Reaction Rates and Rate Laws: Quiz

Question 1EasyWhy does increasing the concentration of a reactant usually increase the rate?

Each collision has more energyCollisions between particles happen more oftenThe activation energy decreasesThe rate constant increases

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Answer: Collisions between particles happen more often

More particles per litre means more frequent collisions. Energy per collision, Ea and k are unchanged.

Question 2MediumFor rate = k[A][B]², what happens to the rate when [B] is tripled?

It triplesIt increases 6 timesIt increases 9 timesIt does not change

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Answer: It increases 9 times

The reaction is second order in B, so the rate is multiplied by 3² = 9.

Question 3MediumWhat are the units of k for a reaction with rate = k[A]²?

s⁻¹M s⁻¹M⁻¹ s⁻¹M⁻² s⁻¹

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Answer: M⁻¹ s⁻¹

k = rate ÷ [A]² = (M s⁻¹) ÷ M² = M⁻¹ s⁻¹.

Question 4MediumDoubling [A] (with [B] constant) multiplies the initial rate by 4. Doubling [B] (with [A] constant) leaves it unchanged. What is the rate law?

rate = k[A][B]rate = k[A]²rate = k[A]²[B]rate = k[B]²

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Answer: rate = k[A]²

×4 for doubling means second order in A; no change means zero order in B, so [B] does not appear.

Question 5MediumA first-order reaction has k = 0.0231 min⁻¹. What is its half-life?

0.0160 min30.0 min43.3 min15.0 min

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Answer: 30.0 min

t½ = 0.693 ÷ 0.0231 min⁻¹ = 30.0 min. The answer 43.3 min is 1 ÷ k.

Question 6EasyA first-order reactant starts at 0.800 M. What is its concentration after 3 half-lives?

0.267 M0.200 M0.100 M0 M

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Answer: 0.100 M

0.800 M → 0.400 M → 0.200 M → 0.100 M: (½)³ × 0.800 M = 0.100 M.

Question 7HardIn 2N₂O₅ → 4NO₂ + O₂, N₂O₅ disappears at 0.0060 M/s. How fast does O₂ form?

0.0030 M/s0.0060 M/s0.012 M/s0.0015 M/s

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Answer: 0.0030 M/s

1 mol O₂ forms per 2 mol N₂O₅: 0.0060 M/s × (1/2) = 0.0030 M/s. The answer 0.012 M/s is the rate of NO₂ formation.