202 exam-style questions on Reactivity 2: How much, how fast and how far? with answers and explanations. Six real samples below — the full set is free with an account.
A compound contains 40.0% C, 6.7% H and 53.3% O by mass. (Ar: C = 12.0, H = 1.0, O = 16.0.) What is its empirical formula?
Answer: CH2O
Divide each percentage by its Ar: C = 40.0/12.0 = 3.33, H = 6.7/1.0 = 6.7, O = 53.3/16.0 = 3.33. Divide by the smallest (3.33): C = 1, H = 2, O = 1. The empirical formula is CH2O.
For N2 + 3H2 -> 2NH3, if 2.0 mol N2 reacts with 3.0 mol H2, which is the limiting reactant and how much NH3 forms?
Answer: H2 limiting, 2.0 mol NH3
The ratio needed is 1 N2 : 3 H2. 2.0 mol N2 would need 6.0 mol H2, but only 3.0 mol H2 is available, so H2 is limiting. 3.0 mol H2 x (2 NH3 / 3 H2) = 2.0 mol NH3.
At STP (0 degrees C, 100 kPa), the molar volume of an ideal gas is 22.7 dm3/mol. What volume does 0.50 mol of oxygen gas occupy at STP?
Answer: 11.35 dm3
At STP the volume of any ideal gas = moles x molar volume = 0.50 x 22.7 = 11.35 dm3. The identity of the gas does not matter for an ideal gas.
A fixed amount of gas at constant temperature has volume 2.0 dm3 at 100 kPa. What is its volume when the pressure is increased to 250 kPa?
Answer: 0.80 dm3
By Boyle's law, P1V1 = P2V2 at constant T. V2 = (P1V1)/P2 = (100 x 2.0)/250 = 200/250 = 0.80 dm3.
On a Maxwell-Boltzmann energy distribution, what happens when temperature is increased?
Answer: The peak shifts to higher energy and more molecules exceed the activation energy
At higher temperature the distribution broadens and its peak shifts to higher energy (and becomes lower). A larger fraction of molecules has energy greater than or equal to the activation energy, so the reaction rate increases. The total area (number of molecules) stays constant.
For a reaction A + B -> products, doubling [A] doubles the rate and doubling [B] quadruples the rate. What is the overall order of reaction?
Answer: 3
Doubling [A] doubles the rate: first order in A. Doubling [B] quadruples the rate (2^2): second order in B. Rate = k[A][B]^2. Overall order = 1 + 2 = 3.
That was 6 of 202 Reactivity 2: How much, how fast and how far? questions.
Practise all 202 freeA student writes the equilibrium constant for N₂(g) + 3H₂(g) ⇌ 2NH₃(g) as Kc = [N₂][H₂]³ / [NH₃]² and concludes that a large value would mean a high yield of ammonia.
See the correction →StateAO1 · 1–2 marks
One word or one short sentence. No explanation, no working: the mark is for the fact itself.
DefineAO1 · 1–2 marks
Give the precise meaning — one sentence, the key terms in it, no example needed unless asked.
IdentifyAO1 · 1–2 marks
Name or pick out the thing asked for from the information given. A label, not a paragraph.
202 Reactivity 2: How much, how fast and how far? questions — free with an account. Spaced repetition, streaks and full exam simulations included.
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