For reactions between gases it is easier to measure pressures than concentrations. Kp is the same idea as Kc, built from the partial pressure of each gas, and it follows exactly the same rules.
The partial pressure of a gas is the pressure it would exert alone in the container: p = mole fraction × total pressure, where mole fraction = moles of that gas ÷ total moles of gas. The partial pressures add up to the total pressure. Kp is written like Kc but with partial pressures: for N2O4 ⇌ 2NO2, Kp = p(NO2)2 ÷ p(N2O4). Units come from the pressure units (Pa, kPa or atm). Like Kc, Kp changes only with temperature.
In the simulator, change the total pressure of an equilibrium mixture and watch each gas's share of the pressure. Then write Kp from the partial pressures and check it against the displayed value.
| Equilibrium | Kp | Units (kPa) |
|---|---|---|
| N2O4 ⇌ 2NO2 | p(NO2)2 ÷ p(N2O4) | kPa |
| N2 + 3H2 ⇌ 2NH3 | p(NH3)2 ÷ p(N2)p(H2)3 | kPa−2 |
| 2SO2 + O2 ⇌ 2SO3 | p(SO3)2 ÷ p(SO2)2p(O2) | kPa−1 |
Follow the chain in Fig. 1: equilibrium moles, then mole fractions, then partial pressures, then substitute.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
The partial pressure of a gas in a mixture equals:
A mixture contains 1.0 mol N2 and 3.0 mol H2 at a total pressure of 200 kPa. What is the partial pressure of N2?
What is the Kp expression for N2O4(g) ⇌ 2NO2(g)?
Which change alters the value of Kp?
Exactly the same rule as for Kc: temperature only.
Partial pressure uses the mole fraction of the equilibrium mixture, so first find the equilibrium moles and the new total. A common slip is dividing by the initial number of moles. Also remember that increasing the total pressure changes the position of equilibrium but not the value of Kp.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
At equilibrium, 0.80 mol N2O4 and 0.40 mol NO2 are present at a total pressure of 100 kPa. What is Kp for N2O4 ⇌ 2NO2?
What are the units of Kp for N2(g) + 3H2(g) ⇌ 2NH3(g), with pressures in kPa?
For 2SO2(g) + O2(g) ⇌ 2SO3(g), the total pressure is increased at constant temperature. Which statement is correct?
At equilibrium p(SO2) = 20 kPa, p(O2) = 10 kPa and p(SO3) = 70 kPa. What is Kp for 2SO2 + O2 ⇌ 2SO3?
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