The balanced equation for a reaction is A + 2B → C. A student concludes that the reaction is first order in A and second order in B. What is wrong with this reasoning?
BThe student should have used molar masses, not coefficients, to determine the orders
CReaction orders must be determined experimentally; the balanced equation only gives stoichiometry, not mechanism
DThe overall order should equal the number of reactants, making this first order overall
Reaction orders cannot be read from stoichiometric coefficients except for elementary reactions — reactions that occur in a single molecular step exactly as written. Most reactions studied in general chemistry are not elementary; they proceed through multi-step mechanisms with a rate-determining step. The observed rate law reflects that slow step, not the overall stoichiometry. A reaction written as A + 2B → C could be zero order in B (if B doesn't appear in the rate-determining step), first order in A and zero order in B, or any combination — only the method of initial rates can determine this.
Question 2 Multiple Choice
Three experiments are run. In Exp 1: [A] = 0.10 M, [B] = 0.10 M, rate = 2.0 × 10⁻³ M/s. In Exp 2: [A] = 0.20 M, [B] = 0.10 M, rate = 8.0 × 10⁻³ M/s. What is the order with respect to A?
AFirst order — doubling [A] doubled the rate
BSecond order — doubling [A] quadrupled the rate
CZero order — [B] was held constant so we cannot determine the order in A
DThird order — the rate increased by a factor of 4, and 4 = 2³
Comparing Exp 1 and Exp 2: [A] doubled while [B] was held constant, and the rate quadrupled (8.0/2.0 = 4). Using the ratio method: (rate₂/rate₁) = ([A]₂/[A]₁)ᵐ → 4 = 2ᵐ → m = 2. Second order. Option A inverts the logic: doubling [A] and doubling the rate would mean first order (2¹ = 2), but the rate quadrupled (2² = 4), indicating second order. Keeping [B] constant is the point — it isolates the effect of A so you can determine its order unambiguously.
Question 3 True / False
For a reaction A + B → products, the balanced equation shows a coefficient of 2 for reactant B, so the reaction should be second order in B.
TTrue
FFalse
Answer: False
Stoichiometric coefficients reflect amounts consumed, not the reaction mechanism. Reaction orders are experimental quantities determined by the method of initial rates. A coefficient of 2 for B means two moles of B are consumed per mole of product formed, but B could be zero order (not involved in the rate-determining step), first order, second order, or even fractional order — depending entirely on the mechanism. The equation 2HA → H₂ + A₂ looks second order, but many dimerizations have first-order kinetics because of their mechanism.
Question 4 True / False
The rate constant k for a given reaction has the same numerical value at 25°C and at 75°C.
TTrue
FFalse
Answer: False
The rate constant k depends strongly on temperature through the Arrhenius equation: k = A·e^(−Ea/RT). Increasing temperature increases k because a greater fraction of molecular collisions have enough energy to overcome the activation energy barrier. A 10°C temperature rise often roughly doubles the rate constant for reactions with typical activation energies. 'Constant' in 'rate constant' means k is fixed at a given temperature for a given reaction — it does not mean k is invariant across temperatures. This is a common source of confusion: k is constant in the sense of not depending on concentration, but it does depend on temperature.
Question 5 Short Answer
Why can't you determine reaction orders directly from a balanced chemical equation, and what experimental approach is used instead?
Think about your answer, then reveal below.
Model answer: The balanced equation shows overall stoichiometry — how much of each reactant is consumed — but not the mechanism by which the reaction occurs. Reaction orders reflect the rate-determining step of the mechanism, which typically involves only a subset of reactants, sometimes at different stoichiometries than the overall equation. The method of initial rates isolates one reactant at a time by varying its concentration while holding all others constant, then comparing how the initial rate changes to determine each reactant's order experimentally.
The disconnect between stoichiometry and kinetics is fundamental. For example, the reaction 2NO₂ → 2NO + O₂ has a stoichiometric coefficient of 2 for NO₂, but the rate law is rate = k[NO₂]² — second order, which in this case happens to match. But the reason it is second order is that the rate-determining step involves two NO₂ molecules colliding, not because the coefficient is 2. Coincidence of stoichiometry and order in elementary reactions (single-step mechanisms) is why the textbook sometimes says 'you can read the orders from elementary steps' — but this only works when you already know the mechanism is elementary.