spot, it will take more work than if I push my couch directly to the spot in a straight line.
Though heat is closely related to enthalpy, there is a slight distinction between the two. I personally think of Enthalpy as the energy associated with a temperature change. While heat is specifically the temperature change. In simpler terms, Enthalpy is heat energy.
To the left, I've got a picture to help clarify why the movement of heat is path-dependent.
The left side of the image shows three different metals. If the three metals are touching and I apply heat to the first, aluminum; the heat will be transferred through the metals. Eventually, the metals will all be the same temperature. Meaning the heat will have spread equally throughout the metals (depending on their specific heat, of course).
On the right side of the image, I have eliminated the copper metal between aluminum and gold. If I apply the same amount of heat to the aluminum, the temperature will again reach equilibrium, but it won't be the same as in the first example.
If heat were a state function, no matter how many metals I placed between aluminum and gold, the heat transferred would always be the same.
Summary
- State functions are variables in chemistry or physics which are path-independent. A path-independent variable only depends on the starting value and the final value.
- If you are taking a test or quiz and don't know for sure if a variable is a state function or not, check to see if it has a capital letter designation.
- Work is not a state function. Work is described as a force multiplied by a distance. The work required to move an object over a distance changes depending on how far the distance is.
- Heat is not a state function. If heat were a state function, no matter how many metals I placed between aluminum and gold, the heat transferred would always be the same.
These explanations have been helpful if you'd like to view the video version of this information, I've attached the link to the video on my youtube channel below.
- Saren
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