Checkpoint 3

• Q1 — C: Rate = Δvolume / Δt = 48 mL / 4 min = 12 mL/min.
• Q2 — B: An effective collision requires BOTH (1) kinetic energy ≥ Eₐ (sufficient energy to break bonds and reach the transition state) AND (2) correct orientation (the reactive parts of the molecules must face each other). Temperature and concentration are factors that influence the frequency of collisions meeting these conditions, but they are not themselves the conditions.
• Q3 — B: Eₐ is a fixed energy threshold marked as a vertical line on the x-axis of the Maxwell-Boltzmann diagram. Only particles whose kinetic energy places them to the right of this line can react. The area under the curve to the right of Eₐ represents the fraction of reactive particles.
• Q4 — C: At higher temperature: the distribution shifts rightward (higher average energy), the peak decreases in height and broadens, and the area under the curve remains equal (same number of particles in the system). Option B is wrong — the total area is constant, not increased. Option D is wrong — temperature does not change Eₐ, which is a property of the reaction.
• Q5 — B: A catalyst provides an alternative reaction pathway with a lower activation energy. This means a greater proportion of collisions at the same temperature have sufficient energy to be effective → rate increases. The catalyst is not consumed and does not change the temperature.
• Q6 — B: Heterogeneous catalysis: the catalyst (solid Pt) is in a different phase from the reactants (gaseous CO and O₂). The reaction proceeds by the gas molecules adsorbing onto the platinum surface, reacting there, then desorbing as CO₂.
• Q7 — B: The 10°C rule: for many reactions near room temperature, rate approximately doubles per 10°C rise. A 20°C increase → approximately 2 × 2 = 4-fold increase. This occurs because the M-B distribution shifts such that the area beyond Eₐ roughly doubles per 10°C. The curve does not become taller (option D) — area is constant.
• Q8 — B: A catalyst provides an alternative pathway involving a lower-energy transition state, but the same reactants produce the same products. Because ΔH is determined by the energy difference between reactants and products (which are identical in both pathways), ΔH is unchanged. The catalyst only lowers the peak height (Eₐ), not the reactant or product energy levels.
• Q9 — C: Catalyst poisoning: lead compounds (from leaded petrol) coat the active sites on the platinum surface, blocking reactant molecules (CO, O₂, NO) from adsorbing. With active sites blocked, the heterogeneous catalytic cycle cannot proceed. This is why leaded petrol was banned in most countries — it destroys catalytic converters.
• Q10 — B: Rate determines how quickly the reaction proceeds, not how much product is formed in total. The total amount of CO₂ depends on the amount of CaCO₃ used (equal masses = same moles). Both reactions produce the same total CO₂. At 40°C, a higher proportion of HCl particles exceed Eₐ → more effective collisions per second → steeper initial rate gradient → reaction completes first.

SA1: Reaction rate is the change in concentration, mass, or volume of a reactant or product per unit time (e.g. mol/L/s, g/s, mL/s). As the reaction proceeds, HCl is consumed — its concentration decreases over time. With fewer H⁺ ions per unit volume in solution, the average distance between H⁺ ions and the CaCO₃ surface increases, so collisions between H⁺ and the solid surface occur less frequently. The proportion of those collisions that are effective (energy ≥ Eₐ and correct orientation) is unchanged at constant temperature. Therefore the number of effective collisions per second decreases → rate decreases. This continues until CaCO₃ or HCl is fully consumed and the rate reaches zero.

SA2: (a) When a catalyst is added at constant temperature, the Maxwell-Boltzmann distribution curve is unchanged — the same spread of particle energies exists at the same temperature. A second vertical line is drawn to the left of the original Eₐ line, at a lower energy value (Eₐ(cat)). This represents the lower activation energy of the catalysed pathway. (b) The area under the distribution curve to the right of Eₐ(cat) is larger than the area to the right of Eₐ(uncat). This means a greater proportion of particles now have sufficient kinetic energy to undergo effective collisions. At the same temperature and collision frequency, more effective collisions occur per second → rate increases. (c) ΔH is unchanged because the catalyst provides an alternative reaction mechanism that still starts from the same reactants and ends with the same products. ΔH = energy of products − energy of reactants; since both are fixed (same chemical species), the energy difference is the same in both the catalysed and uncatalysed pathways. The catalyst only changes the height of the energy barrier (Eₐ), not the energy levels of reactants or products.

SA3: (a) 2NO(g) → N₂(g) + O₂(g). [Check: left = 2N, 2O; right = 2N, 2O ✓]. (b) This is heterogeneous catalysis because the catalyst (solid platinum or palladium) is in a different physical phase from the reactants (gaseous NO). The mechanism involves: (1) gaseous NO molecules adsorbing onto the solid platinum surface at active sites; (2) reaction on the surface, breaking N–O bonds and forming N–N and O–O bonds; (3) desorption of N₂ and O₂ products into the gas phase. The platinum surface is regenerated and not consumed. (c) Heterogeneous catalysis requires reactant gas molecules to adsorb onto the catalyst surface and be activated. Below the operating temperature (~300–400°C), the platinum surface does not have sufficient thermal energy to facilitate effective adsorption of NO molecules — the molecules cannot bind to the active sites and the catalytic cycle cannot begin. Therefore NO passes through the cold converter unreacted, and emissions remain high until the converter warms up (typically within 30–90 seconds of starting the engine).