AP Chemistry · Exam guide
AP Chemistry Equilibrium Question Explained: ICE Tables, Q Versus K and Le Chatelier
Equilibrium questions mix calculation with explanation, which is the pattern of the whole AP Chemistry free-response section. You write an expression for K, find an equilibrium concentration, and then explain what happens when the system is disturbed. The example below uses made-up numbers.
The ideas that carry the question
| Idea | What to know |
|---|---|
| Writing K | Products over reactants, each raised to its coefficient. Leave out solids and pure liquids. K has no units on the exam. |
| Q versus K | Q has the same form as K but uses the concentrations right now. If Q is less than K, the reaction shifts toward products. If Q is greater than K, it shifts toward reactants. |
| ICE table | Initial, change, equilibrium. The change row uses the coefficients as multipliers of x. |
| Le Chatelier | A disturbance changes Q, and the system shifts to bring Q back to K. Only a change in temperature changes K. |
A worked example (made-up numbers)
Hydrogen and iodine react: H2(g) + I2(g) in equilibrium with 2 HI(g), and Kc is 50 at the temperature of the experiment. A 1.00 L container starts with 0.100 mol of H2 and 0.100 mol of I2 and no HI. Find the equilibrium concentration of HI.
| [H2] | [I2] | [HI] | |
|---|---|---|---|
| Initial | 0.100 | 0.100 | 0 |
| Change | -x | -x | +2x |
| Equilibrium | 0.100 - x | 0.100 - x | 2x |
- Write K: Kc = [HI]^2 / ([H2][I2]) = (2x)^2 / (0.100 - x)^2 = 50.
- Both sides are perfect squares, so take the square root: 2x / (0.100 - x) = 7.07.
- Solve: 2x = 0.707 - 7.07x, so x = 0.707 / 9.07 = 0.0780 M.
- Equilibrium [HI] = 2x = 0.156 M, and [H2] = [I2] = 0.100 - 0.0780 = 0.022 M.
Check by substituting back: (0.156)^2 / (0.022)^2 is about 50, which matches Kc.
Explaining a shift
A follow-up asks what happens if more H2 is added to a mixture with [H2] = 0.20 M, [I2] = 0.20 M and [HI] = 0.40 M.
It names a principle but gives no reasoning about Q and K.
It compares Q with K and says what that means for the direction of change.
Common mistakes
- Putting a solid or a pure liquid in the K expression.
- Forgetting to raise a concentration to its coefficient.
- Using the initial row as the equilibrium row in the K expression.
- Saying that adding a catalyst or changing pressure changes K. Only temperature does.
- Giving a direction of shift with no mention of Q.
Frequently asked questions
Does K have units on the AP exam?
No. Write K without units unless the question asks otherwise.
When can I skip the quadratic?
When x is very small compared with the starting concentration, so that subtracting it changes almost nothing. Check that the approximation works before you rely on it.
What changes the value of K?
Only temperature. Changes in concentration, pressure or volume change Q, not K.
How do I justify a shift?
Compare Q with K after the disturbance, and say which way the reaction must go to make Q equal K.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every lesson in the AP Chemistry course ends with practice items and an Exam focus box, and every unit has a ten-question practice set with worked answers. These free lessons also build the skills the free-response section tests:
See the AP Chemistry course Free lessons and lesson list
Other free-response guides:
- AP Chemistry FRQ Guide: Long and Short Questions, Units and Particle Explanations
- AP Chemistry Acid-Base and Buffer Question Explained: pH, Ka and the Henderson-Hasselbalch Equation
- AP Chemistry Particle-Level Explanations: How to Explain Trends, Boiling Points and Bonding
- AP Chemistry Lab and Data Questions Explained: Graphs, Beer's Law and Sources of Error
AP is a registered trademark of the College Board, which is not affiliated with and does not endorse this site. This guide is our own explanation of the question types and scoring. For the official scoring guidelines, see the College Board.