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Posted by on Jul 11, 2011 in Physical Chemistry | 2 comments

Open, Closed and Isolated Systems in Physical Chemistry

Definitions are the key especially at the beginning of learning a new subject. We define a system as something which we identify or prepare in order to do experiments and make observations. Nothing is perfectly isolated from its surroundings, but we can get quite close even though, eventually, all barriers break down.  Thermodynamics and physical chemistry make use of different types of systems which allow interaction or coupling with the surroundings.

System and surroundings

First the definitions:

A system:  that part of the universe we wish to study.

The surroundings:  the rest of the universe.

If a system is isolated, then nothing can enter or leave.  Its energy and matter remain the same.  Any changes go on inside the system, and since it is isolated, we cannot know anything about an isolated system from the outside.

We might think of the Earth as a system or we might focus on a chemical reaction. Usually energy and matter can change in a system.  Imagine your body as a complex biological system which interacts with the surroundings, or think of the burning of a cookie in a bomb calorimeter.  Systems can be complicated or  simple.  A bomb calorimeter only allows heat to be exchanged.  Such a system is called closed.

An example of a closed system is a balloon being heated so the gas inside expands it, or a piston like in the movie : no matter is exchange, only heat.

httpvh://www.youtube.com/watch?v=tH3NfRL1Ncc

The piston responds to heat changes.

When both matter and energy can be exchanged, the system is called open. Clearly when you do a titration matter is added. In this case the system is the beaker.

Interacting with a system

Suppose we have a system which is a drink in a glass.  Certainly heat flows in and warms the cold drink, and mass changes when we add to the glass or drink the contents. The system is open.

If you want to cool your drink, you can add some ice and the temperature goes down as the ice cools the drink.  If however, we dropped an ice cube into the ocean, there would be no noticeable change.

Generally speaking a system undergoes changes when heat and matter are exchanged, but the great outdoors remain at equilibrium.

Of course we know that this is an approximation, just think of global warming, but in science in order to simplify, we  think of the system as small with noticeable changes, and the surroundings as big, which remains at equilibrium.

The images and movies are part of my Physical Chemistry ebook and from my other science tutorials.

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2 Comments

  1. I was pleased with the introductory paragraph. It is clear meaning that produces the axiomatic concepts in physics. In this regard I would refer to the common use of energy inappropriately. If energy leaves a closed system, that system has been reduced as to its total material content. This would mean that matter and energy are different manifestations of the same thing. Physics speaks of change, but if we do not have a definition that “same thing” we do not have a clear definition of energy or matter. Without a clear definition we are speaking of magic when we talk of conversion.

    The solution to the use of energy is to say that matter is converted to light and light is converted to matter. With this epistemological change both light and matter carry energy and momentum. The conversion is in momentum rather than the substance that carries the momentum. Do we ever say that matter can be converted to momentum? No we do not. Since energy is a derivative of momentum, how can we convert matter into energy? Matter is converted to light and light has linear momentum while the particle has angular momentum—even at rest to the observer. The “same thing” referred to is the electric field having total angular momentum while light has a spiral linear motion. This solves the problem epistemologically.

    Just change the word energy to light and you do not need to explain. Defining terms properly is important to every student.

  2. Thank you for your comment. Indeed the real world differs from the ideal case I was talking about. It was this question you raise: what really is a closed system? that I wrote this blog. Only heat can come out of a closed system. Inside the closed system, the states of the material are changed, coal burned etc., but no matter gets out. Rather the heat stored there is lost through conduction, convection, and radiation. Inside the closed reservoir, the kinetic energy has dropped by an equal amount that leaves.

    Heat of course is infrared EM radiation and light is just the visible part of the same EM spectrum. Photons, the quantum form of EM radiation, are massless particles with a spin. Even though they do have relativistic momentum, the amount of matter involved is not relevant.

    Think of an incandescent light bulb. You put in electricity. Because tungsten (W) has a high resistance, it gets hot. Luckily it melts at 3,600 C (I think) so it glows white hot. It does not burn because the bulb is sealed and the gas inside is inert. So for the lifetime of that bulb (8000 hours), the W constantly vibrates and produces Black Body radiation (Max Planck 1898) and the mass change of the bulb is negligible.

    I would be interested in your views.

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