Study guides › C/P › General Chemistry

Chemical Equilibrium

This chapter covers dynamic equilibrium, writing and manipulating K, Kc versus Kp, Q versus K, ICE-table calculations, Le Chatelier's principle, and the link between K and free energy. You will predict shifts and solve for equilibrium concentrations.

Study this chapter in PrepMed (free)

What this chapter covers

• Dynamic Equilibrium & the Equilibrium Constant. Writing K correctly, and knowing what it does and does not tell you, is the base for every equilibrium question.
• Kp, Kc & the Reaction Quotient. Comparing Q with K tells you the direction of a reaction, and the Kp to Kc conversion turns on one number, Δn.
• ICE Tables & Equilibrium Calculations. ICE tables convert starting amounts and K into equilibrium concentrations, and the MCAT keeps the algebra clean.
• Le Chatelier's Principle. Predicting which way a system shifts, and whether K changes, is a reliable MCAT question type.
• Equilibrium & Free Energy. ΔG°, K, and Q connect thermodynamics to equilibrium, and the MCAT expects the qualitative link and the log-10 shortcut.

Key points to know

• K = [products]/[reactants], each to its coefficient
• Solids and pure liquids are omitted from K
• Kp = Kc(RT)^Δn; Δn counts gas moles only
• Kp = Kc when Δn = 0
• ICE: initial, change (−x, +x times coefficients), equilibrium
• Look for perfect squares; use the small-x approximation for small K
• The system shifts to oppose the disturbance
• Only temperature changes K

Try a practice question

What is the equilibrium expression for 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g)?

1. [SO₃] / ([SO₂][O₂])
2. [SO₂]²[O₂] / [SO₃]²
3. [SO₃]² / ([SO₂]²[O₂])
4. 2[SO₃] / (2[SO₂] + [O₂])

Common questions

24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.