What is Equilibrium?
Date: Spring 2012
What is equilibrium? Are molecules still moving at equilibrium?
Imagine a seesaw (teeter-totter). If we start the seesaw parallel to the ground, put two objects, one on each end, and the objects have the same mass, the seesaw does not move. This is what is considered a static equilibrium - a balance. Nothing is changing. The object may move around, but as long as they stay on the same side of the seesaw, the balance is maintained.
There is another way for the balance to be maintained. Suppose we put two jugglers, one on each end of the seesaw. Let's say the jugglers have the same mass and each hold four balls. The seesaw will also not move, remain parallel to the ground, when the jugglers toss the balls to each other - as long as they pass the balls to each other at the same rate so that the weights essentially remains the same on both ends. This is considered a dynamic equilibrium - things are moving back and forth, but a balance is maintained. In dynamic equilibrium, there is change (there is transfer between the two jugglers), but there is no net change (each juggler still ends up with the same overall mass).
Suppose now that the two jugglers have different weights. The seesaw will lean over to the juggler with the higher weight. However, if that juggler starts passing all of his/her balls to the lighter weight juggler, that side becomes too heavy. So the way they can maintain the balance is to vary the speed at which they transfer the balls. The heavier juggler would have to pass the balls back faster, and the lighter juggler will have to hold on to the balls a little longer than the other. In this dynamic equilibrium, the rate of transfer between the two ends are not the same. Again while there is change, there is no net change.
Chemical reactions are like the third situation. For example if we imagine that N2 + 3H2 = 2NH3 were an equilibrium, then there are N2's and H2's becoming NH3 (moving forward) and there are NH3's becoming N2's and H2's (going backward). A dynamic equilibrium is established when the rate of transfer forward is the same as the rate of transfer backward so that there is no net change in the concentration of either reactant or product. There is change, but no net change.
Greg (Roberto Gregorius)
In equilibrium the sum total of things stay the same but that does not mean everything stays the same.
For example, a class would be in equilibrium if for every student that left the room, another entered. The number of students would stay the same but individual students would definitely change.
If the temperature between two bodies, say an ice cube and water, is in equilibrium the temperature of the water will stay at zero degrees (or 32 in Fahrenheit) but the ice would slowly melt. The ice definitely changes although the temperature does not. The molecules in the water and even in the ice are definitely moving (just like the students going in and out of the classroom. Molecules do not stop moving until absolute zero, about 273 degrees below zero centigrade.
Hope this helps a little bit. Equilibrium is a tricky concept
R. W. "Mr. A." Avakian
There is no single simple definition of the term “equilibrium” because to depends upon the context. For example, mechanical equilibrium, thermal (that is, heat) equilibrium, chemical equilibrium, other contexts, and combinations of the contexts. Each type of system requires its own definition. In addition, there is thermodynamic equilibrium. This is a very general criterion – that’s the “good” news. The “bad” news is that this definition is rather abstract and not always easy to apply. See, for example, http://www.quantumthermodynamics.org/ . But these issues are far above the question you posed.
What can be said definitively about your question, “Are molecules still moving at equilibrium?” The answer is a resounding “NO!!” Molecules continue to move even at absolute zero. The have “zero point energy”. You can do a search on that term also.
As an answer to your first question, “What is equilibrium?” that question does not have a single simple answer.
Jeepers! These are good questions! The second one is easy: The molecules are moving. They are always moving. Moving molecules is a requirement of dynamic equilibrium.
Let us think about a soccer game. Suppose players that are on the field are wearing blue breezers, those floppy tee-shorts that look like netting. That is how we know they are playing the game and not resting. Resting persons are on the sideline.
When a person goes on the field to play, they put on the breezer and are now a player. When players come off the field, they take their breezer off (becoming a resting person) and hand them to the resting person coming on, they now become a player on the blue team. Persons keep coming on, putting on the breezer, becoming a player, and go off removing it, becoming a person. This way, there is no doubt that the players wearing blue are different from the rest of the team.
This keeps happening: resting persons walk on, becoming blue players… and blue players walk off becoming a resting person.
Notice how we came to the game with a set number of persons that could fit in our van, exactly 20 people. The rules of the game only allow 11 blue players at any one time. So the exchange of people is constantly in motion in accordance to the rules, but we still have 11 blue players and 9 resting persons. We would say that this soccer game is at equilibrium for players and resting persons. The game is still going! The players and resting persons are all exchanging… it is just that they are at dynamic equilibrium.
Think of the blue players as product molecules, the resting persons as reactant molecules and the playing field as the reaction itself.
The molecules are in motion. They are in a constant state of change: back and forth through the reaction. The reaction is at equilibrium: Start with a total of 20 molecules and form 11 of product and 8 of reactant--- these are working back and forth, freely exchanging according to the rules of chemistry… but we still have 11 of product and 8 of the product left. We would say that this reaction is at equilibrium. The reaction is still going! The molecules are still reacting, but they have found a state of dynamic equilibrium.
In chemistry, the rules that lead to equilibrium are many. That is why we need people with your curiosity to learn chemistry.
From an ex-NCSU soccer player
Peter E. Hughes, Ph,D.
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Update: June 2012