Rates and equilibrium, Year 11: collision theory and catalysts

frequent

HintUse the word that means 'happening more often'.

The whyWith the particles crowded into a smaller space, each one meets others more often. More collisions each second means more successful collisions each second, so the rate rises.

The collisions are more energetic, so more of them reach the activation energy

HintHotter particles move faster; consider what that does to each impact.

The whyTemperature is the only factor that changes two things at once: how often particles collide and how hard. Concentration, pressure and surface area change only how often.

48

HintWork out the area of one small cube's six faces first.

The whyEach 1 cm cube has 6 faces of 1 cm², making 6 cm²; eight of them give 8 × 6 = 48 cm². The volume is still 8 cm³, so the surface area to volume ratio has doubled from 3 : 1 to 6 : 1, giving acid particles twice as much surface to collide with.

It doubles

HintTwice as many particles are crowded into the same volume.

The whyThe frequency of collisions is proportional to the number of particles in a given volume. Twice the particles means twice the collisions each second, and so roughly twice the rate.

Collision theory

HintIt is named after the event in which two particles hit each other.

The whyThe theory says that particles can react only when they collide, and only if the collision has at least the activation energy. Every factor that changes a rate works by changing how often such collisions happen.

activation

HintIt is the name of the energy barrier that colliding particles must get over.

The whyWith a lower barrier, a larger share of collisions have enough energy to react, so the reaction goes faster at the same temperature. The catalyst is not used up in doing this.

The one with the lower peak

HintThe height of the hump above the reactants is the energy barrier.

The whyThe height from the reactants level up to the top of a curve is the activation energy. A catalyst provides a route with a smaller activation energy, so its curve has the smaller hump.

No: the reactants and products levels stay the same

HintCompare where the two curves begin and where they finish.

The whyThe overall energy change is the gap between the reactants level and the products level. A catalyst alters only the route between them, so that gap is unchanged: an exothermic reaction gives out just as much energy with a catalyst as without.

It does not appear in the equation as a reactant or a product

HintA catalyst is still there, unchanged, when the reaction is over.

The whyBecause a catalyst is not used up, it is neither a reactant nor a product. If it is shown at all, its name or formula is written above the arrow.

No: different reactions need different catalysts

HintThink of keys and locks.

The whyA catalyst works by providing a new pathway for one particular reaction. A substance that does this for one reaction will usually do nothing for another, which is why industry uses many different catalysts.