Le Chatelier tells you which way an equilibrium moves. The equilibrium constant tells you exactly where it sits. Write the expression from the equation, fill in the equilibrium concentrations, and you have a value that only temperature can change.
For aA + bB ⇌ cC + dD, Kc = [C]c[D]d ÷ [A]a[B]b, using equilibrium concentrations in mol dm−3. Products go on top, and each concentration is raised to the power of its coefficient. The units come from cancelling the concentration units, and may be none at all. A large Kc means the position lies to the right. Kc changes only with temperature; concentration, pressure and catalysts leave it unchanged.
Build Kc expressions in the simulator, then vary the starting amounts. The equilibrium concentrations change every time, but the value of Kc stays the same at a fixed temperature.
| Equilibrium | Kc | Units |
|---|---|---|
| H2 + I2 ⇌ 2HI | [HI]2 ÷ [H2][I2] | none |
| N2O4 ⇌ 2NO2 | [NO2]2 ÷ [N2O4] | mol dm−3 |
| N2 + 3H2 ⇌ 2NH3 | [NH3]2 ÷ [N2][H2]3 | mol−2 dm6 |
| 2SO2 + O2 ⇌ 2SO3 | [SO3]2 ÷ [SO2]2[O2] | mol−1 dm3 |
If you are given equilibrium concentrations, substitute directly. If you are given starting amounts, build a table: initial, change (using the equation's ratio), equilibrium, then divide by the volume.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the expression for Kc for N2 + 3H2 ⇌ 2NH3?
What are the units of Kc for H2 + I2 ⇌ 2HI?
Which change alters the value of Kc?
A reaction has Kc = 1 × 1010. At equilibrium the mixture is:
Only temperature. Whether Kc rises or falls depends on the sign of ΔH.
Use equilibrium concentrations, not initial amounts, and convert moles to concentrations by dividing by the volume unless the volumes cancel. Products always go on top. Work out the units separately each time by cancelling, rather than guessing.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
At equilibrium [H2] = [I2] = 0.0200 mol dm−3 and [HI] = 0.140 mol dm−3. What is Kc for H2 + I2 ⇌ 2HI?
0.500 mol of PCl5 is heated in a 2.00 dm3 vessel; at equilibrium 40.0% has dissociated into PCl3 and Cl2. What is Kc?
For an exothermic forward reaction, what happens to Kc when the temperature is raised?
What are the units of Kc for 2SO2(g) + O2(g) ⇌ 2SO3(g)?
This skill is now recorded on your profile. Keep the chain going.