Why collisions matter — answer key

1. UQ14 · Practice

Model answer. Particles react only when they collide, and only if the collision has at least the activation energy, Ea. Gentler collisions just bounce the particles apart, whichever way they hit.

A full answer includes

• I chose: they must collide with at least the activation energy.
• I did not choose a collision of any energy.

Also accept

• Collide with at least Ea

Watch for

• “Colliding particles need at least the energy of the products.” — The energy needed is the activation energy: the hump above the reactants. The level of the products does not decide whether a collision works.
• “Any head-on collision works.” — A head-on collision that is too gentle still just bounces the particles apart. What matters is at least Ea.
• “Particles must hit a catalyst to react.” — Most reactions happen without a catalyst. A catalyst only lowers Ea; it is not needed for a collision to work.

2. UQ15 · Practice

Options as numbered on the pupil sheet: 1 — A lower activation energy, so more of the collisions succeed; 2 — Particles move faster: more frequent collisions, and more of them have at least Ea; 3 — More particles exposed on the surface, so collisions are more frequent; 4 — More particles per unit volume, so collisions are more frequent.

Model answer. Concentration and the pressure of a gas both put more particles in each unit of volume, and surface area exposes more of them: all three give more collisions each second. A higher temperature gives more collisions too, but mainly more energetic ones: a larger share have at least Ea. A catalyst lowers Ea, so a larger share of the collisions succeed.

A full answer includes

• I matched 1, more concentrated acid, to more particles per unit volume.
• I matched 2, powder, to more particles exposed on the surface.
• I matched 3, a higher temperature, to faster particles and more collisions with at least Ea.
• I matched 4, a catalyst, to a lower activation energy.
• I matched 5, a higher pressure of gases, to more particles per unit volume.
• I did not match a higher temperature to more frequent collisions alone.

Also accept

• Concentration, pressure and surface: more frequent; temperature: more frequent and more energetic; catalyst: lower Ea

Watch for

• “Heating only makes the particles collide more often.” — They do collide more often, but the bigger reason is energy: a larger share of the collisions have at least the activation energy.
• “A powder has more particles than lumps of the same mass.” — The same mass has the same number of particles. In a powder far more of them are on a surface, where the acid can reach them.
• “A higher pressure gives the particles more energy.” — At the same temperature the particles have the same energy. A higher pressure only puts more of them in each unit of volume.
• “A catalyst makes the particles collide more often.” — With a catalyst the particles collide just as often; a larger share of the collisions succeed, because Ea is lower.

3. UQ16 · Practice

Model answer. At 45 °C the particles move faster (more kinetic energy). They collide more often, and, more importantly, a larger share of the collisions have at least Ea. So there are more successful collisions each second, and the tablet fizzes faster.

A full answer includes

• I wrote that at 45 °C the particles have more kinetic energy and move faster.
• Reason 1: I wrote that they collide more often.
• Reason 2: I wrote that a greater proportion of the collisions have at least the activation energy.
• I wrote that both give more successful collisions each second.
• I did not give "more frequent collisions" as my only reason.

Also accept

• More frequent collisions; a greater proportion with at least Ea

Watch for

• “Heating only makes the particles collide more often.” — They do collide more often, but the bigger reason is energy: a larger share of the collisions have at least the activation energy.
• “Warm water lowers the activation energy.” — The reaction is the same, so Ea is the same. Warm particles have more energy, so more of them reach it.

4. UQ17 · Practice

Model answer. A catalyst gives a route with a lower activation energy, so Q, the lower hump, is the route with the catalyst. Its Ea is the rise from the reactants line (120 kJ/mol) to the top of its hump (210 kJ/mol): 90 kJ/mol. Route P needs 270 − 120 = 150 kJ/mol. Both routes start and end on the same two lines, so ΔH is the same for both: 60 − 120 = −60 kJ/mol, negative because the products are lower and energy is given out. Row A.

A full answer includes

• I chose row A.
• I picked Q, the route with the lower hump, as the route with the catalyst.
• I took Ea as the rise from the reactants line to the top of the hump: 210 − 120 = 90 kJ/mol.
• I took ΔH from the reactants line to the products line: 60 − 120 = −60 kJ/mol.
• I did not read Ea as the energy at the top of the hump.

Also accept

• Row A

Watch for

• “Row B.” — Ea is not the energy at the top of the hump. It is the rise from the reactants line to the top: 210 − 120 = 90 kJ/mol.
• “Row C.” — P has the higher hump, so it is the route without a catalyst. A catalyst lowers the activation energy: the route with the catalyst is Q.
• “Row D.” — The products line is below the reactants line, so energy is given out and ΔH is negative: −60 kJ/mol.
• “A catalyst changes the energy given out or taken in.” — A catalyst gives the reaction a route with a lower activation energy. The reactants and products are the same, so ΔH is the same.

5. UQ18 · Practice

Model answer. A higher pressure puts the same gas particles into a smaller space: more particles per unit volume, so they collide more often. The temperature is the same, so the particles have the same kinetic energy and the same share of their collisions have at least Ea. More collisions each second, the same share successful: more successful collisions each second. Row A.

A full answer includes

• I chose row A.
• I wrote that squeezing puts more particles in each unit of volume.
• I wrote that this makes collisions more frequent.
• I wrote that the share of collisions with at least Ea stays the same, because the temperature is the same.
• I did not choose row B, more energetic collisions.

Also accept

• Row A

Watch for

• “Row B.” — At the same temperature the particles have the same kinetic energy, so the share of collisions with at least Ea does not change. Only how often they collide changes.
• “Row C.” — Squeezing does not make the particles faster. It puts the same particles into a smaller space: more of them per unit volume.
• “Row D.” — Pressure does not change Ea or the share that succeed; that is what a catalyst does. Pressure makes collisions more frequent.

6. UT3 · Apply

Model answer. (a) Thin strands have an enormous surface area compared with a nail. (b) Far more iron particles are exposed to the oxygen, so there are many more collisions between iron and oxygen particles each second, and the burning spreads very fast. (c) At room temperature too few collisions have at least Ea. The flame heats the first particles so that their collisions have enough energy; the energy given out then heats the next ones.

A full answer includes

• (a) I named surface area.
• (b) I wrote that the thin strands expose far more iron particles to the oxygen.
• (b) I wrote that collisions between oxygen and iron particles are more frequent.
• (b) I did not write that the steel wool is made of a different kind of particle.
• (c) I wrote that particles react only if they collide with at least the activation energy.
• (c) I wrote that the flame gives the first particles that energy.
• (c) I did not write that the flame is a catalyst.

Also accept

• Surface area; more frequent collisions with oxygen; the flame supplies Ea

Watch for

• “Steel wool burns because steel is a different substance from iron.” — Steel is mostly iron, like the nail. It burns faster because far more of its particles are on a surface, where the oxygen can reach them.
• “The flame is a catalyst.” — A flame is not a substance that is unchanged at the end. It heats the particles so that their collisions have at least Ea.