Lessons 24 · 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
24
Reaction rate problems and their solutions
Textbook: pp. 109–111
GoalSolve problems on reaction rate, duration and vessel volume by converting units correctly and leaving solids out of the rate expression.
New words
average rate · o‘rtacha tezlikunit conversion · birliklarni o‘tkazishchange in amount of substance · modda miqdori o‘zgarishirate expression · tezlik ifodasi
Explanation
In rate problems first identify the substance for which the initial and final amount (or concentration) is given. If a mass is given, convert to moles with n = m/M and then find Δn = n₁ − n₂. Match time and volume to the unit of rate: minutes for mol/(L·min), seconds for mol/(L·s); 1 m³ = 1000 L. From v = Δn/(V·Δt) we get the duration Δt = Δn/(V·v) and the volume V = Δn/(v·Δt). If the concentration is given, the vessel volume is not needed: v = ΔC/Δt. The rates for different substances in one reaction are proportional to the coefficients: in N₂ + 3H₂ → 2NH₃, v(H₂) = 3·v(N₂) and v(NH₃) = 2·v(N₂). In problems on the effect of pressure only gases and dissolved substances are written in the rate expression, pure solids are omitted; if the reaction is taken as one-step, the exponents equal the coefficients. Check the answer for units and sensible size.
Worked examples
In an 8 L vessel the amount of O₂ falls from 30 mol to 18 mol in 30 s. Δn = 12 mol, t = 0.5 min: v = 12/(8·0.5) = 3 mol/(L·min).
In a 4 L vessel 0.8 mol of NH₃ forms in 2 min by N₂ + 3H₂ → 2NH₃: v(NH₃) = 0.8/(4·2) = 0.1 mol/(L·min); by the coefficients v(N₂) = 0.05 and v(H₂) = 0.15 mol/(L·min).
Class activity
Pair work: pick one problem and, keeping its data, solve it three ways — find the rate, the time and the volume; explain how the formulas follow from one another.
Practice
1
In a 5 L vessel the amount of CO falls from 40 mol to 16 mol in 24 s. What is the rate in mol/(L·min)?
12
2
In a 2 L vessel 1.2 mol of SO₃ forms in 3 min by 2SO₂ + O₂ → 2SO₃. Find the rate of formation of SO₃ and the rate of consumption of O₂ (mol/(L·min)).