What is Reaction Rate and Kinetics?
Reaction rate is the speed at which reactants are consumed or products are formed, measured in concentration change per unit time. Chemical kinetics is the study of reaction rates and the mechanisms by which reactions occur.
Reaction rate is Δ[concentration]/Δt, typically in mol/(L·s). Kinetics examines how fast reactions proceed and what factors control their speed.
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Step-by-step worked examples
A reaction consumes 0.8 mol/L of reactant in 40 seconds. What is the reaction rate?
r = Δ[concentration]/Δt r = 0.8 / 40 = 0.02 mol/(L·s)
In the first 10 s, [A] drops from 1.5 to 0.9 mol/L. Rate?
r = Δ[A]/Δt r = (1.5 - 0.9) / 10 = 0.6 / 10 = 0.06 mol/(L·s)
A decomposition reaction produces 0.3 mol/L of product in 15 minutes. Average rate?
Convert: 15 min = 900 s r = 0.3 / 900 = 0.000333 mol/(L·s) ≈ 3.3 × 10^-4 mol/(L·s)
Flashcards
Quick quiz
Q1.0.5 mol/L of reactant consumed in 25 s. Rate?
Q2.Which is NOT a factor affecting reaction rate?
Q3.A fast reaction has a high rate because…
Q4.Instantaneous rate differs from average rate because…
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Common mistakes
Reaction rate is always the same. — Correct: Rate changes as concentration decreases; it's highest at the start.
Fast reaction = high rate always. — Correct: A 'fast' reaction may have high initial rate that drops quickly.
Rate only depends on temperature. — Correct: Temperature, concentration, catalyst, and surface area all matter.
Producing 1 mole of product = rate 1. — Correct: Rate depends on concentration change per unit time, not absolute amount.
FAQ
What is the difference between reaction rate and rate constant?
Reaction rate (r) changes with concentration; rate constant (k) is fixed at a given temperature.
Why does heating increase reaction rate?
Molecules move faster at higher T, colliding more frequently and with higher energy.
Can a catalyst change the final equilibrium?
No — it only speeds up the approach to equilibrium, not the position.
How is reaction rate measured experimentally?
By measuring concentration change over time using techniques like spectrophotometry or gas chromatography.