How fast? The rate of chemical change

The **change in concentration** of a reactant or product **per unit time**.

**mol dm⁻³ s⁻¹** — a concentration (mol dm⁻³) divided by a time (s).

It is the **gradient** (steepness) of the curve — the tangent at a point gives the instantaneous rate.

The **reactant concentration is highest** at t = 0, so effective collisions are most frequent and the curve is **steepest**.

**Average** = total change ÷ total time (slope of the **chord**); **instantaneous** = slope of the **tangent** at one moment.

Particles must **collide** to react, but only **effective** collisions (enough energy + correct orientation) lead to a reaction.

Energy **≥ the activation energy Eₐ**, AND the particles collide in the **correct orientation**.

The **minimum energy** that colliding particles must have for a reaction to occur.

Reactants are **used up**, so their concentration falls and effective collisions become **less frequent**; rate drops to zero when reactants run out.

Measure the **volume of gas** collected vs time, or the **mass lost** vs time.

Use a **colorimeter** to measure the **light absorbed** as it changes with time.

The rate at t = 0 — the **slope of the tangent drawn at the start** of a concentration–time graph (the steepest point).

Energy **≥ the activation energy (E_{a})** AND the **correct orientation**.

The **minimum** energy a colliding pair of particles must have for a reaction to occur.

**Concentration, pressure, surface area, temperature** and a **catalyst**.

They put more particles in the reaction space, so collisions are **more frequent** (the energy per collision is unchanged).

Particles move faster (collisions **more frequent**) AND the distribution shifts right so a **greater fraction** have energy ≥ E_{a} — the second effect is the main one.

A substance that speeds up a reaction by providing an **alternative pathway of lower E_{a}**, and is **not used up** itself.

**No** — the reactant and product energy levels are unchanged, so ΔH is the same.

How the **kinetic energies** of particles are **spread out**; only those to the right of E_{a} can react.

**Lower and shifted to the right** (broader/flatter), but with the **same area** underneath.

The **fraction of particles** with enough energy to react (energy ≥ E_{a}).

The curve is **unchanged**; the **E_{a} line moves left**, so a larger fraction lies to the right of it.

The reaction goes **faster**, AND the solid is **recovered unchanged** (same mass/nature) at the end.