This lab uses color changes in a reversible chemical system as visible evidence that equilibrium is real, dynamic, and responsive to stress. You are not just watching a color change for fun. You are building the conceptual foundation for Topics 7.1, 7.2, and 7.3, and you are practicing the kind of claim-evidence-reasoning that shows up directly on the AP exam.

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Why This Lab Matters for the AP Exam

Equilibrium questions are some of the most common on the AP Chemistry exam, and they show up in multiple-choice and free-response. The exam will ask you to predict the direction a reaction shifts, write an equilibrium expression, compare Q to K, and interpret graphs of concentration versus time. This lab gives you a concrete, visual experience to anchor all of that abstract reasoning. When you see a solution turn from orange to yellow after adding a base, you are watching a shift in equilibrium happen in real time. That visual memory will help you reason through exam questions much more reliably than just memorizing rules.

CED Connections

This lab directly supports Unit 7: Equilibrium and connects to three specific topics.

Topic 7.1 - Introduction to Equilibrium (LO 7.1.A)

The lab demonstrates 7.1.A.1 through 7.1.A.4. You observe a reversible reaction reaching a state where no further visible change occurs, even though both the forward and reverse reactions are still happening. The color stabilizing is your observable evidence that the system has reached dynamic equilibrium, which is exactly what 7.1.A.2 and 7.1.A.3 describe.

Topic 7.2 - Direction of Reversible Reactions (LO 7.2.A)

When you add a stress and the color shifts, you are watching 7.2.A.1 in action. The forward reaction rate temporarily exceeds the reverse rate (or vice versa), causing a net change until the rates equalize again at a new equilibrium position.

Topic 7.3 - Reaction Quotient and Equilibrium Constant (LO 7.3.A)

Every time you add a stress, you are essentially pushing Q away from K. The system shifts to bring Q back toward K. Writing the equilibrium constant expression for the reaction in this lab and reasoning about Q versus K is a core skill this lab builds.

What You Need to Be Able to Do

These are the concrete skills this lab develops. Expect to use all of them on the exam.

Identify evidence of equilibrium from observable data (color, concentration, or partial pressure staying constant over time)
Explain the dynamic nature of equilibrium rather than describing it as a reaction that has stopped
Predict the direction of a shift when a stress is applied to a system at equilibrium
Write an equilibrium constant expression (Kc) for a given reversible reaction
Compare Q to K and use that comparison to predict whether the reaction will shift toward products or reactants
Interpret concentration-versus-time graphs and identify when equilibrium has been established
Construct a claim-evidence-reasoning response using color change as evidence for a shift in equilibrium position

Core Concepts

Reversible Reactions and Equilibrium

A reversible reaction is one where reactants can form products and products can re-form reactants. You write it with a double arrow (⇌). Not all reactions are reversible under normal conditions, but many important ones are, including acid-base reactions, dissolution reactions, and the reaction system you use in this lab.

Chemical equilibrium is the state a reversible reaction reaches when the rate of the forward reaction equals the rate of the reverse reaction. At that point, the concentrations of all species stop changing. This does not mean the reaction has stopped. Both directions are still happening, just at the same rate. That is why we call it dynamic equilibrium.

A closed system is essential here. Equilibrium can only be maintained if matter cannot escape. If the system were open, products could leave and the reverse reaction would never catch up.

The Reaction in This Lab

The specific reaction used in this lab involves the iron(III) thiocyanate system or a similar chromate/dichromate equilibrium, depending on your teacher's version. The chromate/dichromate system is a common choice because the color difference is dramatic and easy to observe.

The chromate/dichromate equilibrium looks like this:

$2 \text{CrO}_4^{2-}_{(aq)} + 2\text{H}^+_{(aq)} \rightleftharpoons \text{Cr}_2\text{O}_7^{2-}_{(aq)} + \text{H}_2\text{O}_{(l)}$

Chromate ion (CrO4 2-) is yellow
Dichromate ion (Cr2O7 2-) is orange

When you add acid (H+), the equilibrium shifts toward the right (toward products), and the solution turns more orange. When you add base (OH-), it removes H+ from the system, and the equilibrium shifts left (toward reactants), turning the solution more yellow. The color is your direct evidence of which species is more concentrated at any given moment.

Reaction Quotient and Equilibrium Constant

The equilibrium constant (K) is a number that tells you the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to the power of its stoichiometric coefficient. For the general reaction:

$a\text{A} + b\text{B} \rightleftharpoons c\text{C} + d\text{D}$

The equilibrium constant expression is:

$K_c = \frac{[\text{C}]^c[\text{D}]^d}{[\text{A}]^a[\text{B}]^b}$

Pure liquids and pure solids are left out of this expression because their concentrations do not change.

The reaction quotient (Q) uses the exact same expression but plugs in concentrations from any point in time, not just at equilibrium. Comparing Q to K tells you which way the reaction will shift:

If Q < K: the reaction shifts toward products (forward reaction is faster)
If Q > K: the reaction shifts toward reactants (reverse reaction is faster)
If Q = K: the system is already at equilibrium, no net shift

Le Chatelier's Principle

Le Chatelier's principle says that when a stress is applied to a system at equilibrium, the system will shift in the direction that partially relieves that stress. Stresses include changes in concentration, temperature, and (for gases) pressure or volume.

