What is Reaction Rate?
Reaction rate measures how fast reactants turn into products — the change in concentration over time. It depends on concentration, temperature, surface area and catalysts, and understanding it is key to controlling industrial and biological reactions.
Reaction rate is the change in concentration of a reactant or product per unit time, often written as rate = k[A]^m[B]^n, where k is the rate constant and m, n are experimentally determined reaction orders.
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Step-by-step worked examples
For rate = k[A][B], k = 0.020 M⁻¹s⁻¹, [A] = 0.50 M, [B] = 0.30 M. Find the rate.
rate = k[A][B] rate = 0.020 × 0.50 × 0.30 rate = 0.003 M/s
A reactant's concentration drops from 0.80 M to 0.60 M in 50 s. Find the average rate.
average rate = −Δ[A]/Δt average rate = −(0.60 − 0.80) / 50 average rate = 0.20 / 50 = 0.004 M/s
A reaction's rate roughly doubles for every 10°C rise. At 20°C the rate is 0.010 M/s. Estimate the rate at 40°C.
Temperature rises by 20°C = two 10°C steps Each step doubles the rate: ×2 × 2 = ×4 rate at 40°C ≈ 0.010 × 4 = 0.040 M/s
Flashcards
Quick quiz
Q1.The typical units of reaction rate are:
Q2.Doubling [A] in a first-order reaction (rate = k[A]) will:
Q3.In rate = k[A]^2[B], the overall reaction order is:
Q4.Which factor speeds up a reaction by lowering activation energy?
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Common mistakes
Reaction order always equals the stoichiometric coefficient. — Correct: Order must be found experimentally — it doesn't always match the balanced equation's coefficients.
Catalysts are consumed in the reaction. — Correct: Catalysts are not consumed — they're regenerated by the end of the reaction.
Higher temperature always exactly doubles the rate. — Correct: Rate increases with temperature (Arrhenius equation), but the exact factor depends on activation energy — 'doubles every 10°C' is just a rough rule of thumb.
Reaction rate stays constant throughout a reaction. — Correct: Rate usually decreases over time as reactant concentration drops (except in zero-order reactions).
FAQ
What is reaction rate?
It's how fast a reactant is consumed or a product is formed, measured as concentration change per unit time.
What is the formula for reaction rate?
rate = k[A]^m[B]^n, where k is the rate constant and m, n are experimentally determined orders.
How do you calculate reaction rate from concentration data?
Divide the change in concentration by the time interval: rate = −Δ[reactant]/Δt or Δ[product]/Δt.
What are examples of factors affecting reaction rate?
Higher concentration, higher temperature, greater surface area, and catalysts all typically increase reaction rate.