☰ Contents · Chemistry (General chemistry)

Chemical equilibrium problems

Lessons 27 · 1 lessons · S. Masharipov, A. Mutalibov, E. Murodov, H. Islomova. General chemistry, Grade 11, 1st edition. G‘afur G‘ulom Publishing and Printing House, Tashkent, 2018
27

Chemical equilibrium problems and their solutions

Textbook: pp. 121–126
GoalSolve equilibrium problems with an “initial – change – equilibrium” table: find starting amounts, equilibrium concentrations and K.
New words
initial concentration · boshlang‘ich konsentratsiyaamount consumed · sarflangan miqdorequilibrium concentration · muvozanat konsentratsiyasireaction table · reaksiya jadvali
Explanation

In equilibrium problems first balance the equation and set up a three-row table: initial, change (consumed or formed) and equilibrium concentrations. The changes are in the ratio of the coefficients: if 2 mol/L of N₂ is consumed, 6 mol/L of 3H₂ is consumed and 4 mol/L of 2NH₃ is formed. Initial concentration = equilibrium + consumed (for a reactant); if no product is present at the start, its initial concentration is 0 and its equilibrium value equals the amount formed. The amount of substance is n = C·V (V is the vessel volume in L). If K is given, call the unknown change x and write the equilibrium concentrations (initial − coefficient·x) into the K expression to get an equation. Check the result by putting the concentrations back into the K expression.

Worked examples
In a 5 L vessel N₂ + 3H₂ ⇌ 2NH₃: at equilibrium [N₂] = 3, [H₂] = 2, [NH₃] = 4 mol/L. 4 mol/L of NH₃ formed, so 2 of N₂ and 6 of H₂ were consumed. Initial: N₂ 3 + 2 = 5, H₂ 2 + 6 = 8 mol/L; amounts 25 and 40 mol.
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O, K = 4, 3 mol/L of each at the start, no product. x/(3 − x) = 2 (take the square root of K), x = 2 mol/L: ester 2 mol/L, acid and alcohol 1 mol/L each; check: 2·2/(1·1) = 4.
Class activity

Pair work: write each other a problem solvable with the three-row table (choose the reaction and the equilibrium concentrations yourselves), swap, solve, and check the answer through the K expression.

Practice
1
In a 4 L vessel 2SO₂ + O₂ ⇌ 2SO₃: at equilibrium [SO₂] = 2, [O₂] = 1, [SO₃] = 6 mol/L. What was the initial amount of O₂ in mol?
2
A + B ⇌ C + D, K = 9, initially [A] = [B] = 4 mol/L, no products. What is [C] at equilibrium in mol/L?
3
PCl₅ ⇌ PCl₃ + Cl₂, K = 5, [PCl₃] = [Cl₂] = 10 mol/L. Find [PCl₅].
4
Why are the consumed concentrations in the ratio of the coefficients?