In this lab, you are primarily working with concentration as the stress. Adding a reactant pushes Q below K, so the system shifts forward. Removing a reactant (by adding something that reacts with it) pushes Q above K, so the system shifts in reverse.

Reaction Rate and How Equilibrium Is Established

When a reaction first starts, only reactants are present. The forward reaction rate is high and the reverse rate is zero. As products build up, the reverse rate increases. As reactants are consumed, the forward rate decreases. Eventually the two rates meet and become equal. That is the moment equilibrium is established.

The reaction rate depends on concentration. Higher concentration means more collisions, which means a faster rate. This is the connection between rate laws and equilibrium: the equilibrium constant K is actually equal to the ratio of the forward rate constant to the reverse rate constant (kf/kr = K), though you do not need to derive this on the AP exam.

How the Lab Works

The investigation logic is straightforward. You start with a system at equilibrium, observe its color, apply a stress, observe the new color, and then reason about what happened at the molecular level.

You are essentially asking: "If I change one thing, what does the system do to compensate?" The color change is your evidence. A shift toward the orange dichromate side means the forward reaction dominated temporarily. A shift toward the yellow chromate side means the reverse reaction dominated temporarily.

The guided-inquiry version of this lab often asks you to design your own tests. For example, you might be asked to predict what will happen when you add a specific reagent, then test your prediction. This is where your understanding of Q versus K becomes practical. Before you add anything, you should be able to say: "Adding this will increase [H+], which means Q will drop below K, so the system will shift right, and I expect the solution to become more orange."

After each stress, the system reaches a new equilibrium. The value of K does not change (as long as temperature stays constant), but the equilibrium position (the actual concentrations) does change.

Data and Analysis Moves

Recording Observations

Your primary data in this lab is qualitative: color. But you need to be precise about it. Do not just write "it changed." Write what color it was before, what color it became, and what that tells you about which species increased in concentration.

Connecting Color to Concentration

Color intensity is related to absorption. A more intensely colored solution contains a higher concentration of the colored species. If the solution gets more orange, the concentration of dichromate increased. If it gets more yellow, chromate concentration increased.

Concentration-vs-Time Graphs

You should be able to sketch and interpret a graph showing how concentrations change as a system approaches equilibrium. The key features are:

Reactant concentration starts high and decreases, leveling off at a constant value
Product concentration starts at zero (or low) and increases, leveling off at a constant value
Both lines become flat at the same time, which is when equilibrium is reached
The flat region shows that concentrations are constant, not that the reaction stopped

After a stress is applied, the graph shows another period of change before leveling off again at new constant values.

Rate-vs-Time Graphs

A rate-versus-time graph shows the forward and reverse rates starting at different values and converging. When they meet and stay equal, that is equilibrium. After a stress, the rates temporarily diverge again before converging at a new equilibrium.

Writing and Using the Equilibrium Expression

For the chromate/dichromate system, the equilibrium expression (leaving out water as a pure liquid) is:

$K_c = \frac{[\text{Cr}_2\text{O}_7^{2-}]}{[\text{CrO}_4^{2-}]^2[\text{H}^+]^2}$

If you add H+ (acid), the denominator increases, so Q drops below K. The system shifts right to increase the numerator and decrease the denominator until Q = K again.

Identifying Controls and Variables

Independent variable: the type of stress applied (which reagent you add)
Dependent variable: the observed color change
Control: the original equilibrium solution with no stress applied

Common Mistakes

Saying equilibrium means the reaction stopped. This is the most common misconception and it will cost you points. Equilibrium is dynamic. Both reactions are still happening. The rates are just equal, so there is no net change.

Confusing equilibrium position with the equilibrium constant. K stays the same at constant temperature. The equilibrium position (the actual concentrations) changes when you apply a stress. These are not the same thing.

Including water or solids in the equilibrium expression. Pure liquids and pure solids are left out of Kc and Kp expressions. In the chromate/dichromate system, water is a product but it does not appear in the expression.

Saying the system "tries to restore the original equilibrium." Le Chatelier's principle says the system shifts to reduce the stress. It reaches a new equilibrium position, not the original one. The concentrations at the new equilibrium will be different from the original.

Mixing up Q < K and Q > K directions. Remember: if Q < K, the numerator needs to get bigger (more products), so the reaction shifts forward. If Q > K, the denominator needs to get bigger (more reactants), so the reaction shifts in reverse.

Describing color change without connecting it to concentration. On the AP exam, a color change observation is only useful if you connect it to which species increased or decreased in concentration and why.

Quick Review Checklist

Dynamic equilibrium means forward and reverse reaction rates are equal, not that the reaction has stopped.
At equilibrium, concentrations of all species remain constant but are not necessarily equal to each other.
The equilibrium constant expression (Kc) puts products over reactants, each raised to their stoichiometric coefficient, and excludes pure solids and pure liquids.
Q compared to K tells you the direction of shift: Q < K shifts forward, Q > K shifts reverse, Q = K means no shift.
Le Chatelier's principle predicts that a system at equilibrium will shift to partially offset any applied stress.
Color change in this lab is evidence of a shift in equilibrium position, which you connect to changes in concentration of the colored species.
K only changes when temperature changes. Concentration and pressure changes shift the equilibrium position but do not change K.
Concentration-vs-time graphs flatten out when equilibrium is reached. Both reactant and product lines become horizontal at the same time.