Rate law and reaction order
A rate law links rate to concentrations: rate = k[A]ᵐ[B]ⁿ. The exponents m and n are the orders, found by experiment, not from the balanced equation. k is the rate constant. The overall order is m + n.
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Analogy
A recipe for speed: how sharply the pace changes when you add more of each ingredient.
Example
rate = k[A]¹[B]²Order in A: 1Order in B: 2Overall order: 3
In practice
Knowing the rate law lets engineers predict how fast a drug breaks down in the body or in a bottle.
Practice
Flashcards
- Can reaction orders be read from the balanced equation?No. They must be found by experiment (unless the reaction is one elementary step).
Interview questions
- The initial rate of a reaction was 1.0, 2.0 and 4.0 (×10⁻³ M/s) when [A] was 0.10, 0.20 and 0.40 M. Find the order in A and write the rate law.
- Rate doubles when [A] doubles
- The order in A is 1
- rate = k[A]
- k = rate/[A] = 0.010 s⁻¹
Questions people ask
Can I find the order from the balanced equation?
No. Orders must be found by experiment unless the reaction is a single elementary step. The reaction 2NO₂ + F₂ → 2NO₂F is first order in each reactant, not second order in NO₂ as the coefficient 2 might suggest. The balanced equation shows overall change, not how the reaction proceeds.
What are the units of k?
The units of k depend on the overall order. For zero order, k is in M/s. For first order, it is s⁻¹. For second order, it is M⁻¹s⁻¹. You can work them out by making the units of rate equal M/s on both sides.