AP Chemistry Equilibrium Explained: K, Q, ICE Tables, and More

Chemical equilibrium is the state in which the forward and reverse reactions happen at the same rate, so the concentrations of reactants and products stop changing, though they are generally not equal. In AP Chemistry, nearly every equilibrium question comes back to one comparison: the reaction quotient Q against the equilibrium constant K. If you can write K, compare Q to K, and run an ICE table, you can handle most equilibrium questions on the exam, including the ones that show up in acid-base and solubility problems.

What is chemical equilibrium?

Picture a reversible reaction in a closed container. At first, the forward reaction is fast because reactant concentrations are high. As products build up, the reverse reaction speeds up. Eventually the two rates are equal, and the concentrations stay constant from that point on.

Two misconceptions cost students points:

• Equilibrium does not mean equal concentrations. It means constant concentrations, because the rates are equal.
• The reactions do not stop. Equilibrium is dynamic: both reactions continue at the same rate.

Equilibrium is Unit 7 of AP Chemistry, weighted at 7–9% of the exam. But its ideas run through Unit 8 (Acids and Bases, 11–15%) and Unit 9 (Thermodynamics and Electrochemistry), so it effectively accounts for much more of your score.

How do you write an equilibrium constant expression?

For a general reaction aA + bB ⇌ cC + dD, the equilibrium constant is:

K = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ

Products go on top, reactants on the bottom, and each concentration is raised to its coefficient. Leave out pure solids and pure liquids, because their concentrations do not change. For gases, you can write Kp using partial pressures instead of molar concentrations.

Example: For CaCO₃(s) ⇌ CaO(s) + CO₂(g), the expression is simply Kc = [CO₂], since both solids are excluded.

What does the size of K tell you?

K tells you where the reaction ends up, not how fast it gets there. Speed is a kinetics question.

How do you manipulate K?

These rules let you find K for a target reaction from reactions with known K values, in the same way Hess's law works for enthalpy.

Q vs K: which way will the reaction shift?

The reaction quotient Q uses the same expression as K, but with the current concentrations instead of equilibrium concentrations.

Q vs K reasoning is also the most rigorous way to justify a shift on a free-response question, as you will see in the Le Chatelier section below.

How to solve equilibrium problems with ICE tables

An ICE table tracks the Initial concentrations, the Change, and the Equilibrium concentrations.

Worked example 1: finding K from equilibrium data

A flask initially contains 1.00 M H₂ and 1.00 M I₂. They react according to H₂(g) + I₂(g) ⇌ 2HI(g). At equilibrium, [HI] = 1.56 M. Find Kc.

Since 2x = 1.56, x = 0.78. So [H₂] = [I₂] = 0.22 M.

Kc = (1.56)² / (0.22)(0.22) = 2.43 / 0.0484 ≈ 50

The key step is using the stoichiometric coefficients in the Change row: HI gains 2x because 2 moles of HI form for every mole of H₂ used.

Worked example 2: the small-x approximation

Find the pH of a 0.10 M solution of a weak acid HA with Ka = 1.8 × 10⁻⁵.

Ka = x² / (0.10 − x) = 1.8 × 10⁻⁵

Because Ka is small, x will be small compared with 0.10, so approximate 0.10 − x ≈ 0.10:

x² = 1.8 × 10⁻⁶, so x = 1.34 × 10⁻³ M = [H⁺]

pH = −log(1.34 × 10⁻³) ≈ 2.87

Check the approximation: 1.34 × 10⁻³ is about 1.3% of 0.10, well under the commonly used 5% guideline, so the approximation holds.

Le Chatelier's principle

When a system at equilibrium is disturbed, it shifts in the direction that partially counteracts the change.

Temperature is the only change in this table that changes the value of K. A useful trick: treat heat as a product of an exothermic reaction and a reactant of an endothermic one.

How to justify a shift for full credit

"The equilibrium shifts right" is a prediction, not a justification. Explain it with Q and K:

Adding H₂ increases the denominator of Q, so Q becomes less than K. To restore Q = K, the reaction proceeds in the forward direction, forming more HI.

That answer names the change, its effect on Q, the comparison with K, and the resulting direction.

Solubility equilibria and Ksp

Slightly soluble ionic compounds establish an equilibrium with their ions. For AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq), Ksp = [Ag⁺][Cl⁻].

To find molar solubility s:

Common-ion effect: dissolving a salt in a solution that already contains one of its ions lowers its solubility. Adding Cl⁻ to an AgCl equilibrium raises Q above Ksp, so the reaction shifts toward the solid.

pH effects: salts with a basic anion, such as hydroxides or fluorides, become more soluble in acidic solution, because H⁺ removes the anion and pulls the equilibrium toward dissolving.

How equilibrium connects to thermodynamics

The standard free energy change and K are linked by ΔG° = −RT ln K, which appears on the equations sheet.

• If ΔG° < 0, then K > 1, and products are favored.
• If ΔG° > 0, then K < 1, and reactants are favored.

Expect questions that ask you to connect these signs without calculating anything.

Common equilibrium mistakes on the AP exam

• Including solids or pure liquids in K or Q.
• Forgetting to raise concentrations to their coefficients, or forgetting the 2x in the Change row.
• Saying a catalyst shifts equilibrium.
• Claiming that adding reactant changes K.
• Using initial concentrations in K instead of equilibrium concentrations.
• Justifying a shift with "Le Chatelier's principle" alone, without explaining Q vs K.

For the broader exam strategy, read how to get a 5 on AP Chemistry, and estimate your practice scores with the AP Chemistry score calculator.

How Ivystar can help

Equilibrium is where AP Chemistry starts to feel abstract for many students, and a tutor who can connect K, Q, and Le Chatelier's principle to clear pictures makes a real difference. Ivystar's AP Exam Prep pairs you with a tutor from schools like Harvard, Penn, Cornell, Stanford, or MIT, with a "Get an A or we work for free" guarantee on your class grade. Book a free 20-minute strategy call to get started.