What a learner can do afterwards
- Calculates mole fractions and partial pressures from equilibrium amounts and a total pressure
- Writes Kp for a gas reaction and works out its units
- Explains why raising the total pressure shifts the position while leaving Kp untouched
- Uses Kp to argue the conditions picked for an industrial process such as ammonia synthesis
1 · Read
Kc put a number on how far a reaction goes; for gases we write that number with pressures instead of concentrations, and it tells industry how to run its plants. In a gas mixture each gas pushes as if the others were not there. Its mole fraction is its moles over the total moles, and its partial pressure is that fraction times the total pressure.
For nitrogen plus hydrogen making ammonia, Kp is the product pressures over the reactant ones, each raised to its balancing number: p(NH3) squared over (p(N2) times p(H2) cubed), with both reactant terms in the denominator. To find the units, put pressure units into that expression and cancel them down.
Raise the total pressure and the system shifts toward the side with fewer gas moles to ease the strain. Kp itself does not move, because Kp answers only to temperature.
That is why ammonia plants run at high pressure: it pushes the yield up without touching Kp. The temperature is a compromise, hot enough for a decent rate but cool enough to protect the yield.
Share out the pressure by mole fraction, write Kp with powers, and shift position with pressure while Kp stays fixed.
The rewritten readings were written for this dataset.
More to read
Writing Kp from gas pressures
Like Kc, Kp divides product terms by reactant terms, with equation coefficients as exponents. For gases, use equilibrium partial pressures in consistent units.
School expressions may have apparent units, found by canceling pressure units for that equation. Thermodynamic Kp is dimensionless: its terms are pressures divided by a reference pressure.
8.11: Partial Pressure and Dalton's Law
In a gas mixture each gas pushes as if the others were absent. That push is its partial pressure. Dalton's law says the total pressure is the sum of all the partial pressures.
To get one gas's share, multiply the total pressure by its mole fraction. The mole fractions of every gas always add to one, which is a handy check on your arithmetic.
Compression and equilibrium
At constant temperature, compressing an equilibrium gas mixture raises pressure and favors the side with fewer gas moles. Compression causes no shift when the balanced equation has equal numbers of gas moles on both sides.
Only temperature changes Kp. Pressure or concentration changes can alter equilibrium without altering Kp. A catalyst speeds the approach to equilibrium but changes neither its position nor Kp.
Finding a gas's share of pressure
Calculate each gas's amount in moles and sum these amounts. Divide a gas's moles by that sum to obtain its mole fraction.
For an ideal gas mixture, partial pressure equals mole fraction multiplied by total pressure. Check that all fractions sum to 1 and all partial pressures sum to the measured pressure.
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2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.