What do the square brackets in Kc mean?

Equilibrium concentration in mol dm⁻³.

Do coefficients multiply or raise to a power in Kc?

They become powers of the concentrations.

What is a heterogeneous equilibrium?

One in which the species are in more than one phase.

What is the mole fraction of a gas?

Moles of that gas ÷ total moles of gas.

How do you find a partial pressure?

Mole fraction × total pressure.

When does Kc have no units?

When there is the same number of moles of gaseous reactants and products.

What is the first step when calculating Kc from experimental data?

Work out equilibrium amounts using the initial amounts and the equation's mole ratio.

Equilibrium constants Kc and KpEdexcel International A Level Chemistry: Flashcards

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Question

Write the Kc expression for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

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Write the Kc expression for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).: Kc = [NH₃]² / ([N₂][H₂]³)
What do the square brackets in Kc mean?: Equilibrium concentration in mol dm⁻³.
Do coefficients multiply or raise to a power in Kc?: They become powers of the concentrations.
What is a heterogeneous equilibrium?: One in which the species are in more than one phase.
Which species are left out of Kc and Kp expressions?: Pure solids and pure liquids.
Write Kp for CaCO₃(s) ⇌ CaO(s) + CO₂(g).: Kp = p(CO₂)
What is the mole fraction of a gas?: Moles of that gas ÷ total moles of gas.
How do you find a partial pressure?: Mole fraction × total pressure.
What pressure units are used for Kp in this course?: atm
What are the units of Kc for 2SO₂ + O₂ ⇌ 2SO₃?: mol⁻¹ dm³
What are the units of Kp for N₂ + 3H₂ ⇌ 2NH₃?: atm⁻²
When does Kc have no units?: When there is the same number of moles of gaseous reactants and products.
What is the first step when calculating Kc from experimental data?: Work out equilibrium amounts using the initial amounts and the equation's mole ratio.
What do the partial pressures of all gases in an equilibrium add up to?: The total pressure.

Exam questions on Equilibrium constants Kc and Kp

In the Haber process nitrogen and hydrogen react reversibly: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). A sealed vessel is held at 700 K until equilibrium is reached. The equilibrium concentrations are: N₂ 0.40 mol dm⁻³, H₂ 1.20 mol dm⁻³ and NH₃ 0.60 mol dm⁻³.
Calculate the value of Kc at 700 K, including units.2 marks
Carbon dioxide reacts with hot carbon: C(s) + CO₂(g) ⇌ 2CO(g). Excess solid carbon is held in a closed vessel at 1000 K and the total pressure at equilibrium is 2.0 atm. The gas mixture at equilibrium contains 0.30 mol of CO₂ and 0.90 mol of CO.
Calculate Kp at 1000 K, including units.2 marks
Dinitrogen tetraoxide dissociates in a sealed flask: N₂O₄(g) ⇌ 2NO₂(g). A student places 0.250 mol of N₂O₄ in a 5.00 dm³ flask at 373 K. At equilibrium 0.050 mol of N₂O₄ remains and the total pressure in the flask is 2.75 atm.
Calculate Kc at 373 K, including units.3 marks

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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).