Definition
Collision frequency is the number of collisions per unit volume of the reaction mixture per unit time.
A successful collision is one that actually leads to a reaction — this requires both the colliding particles to have enough energy (at least the activation energy) and to collide with the correct orientation, so that the right atoms come together to allow bonds to break and re-form.
Activation energy is the minimum energy that colliding particles must have for a collision to be successful and lead to a reaction — you can picture it as an energy 'hill' that reactants must climb over before they can turn into products.
If the reactants possess energy higher than the activation energy, the reaction proceeds (that collision succeeds).
After reactant particles climb over the activation energy hill during a successful collision, they form products — and since products are typically at a lower energy than the top of that hill, the excess energy is released as the particles descend to the product energy level.
Definition
Collision theory states that for a reaction to occur, particles must collide; only a fraction of collisions are successful (having both sufficient energy and correct orientation); and anything that increases the frequency of successful collisions increases the rate of reaction.
Remember
Molecules gain the extra energy needed to reach the higher (activated) energy state mainly from the thermal (kinetic) energy of the reaction mixture itself — faster-moving, more energetic molecules collide harder and more often with enough force to succeed.
Rule
Concentration: increasing the concentration of reactants packs more particles into the same volume, increasing collision frequency and so the rate of reaction — this is why sulphur burns faster in pure oxygen than in air (which is only about oxygen), since pure oxygen means a much higher concentration of the reacting oxygen molecules.
Pressure (for gaseous reactants): increasing pressure squeezes gas molecules closer together, effectively increasing their concentration, which increases collision frequency and rate — this is exactly why, for hydrogen reacting with chlorine (hydrogen in excess), doubling the chlorine gas pressure increases the rate of reaction.
Temperature: increasing temperature gives particles more kinetic energy, so they move faster, collide more often, and — crucially — a much larger fraction of collisions now have enough energy to exceed the activation energy, making a far greater proportion of collisions successful. This combined effect is why even a modest temperature rise can noticeably speed up a reaction.
Surface area: increasing the surface area of a solid reactant (e.g. using powder instead of a lump) exposes more particles at the surface where collisions with the other reactant can actually happen, increasing collision frequency and rate without changing the total amount of reactant.
Catalyst: a catalyst increases reaction rate by providing an alternative reaction pathway with lower activation energy, so a larger fraction of collisions now have enough energy to succeed, without the catalyst itself being used up in the reaction.
Example 1 — magnesium and temperature
Magnesium doesn't react with air at room temperature but reacts very fast at high temperature, giving intense white light. Explain.
- At room temperature, very few collisions between Mg and O particles have enough energy to exceed the activation energy — the reaction rate is negligible.
- At high temperature, particles have much more kinetic energy, so far more collisions exceed the activation energy, and the reaction proceeds rapidly (burning), releasing energy as intense light and heat.
Answer: The reaction needs enough activation energy, which is only reached at high temperature
Remember
A catalyst is often used in powdered form to maximize its surface area, giving reactant particles more contact points on the catalyst's surface where the lower-activation-energy pathway is available, speeding up the overall reaction further.
The reaction rate is often fastest at the very beginning because reactant concentration is at its highest then, giving the greatest collision frequency; as reactants are used up, their concentration falls and the rate slows down accordingly.
To stop a reaction happening at C, you could rapidly cool the mixture (lowering the fraction of successful collisions), dilute it (lowering concentration), or add a substance that removes a key reactant or poisons any catalyst present — any action that reverses one of the rate-increasing factors above.
Definition
A catalyst speeds up a reaction by lowering its activation energy through an alternative reaction pathway/mechanism, without being permanently consumed or changed at the end of the reaction — e.g. catalyses by providing a lower-energy stepwise pathway (via intermediate vanadium compounds) instead of the reactants needing to collide directly with enough energy on their own.
Platinum metal catalyses the addition of hydrogen to ethene by adsorbing both hydrogen and ethene molecules onto its surface, weakening their bonds and holding them close together in the right orientation, so the reaction can proceed with far lower activation energy than if the molecules had to collide freely in the gas phase.
Definition
Enzymes are biological catalysts (proteins) that speed up reactions in living organisms. Two features of enzyme catalytic action: they are highly specific (each enzyme usually catalyses only one particular reaction or type of reaction, due to its precisely shaped active site), and they work efficiently at relatively mild conditions (body temperature and near-neutral pH), unlike many industrial catalysts that need extreme heat or pressure.
Enzymes play an essential role in the body as biological catalysts, enabling digestion (breaking down food into absorbable molecules), metabolism, and virtually every biochemical process — reactions that would otherwise be far too slow to sustain life proceed fast enough at body temperature because enzymes dramatically lower their activation energy.
Remember
Chemical kinetics plays an important role in the food industry through controlling reaction rates for preservation (slowing spoilage reactions via refrigeration, which lowers temperature and so rate) and processing (using enzymes or catalysts to speed up desired reactions like fermentation, or controlling conditions precisely for consistent product quality).
Rule
The law of conservation of mass states that in a chemical reaction carried out in a closed container, the total mass of reactants equals the total mass of products — mass is neither created nor destroyed, only rearranged into different substances, since the same atoms are simply reorganized into new bonds.
Remember
When marble chips react with dilute hydrochloric acid in an open container, the apparent mass decrease is because one of the products, carbon dioxide gas, escapes into the air rather than staying in the container to be weighed — the total mass (including the escaped gas) is actually conserved, but the measured mass of what remains in the container drops.
Some reactions in an open container appear to gain mass instead — e.g. burning magnesium in air (open to oxygen) gains mass, because oxygen from the surrounding air combines with the magnesium to form magnesium oxide, and that added oxygen's mass wasn't part of the original weighed sample.
Definition
Exothermic reactions release energy (usually as heat) to the surroundings, so the surrounding temperature rises — e.g. quicklime reacting with water, , which releases a lot of heat, and combustion reactions in general.
Endothermic reactions absorb energy from the surroundings, so the surrounding temperature falls — e.g. dissolving ammonium chloride in water, which makes the surrounding temperature drop as the process draws in heat from its surroundings.
Quick Recap
- ●Collision theory: reaction needs successful collisions — enough energy (≥ activation energy) correct orientation
- ●Rate increases with: higher concentration, higher pressure (gases), higher temperature, greater surface area, a catalyst
- ●Catalyst: lowers activation energy via an alternative pathway, not consumed; enzymes are specific biological catalysts
- ●Law of conservation of mass: total mass constant in a closed container; apparent gain/loss in open containers is due to gas escaping or entering
- ●Exothermic: releases heat, surroundings warm up (e.g. quicklime + water). Endothermic: absorbs heat, surroundings cool (e.g. NHCl dissolving)
