Every card in Rates and equilibrium, Year 11: collision theory and catalysts
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- Increasing the pressure of reacting gases squeezes the particles closer together, so collisions between them become more ____.
frequent
HintUse the word that means 'happening more often'.
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.
- Raising the temperature makes particles collide more often. What is the second reason, given by collision theory, why it increases the rate of reaction?
The collisions are more energetic, so more of them reach the activation energy
HintHotter particles move faster; consider what that does to each impact.
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.
- A 2 cm cube of marble has a total surface area of 24 cm². It is cut into eight 1 cm cubes. What is the total surface area of the eight small cubes, in cm²? (number only)
48
HintWork out the area of one small cube's six faces first.
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.
- In a simple collision model, the concentration of a reactant in solution is doubled. What happens to the number of collisions each second that involve its particles?
It doubles
HintTwice as many particles are crowded into the same volume.
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.
- The theory that explains rates of reaction by how often reacting particles meet and how much energy they have when they do
Collision theory
HintIt is named after the event in which two particles hit each other.
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.
- A catalyst increases the rate of a reaction by providing a different pathway that has a lower ____ energy.
activation
HintIt is the name of the energy barrier that colliding particles must get over.
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.
- A reaction profile shows two curves. Both start at the same reactants level and end at the same products level, but one curve has a lower peak than the other. Which curve is for the reaction with a catalyst?
The one with the lower peak
HintThe height of the hump above the reactants is the energy barrier.
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.
- A catalyst lowers the activation energy. On a reaction profile, does it change the overall energy change of the reaction?
No: the reactants and products levels stay the same
HintCompare where the two curves begin and where they finish.
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.
- A substance increases the rate of a reaction, which is one sign that it is a catalyst. What is the second sign, seen in the chemical equation for the reaction?
It does not appear in the equation as a reactant or a product
HintA catalyst is still there, unchanged, when the reaction is over.
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.
- Manganese dioxide is a catalyst for the breakdown of hydrogen peroxide. Would you expect it to catalyse every other reaction as well?
No: different reactions need different catalysts
HintThink of keys and locks.
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.
1★ GCSE-CHEM-RTE-0017
2★ GCSE-CHEM-RTE-0018
3★ GCSE-CHEM-RTE-0019
4★ GCSE-CHEM-RTE-0020
5★ GCSE-CHEM-RTE-0021
6★ GCSE-CHEM-RTE-0022
7★ GCSE-CHEM-RTE-0023
8★ GCSE-CHEM-RTE-0024
9★ GCSE-CHEM-RTE-0025
10★ GCSE-CHEM-RTE-0026
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