Difference between an open and closed system?

[yzdm]

I do not quite understand the differences between an open and closed system. 

In my view for a closed system, the amounts of reactants and fluids are given at first, then the reaction goes ahead, resulting in the decomposition of reactants and the formation of products. None of the reactants and fluids is fixed.

As a contrast, one or some components of the reactants and fluids is fixed during the reaction.

I am not sure whether my understanding is correct or not, so any suggestions are welcomed.

 

[Brian Farrell]

Hi,

There might be a little wiggle room in how you define open or closed systems, but you should always be able to justify that the software settings you use match your conceptual model. 

At the most basic level, the program doesn’t add or remove mass unless you tell it to, so by default a simulation is closed. 

A very simple example of a closed system (with respect to mass) is a fluid that’s heated to determine the effect of temperature on species stability. No mass is added to or removed from the system, so it’s closed. In another example, a bioreactor is filled with oxygenated water, amended with a carbon substrate, and inoculated with aerobic microbes, then sealed off from the atmosphere with a stopper and left to sit for several days (the closed system simulation starts after the inoculation and feeding). The microbes will consume oxygen, causing its concentration in the fluid to decrease. Because it’s sealed, nothing is added or removed, so it’s a closed system.

If that bioreactor is left open to the atmosphere, however, O2 will dissolve into the fluid as it’s consumed to maintain a constant fugacity. In the GWB, you’d use a fixed O2 fugacity path to define this type of open system. The program fixes the O2 fugacity at its original value by allowing gas to enter the fluid from an external reservoir (the atmosphere). Thus, it’s an open system.

A configuration that depends on your concept of open vs. closed systems is a titration path. Imagine adding an acid to an alkaline fluid, to see how the pH changes. You might add HCl as a simple reactant. In this example, it seems like a pretty straightforward open system, since you’re gradually adding acid to a fluid. Alternatively, consider a water-rock system composed of halite and water. In our conceptual model, the halite is present in the solution, but it hasn’t reacted at all yet. You could add halite as a simple reactant to the fluid, so that’s its entire mass is gradually exposed to the fluid, or you could set a kinetic rate law, by which it dissolves at a certain rate that can change with time. You might conceptualize the “system” as being composed of the fluid and the rock, in this case, so that the system might be said to be closed.

It can be helpful to plot the mass of various components in the fluid, the rock (all minerals), the sorbate, the system (all of the above) and so on. In GWB12, you can additionally plot the mass reacted for all types of reactants: simple, sliding, fixed, or kinetic. If the model includes a time span, you can additionally plot reaction rates for any type of reactant. It can be very useful to make plots of these as a function of reaction progress to help verify that your numerical model matches your conceptual model.

In the future, please post new topics on the front page of the GWB forum, not in the archive of old posts.

Regards,

Brian Farrell
Aqueous Solutions