Equilibrium · expressions and response
Understand Kc
Build equilibrium expressions, compare Qc with Kc, and watch a Haber-process model respond to concentration, pressure, temperature and catalysts.
Dynamic equilibrium model
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Exothermic forward reaction, ΔH ≈ −92 kJ mol⁻¹. Kc values are illustrative but the direction and equilibrium calculations are internally consistent.
Equilibrium concentrations
Forward→←Reverse
System data
- Volume
- 10.0 dm³
- Temperature
- 400 K
- Kc at this temperature
- 0.500
- Current Qc
- 0.500
The distinction that matters
Kc is fixed for a particular reaction at a particular temperature. Qc has the same algebraic form but uses the concentrations at the present instant. A disturbance usually changes Qc first; the system then shifts until Qc again equals Kc. Only a temperature change alters Kc.