📐 Calculating Kₑq from Concentrations

Now that you can write the expression, let us calculate Kₑq from equilibrium concentration data.

📝 Worked Example (5 marks style)

At equilibrium, a 1.0 L flask contains 0.040 mol N₂O₄ and 0.12 mol NO₂ at 373 K. Calculate Kₑq for the reaction N₂O₄(g) ⇌ 2NO₂(g).

Step 1: Write the Kₑq expression.

Kₑq = [NO₂]² / [N₂O₄]

Step 2: Calculate molar concentrations.

Since volume = 1.0 L: [N₂O₄] = 0.040 / 1.0 = 0.040 mol/L [NO₂] = 0.12 / 1.0 = 0.12 mol/L

Step 3: Substitute into the expression.

Kₑq = (0.12)² / (0.040)

Step 4: Calculate.

Kₑq = 0.0144 / 0.040 = 0.36

Step 5: Interpret.

Kₑq = 0.36 (less than 1). At 373 K, this equilibrium favours the reactants (N₂O₄). There is more N₂O₄ than NO₂ at equilibrium.

💡 Exam Tips for Kₑq Calculations

1. Always write the expression FIRST (even if not asked). Show your working.

2. Check units: concentrations must be in mol/L. If given moles and volume, divide.

3. Kₑq has no units in the HSC (it is dimensionless by convention for this course).

4. Remember to apply the POWER. [NO₂]² means (0.12)² = 0.0144, NOT 0.12 × 2 = 0.24.

5. Interpret your answer: Kₑq > 1 = products favoured. Kₑq < 1 = reactants favoured.

✏️ Your Turn: Calculate Kₑq

📝 Problem

At equilibrium in a 2.0 L vessel at 450 K: 0.10 mol N₂, 0.30 mol H₂, 0.20 mol NH₃.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Calculate Kₑq. Show all working and interpret your answer.