The rate and extent of chemical change

• Rate is a quantity per second — read it off tables, graphs and tangents; collision theory 碰撞理论 explains every factor that changes it.
• Reversible reactions settle into dynamic equilibrium 动态平衡; (HT) Le Chatelier predicts how each condition shifts it.
• Industry optimises rate and yield together — compromise conditions.

Calculating rates: rate of reaction 反应速率 = quantity of reactant used ÷ time, or product formed ÷ time — in g/s or cm³/s (HT also mol/s). Interpret product-vs-time graphs (steeper start, flattening as reactant runs out); draw tangents 切线 and use their gradient as the rate at that instant (HT calculate it).

Factors affecting rate: concentration (solution), pressure (gas), surface area (solid), temperature, catalyst 催化剂s. RP5: investigate concentration by (a) measuring the volume of gas produced and (b) observing a colour/turbidity change — hypothesis, variables, repeats.

Collision theory: reactions occur only when particles collide with at least the activation energy 活化能. Raising concentration/pressure crowds particles — more frequent collisions; smaller solid pieces raise the surface-area-to-volume ratio — more exposed surface, more frequent collisions; raising temperature gives particles more energy — more collisions AND more collisions that pass the activation energy.

Catalysts: change the rate but are not used up; each reaction has its own catalyst; enzymes are biological catalysts. A catalyst offers an alternative pathway with lower activation energy — identify it by speeding the reaction yet never appearing in the equation. Its reaction profile keeps the same overall energy change with a lower hump:

Reversible reactions 可逆反应: the products can react back to the reactants — written with the ⇌ arrow. If exothermic one way, endothermic the other, transferring the same amount of energy.

Equilibrium: in a closed system, equilibrium is reached when the forward and reverse rates are equal — the concentrations stop changing though both reactions continue (dynamic).

(HT) Le Chatelier's principle 勒夏特列原理: a system at equilibrium responds to counteract any change:

• concentration ↑ of a reactant → more product forms (and vice versa);
• pressure ↑ → the position moves to the side with fewer gas molecules;
• temperature ↑ → the position moves in the endothermic direction (↓ for exothermic direction).

A catalyst does not shift the position — it reaches equilibrium faster. Industry picks compromise conditions balancing rate, yield, safety and cost (e.g. the Haber process in topic 10).

• Rate from a table, from a graph's steepness, and (HT) from a tangent gradient.
• Each of the five factors explained by collision theory, naming what happens to collision frequency and energy.
• Catalyst effect on the profile; not used up; enzymes.
• The ⇌ arrow; equilibrium as equal rates; (HT) predict all three condition changes with Le Chatelier; compromise conditions justified.