Rates and equilibrium, Year 11: collision theory and catalysts

frequent

提示Use the word that means 'happening more often'.

为什么With 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

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

为什么Temperature 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

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

为什么Each 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

提示Twice as many particles are crowded into the same volume.

为什么The 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

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

为什么The 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

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

为什么With 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

提示The height of the hump above the reactants is the energy barrier.

为什么The 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

提示Compare where the two curves begin and where they finish.

为什么The 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

提示A catalyst is still there, unchanged, when the reaction is over.

为什么Because 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

提示Think of keys and locks.

为什么A 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.