What is Reaction Kinetics?
Reaction kinetics studies how fast chemical reactions occur and what factors speed them up or slow them down. The rate of reaction is the change in concentration of a reactant or product over time, measured in mol/L·s.
Reaction kinetics examines the rate at which reactants transform into products. Rate depends on concentration, temperature, catalyst presence, pressure and surface area.
Step-by-step worked examples
In 30 s, 0.6 mol/L of reactant is consumed. What is the average rate?
rate = Δc/Δt rate = 0.6 mol/L ÷ 30 s rate = 0.02 mol/L·s
A catalyst speeds a reaction by 50%. If uncatalysed rate is 0.01 mol/L·s, new rate?
Increase = 50% of 0.01 Increase = 0.005 mol/L·s New rate = 0.01 + 0.005 = 0.015 mol/L·s
Temperature rises 10 °C, doubling the reaction rate. Old rate 0.04 mol/L·s, new rate?
New rate = old rate × 2 New rate = 0.04 × 2 = 0.08 mol/L·s
Flashcards
Quick quiz
Q1.0.8 mol/L reactant consumed in 40 s. Average rate?
Q2.Which factor does NOT directly speed a reaction?
Q3.A catalyst works by…
Q4.Reaction rate typically has units…
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Common mistakes
Confusing reaction rate with equilibrium constant. — Correct: Rate (kinetics) is how fast; equilibrium constant (thermodynamics) is the final ratio.
Thinking a catalyst provides energy to the reaction. — Correct: Catalyst lowers the activation barrier; energy is already available in bonds.
Ignoring surface area for solid reactants. — Correct: Increasing surface area (crushing a solid) increases collisions → faster rate.
Assuming all reactions have the same temperature dependence. — Correct: Each reaction's rate sensitivity to temperature is unique (Arrhenius equation).
FAQ
What is the difference between reaction rate and rate constant?
Reaction rate = current speed of reaction (mol/L·s); rate constant k = intrinsic property (changes with temperature), appears in rate law.
How does increasing pressure affect gas-phase reactions?
Higher pressure increases concentration of gases, raising collision frequency → faster rate.
Why do chemists measure reaction rates?
To understand reaction mechanisms, optimize industrial processes, control speed (safety, yield), and predict product formation.
Can a reaction rate ever be negative?
Rate of reactant consumption is positive; mathematically we write −Δ[reactant]/Δt to denote disappearance.