How do you calculate a partial pressure?

Mole fraction × total pressure.

What is the Kp expression for N₂ + 3H₂ ⇌ 2NH₃?

Kp = p(NH₃)² ÷ (p(N₂) × p(H₂)³)

What are the units of Kp for N₂ + 3H₂ ⇌ 2NH₃?

kPa⁻² (or Pa⁻², atm⁻²).

Which equilibria does Kp apply to in this specification?

Homogeneous gas-phase equilibria.

Does a change in pressure change Kp?

No. Kp depends only on temperature.

Which way does the position shift when pressure increases?

Towards the side with fewer moles of gas.

How does Kp change with temperature for an exothermic reaction?

Kp decreases as temperature increases.

How does Kp change with temperature for an endothermic reaction?

Kp increases as temperature increases.

Does a catalyst change the value of Kp?

No. It does not change Kp or the position of equilibrium.

What does a catalyst do to an equilibrium?

It makes the equilibrium be reached faster by speeding up both directions equally.

What is the first step in a Kp calculation from initial amounts?

Work out the amount of each gas at equilibrium and the total moles.

Partial pressures and the equilibrium constant KpAQA A-Level Chemistry: Flashcards

Card 1 of 130 of 13 known

Question

What is mole fraction?

Tap or press Space to reveal

Tap card or press Space to flip

See all 13 cards

What is mole fraction?: The amount of a gas divided by the total amount of all gases in the mixture.
How do you calculate a partial pressure?: Mole fraction × total pressure.
What do the partial pressures in a mixture add up to?: The total pressure.
What is the Kp expression for N₂ + 3H₂ ⇌ 2NH₃?: Kp = p(NH₃)² ÷ (p(N₂) × p(H₂)³)
What are the units of Kp for N₂ + 3H₂ ⇌ 2NH₃?: kPa⁻² (or Pa⁻², atm⁻²).
Which equilibria does Kp apply to in this specification?: Homogeneous gas-phase equilibria.
Does a change in pressure change Kp?: No. Kp depends only on temperature.
Which way does the position shift when pressure increases?: Towards the side with fewer moles of gas.
How does Kp change with temperature for an exothermic reaction?: Kp decreases as temperature increases.
How does Kp change with temperature for an endothermic reaction?: Kp increases as temperature increases.
Does a catalyst change the value of Kp?: No. It does not change Kp or the position of equilibrium.
What does a catalyst do to an equilibrium?: It makes the equilibrium be reached faster by speeding up both directions equally.
What is the first step in a Kp calculation from initial amounts?: Work out the amount of each gas at equilibrium and the total moles.

Exam questions on Partial pressures and the equilibrium constant Kp

Dinitrogen tetroxide decomposes reversibly: N₂O₄(g) ⇌ 2NO₂(g). A sample of 1.00 mol of N₂O₄ is allowed to reach equilibrium in a sealed vessel at constant temperature. At equilibrium the mixture contains 0.60 mol of N₂O₄ and 0.80 mol of NO₂ and the total pressure is 200 kPa.
Calculate the partial pressure of each gas in the equilibrium mixture.2 marks
Ammonia is made in the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. An iron catalyst is used.
State and explain the effect of the iron catalyst on the value of Kp and on the position of equilibrium.2 marks
In a sealed vessel, 1.00 mol of phosphorus(V) chloride is heated at constant temperature until equilibrium is reached: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). The equilibrium mixture contains 0.40 mol of PCl₅, 0.60 mol of PCl₃ and 0.60 mol of Cl₂, and the total pressure is 150 kPa.
Calculate the value of Kp for this equilibrium, and state its units.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).