What affects how fast a reaction goes
Chemistry · Key Stage 3 · The School
Collision theory, in outline
For a reaction to happen, particles must COLLIDE with enough energy to react — a SUCCESSFUL collision. Anything that increases how often particles collide, or how much energy each collision carries, speeds the reaction up. Anything that reduces either of those slows it down. Every factor below works through one of these two routes.
Temperature and concentration
Higher TEMPERATURE makes particles move faster, so they collide more often AND with more energy — both effects speed up the reaction at once. Higher CONCENTRATION packs more particles into the same space, so collisions simply happen more often, though each one carries the same energy as before.
Surface area
A solid can only react where its surface is exposed to the other reactant. Breaking a lump into smaller pieces, or using a powder instead of a lump, exposes far more SURFACE AREA for the same total mass, giving many more places for a collision to happen at once. This is why powdered chalk reacts with acid far faster than a single chalk stick of the same mass.
Start with marble chips reacting steadily with dilute hydrochloric acid. Warm the acid: faster, because particles collide more often and with more energy. Use a more concentrated acid instead: faster, because there are more acid particles packed into the same volume to collide with. Crush the marble chips into powder: faster, because far more surface area is now exposed to the acid at once. Add a catalyst, if one existed for this particular reaction: faster too, because the energy needed for a successful collision would drop. Four different changes, each working through the same two levers collision theory names: how often particles meet, and how much energy each meeting carries.