Method of initial rates
To find orders, run several experiments with different starting concentrations and measure the initial rate. Change one concentration at a time. Compare how the rate changes with how the concentration changed.
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Analogy
Changing one dial at a time while watching the speedometer shows which dial matters.
Example
Run 1: [A] = 0.10 M, rate = 2.0×10⁻³ M/sRun 2: [A] = 0.20 M, rate = 8.0×10⁻³ M/sRate × 4 when [A] × 2So order in A is 2.
In practice
Researchers use this to figure out how a pollutant reacts, by testing it at several concentrations.
Practice
Flashcards
- How do you find an order from initial-rate data?Change one concentration and see how the rate changes. Doubling that gives ×2 means first order, ×4 means second.
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
Why use the initial rate and not a later one?
At the start, the concentrations are known exactly and almost no product has formed. Later, concentrations change and reverse reactions or product interference can alter the rate. The initial rate keeps the data clean so you can compare the runs fairly.
How do I find the order if the rate does not change when I double a concentration?
If doubling a concentration leaves the rate unchanged, the order in that reactant is zero. Its concentration does not appear in the rate law. If doubling doubles the rate, the order is one. If doubling quadruples the rate, the order is two.