Reaction types, rates, exo/endothermic

Year 10 answers

Fluency

(a) Decomposition. (b) Synthesis. (c) Displacement.
(a) No: silver is below copper, so Ag cannot displace Cu. (b) Yes: magnesium is above zinc in the reactivity series, so Mg displaces Zn.
$2\text{Mg} + \text{O}_2 \to 2\text{MgO}$ .
$\text{CaCO}_3 \to \text{CaO} + \text{CO}_2$ .

Fluency

Particles must collide, with enough energy (activation energy), and with the correct orientation.
Higher temperature, higher concentration, greater surface area, adding a catalyst.
No — “concentration” refers to dissolved or gaseous reactants. For a solid, surface area is the equivalent lever.
Powdered sugar has a far greater surface area exposed to oxygen; more collisions per second support rapid combustion.
A substance that speeds up a reaction by providing a lower-activation-energy pathway without being consumed overall.

Fluency

Exothermic: releases energy (e.g. combustion of methane). Endothermic: absorbs energy (e.g. photosynthesis).
The products are at a lower energy than the reactants — the curve ends below where it started.
The minimum energy that colliding particles need to react; it is the peak of the energy profile above the reactant level.
False. A catalyst lowers activation energy; the overall energy change between reactants and products is unchanged.

Reasoning

Lower temperature means slower particles, fewer successful collisions per second, so spoilage (a reaction) is slower.
Rate increases — more marble surface is exposed to acid, so more particle collisions per second.
Higher concentration means more reactant particles per unit volume, giving more collisions per second with the metal.
A small rise in average kinetic energy roughly doubles the fraction of particles above the activation-energy threshold, even though $E_a$ itself is unchanged. (The Maxwell-Boltzmann distribution steepens in the high-energy tail with temperature.)
The catalyst participates in intermediate steps but is regenerated by the end of the reaction. It offers a new route with a lower energy barrier.

Problem solving

The sparkler has fine metal particles (large surface area) and burns at high temperature, meeting many oxygen molecules per second; the lump of iron has very little exposed surface and stays cool.
Endothermic — it absorbs solar energy. Respiration (combustion of glucose) is the reverse, so it is exothermic, releasing the stored energy.
(a) $40\,^\circ\text{C}$ : more CO $_2$ per second on average. (b) Yes — same amount of CaCO $_3$ , same amount of HCl, same total CO $_2$ .
(a) MnO $_2$ is a catalyst — it lowers the activation energy for H $_2$ O $_2$ decomposition. (b) Its mass is unchanged; it is not consumed.

Reasoning

Doubling concentration does not change the distribution of molecular speeds, so the fraction of collisions with enough energy is the same. However, the total number of collisions per second doubles, so the rate doubles.
The catalyst surface adsorbs exhaust gases and brings them into reactive proximity at a lower activation energy. Pt/Pd/Rh are used because they resist high temperatures, bind the gases with ideal strength (strong enough to hold but weak enough to release products), and are not consumed.
Exothermic: curve from high reactants, up to $E_a$ , down to low products; with catalyst the peak is lower but start and end heights unchanged. Endothermic: curve from low reactants, up to $E_a$ , down to a level higher than start.
Higher T speeds up the reaction (collision theory) but reduces equilibrium yield because the forward reaction is exothermic (Le Chatelier). The $400$ - $500\,^\circ\text{C}$ range is a compromise — fast enough to produce useful amounts per hour while keeping yield economically worthwhile, often combined with an iron catalyst and high pressure.