Finding reaction order from rate data
Orders come from experiments, not from the balanced equation.
The common mistake
Reading the rate law straight off the balanced equation's coefficients — assuming 2A + B → C means rate = k[A]²[B].
Instead: Compare two trials where only one concentration changes. If doubling it doubles the rate, that reactant is first order; ×4 is second order; no change is zero order.
Worked example with Guide me
Doubling [A] with [B] fixed makes the rate ×4. Doubling [B] with [A] fixed leaves the rate unchanged. Write the rate law.
- Sugar · Doubling A gives ×4. What power of 2 is 4?Student · 2², so A is second order.
- Sugar · Doubling B changes nothing. What order is B?Student · Zero order.
- Sugar · So what's the rate law?Student · rate = k[A]².
Answer: rate = k[A]², overall second order.
Try 3 practice questions
1. Tripling [X] triples the rate. What order is X?
2. Doubling [Y] makes the rate ×8. What order is Y?
3. rate = k[A][B]². What is the overall order?
Quick answers
- What is the most common mistake with finding reaction order from rate data?
- Reading the rate law straight off the balanced equation's coefficients — assuming 2A + B → C means rate = k[A]²[B].
- How do I get finding reaction order from rate data right?
- Compare two trials where only one concentration changes. If doubling it doubles the rate, that reactant is first order; ×4 is second order; no change is zero order.
- Can you show a worked example?
- Doubling [A] with [B] fixed makes the rate ×4. Doubling [B] with [A] fixed leaves the rate unchanged. Write the rate law. rate = k[A]², overall second order.
Related concepts
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Save this as a miss — practice it before your test