Mistake Master
Representations of equilibrium
An equilibrium looks different depending on how you draw it: an equation, a graph that flattens out, a jar of mixed particles. The skill is making all three tell the same story.
§1
One equilibrium, three views.
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An equilibrium can be shown symbolically (the balanced equation and its K expression), graphically (a concentration-versus-time plot), or particulately (a drawing of the mixture of particles).
On a concentration-versus-time graph, the curves change at first and then level off to constant values once equilibrium is reached. The curves flatten; they do not necessarily meet, because the equilibrium concentrations need not be equal.
A faithful set of representations is consistent: the leveling-off point on the graph, the K expression, and the particle picture all describe the same equilibrium mixture.
§2
Reading an equilibrium graph.
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Look for the flattening, not a crossing.
- Identify the changing region. Before equilibrium, concentrations rise and fall.
- Find where curves level off. Constant (flat) curves mark that equilibrium has been reached.
- Do not require curves to meet. Equilibrium concentrations need not be equal, so the curves need not cross.
- Check consistency. The leveled values should match the K expression and the particle picture.
§3
The pieces you'll meet.
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Three consistent views of one equilibrium.
§4
Worked example: read the graph.
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Graph. A reactant curve falls and a product curve rises; after a while, both go flat at different heights.
When is equilibrium reached? At the point where the curves stop changing (level off), not where they might cross.
Are they equal? Not necessarily — here they flatten at different values, meaning unequal equilibrium concentrations, which is perfectly normal.
Consistency. Plugging the flattened (equilibrium) concentrations into the K expression should reproduce K, and a particle drawing would show that same constant mix.
§5
Mistakes that cost real points.
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"Equilibrium is reached where the concentration curves cross."
Equilibrium is reached where the curves level off (become constant), not where they cross. Curves may or may not cross, and crossing does not indicate equilibrium — a crossing just means the two concentrations are momentarily equal, which can happen before equilibrium.
Fix. Identify equilibrium by the flattening of the curves, not by a crossing point.
"At equilibrium, reactant and product concentrations must be equal."
Equilibrium concentrations are constant, not necessarily equal. The curves level off at whatever values K dictates, which are usually different. Requiring them to be equal misreads the graph.
Fix. Read equilibrium as constant (leveled) concentrations, which are generally unequal.
"The three representations can each tell a different story."
The symbolic, graphical, and particulate representations must be consistent — they describe the same equilibrium. If the graph's leveled values do not match the K expression or the particle picture, there is an error, not three valid truths.
Fix. Cross-check the three views; they must agree about the same equilibrium mixture.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.