Hit Rate

Hit Rate: collision theory you can feel

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

What is collision theory?

Collision theory explains how the conditions affect the rate of a reaction. For AQA GCSE Chemistry (spec point 4.6.1.3), chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy. Most collisions in a reaction mixture do not have enough energy, so the particles just bounce off each other.

Hit Rate zooms into a reaction vessel about a billion times so you can see the particles, and puts you in control of one of them. You steer it into the other reactant’s particles, aiming past them so you hit at full speed. A gold ring around a particle means a collision now would have at least the activation energy. Hit it at full speed and it reacts: a product forms and you take over the next particle. Bump it gently and an energy bar shows the collision fell short of the activation energy line, so nothing happens.

Frequency or energy? The collision log

The rate of reaction depends on two separate things: how often the reacting particles collide (the frequency of collisions) and how many of those collisions have at least the activation energy. Students often mix these up, so every collision between the other particles drops a block onto a live collision log (yours don’t count, because you choose how hard you hit). Each block is placed by the energy of the collision, and only the gold blocks past the dashed activation energy line react. How fast the log fills up shows the frequency of collisions. How far right the blocks land shows how energetic they are.

Each test has two runs, before and after one change, followed by a card that puts the two logs side by side with the collisions per second, the share with enough energy and the rate (reactions per second).

Concentration, pressure and surface area

You make each change yourself. In the concentration test (sodium thiosulfate and hydrochloric acid, the reaction from the disappearing cross practical) you drop in more thiosulfate particles: twice as many particles in the same volume, so the log fills about twice as fast while the blocks land in the same places. In the pressure test you drag the plunger of a gas syringe holding hydrogen and chlorine in to half the volume. The same particles in a smaller volume collide more often. In the surface area test, acid particles can only reach the calcium carbonate particles on the surface of a marble lump. You crush it into small pieces: the same amount of marble, but more particles on the surface, because smaller pieces have a larger surface area to volume ratio.

All three increase the frequency of collisions, so the rate increases, but none of them makes the collisions more energetic. A quick check uses proportionality: if the rate is directly proportional to concentration, tripling the concentration triples the frequency of collisions and the rate. Another asks for the surface area to volume ratio of a 2 cm cube: 24 cm² to 8 cm³, which is 3 : 1.

Temperature does both

When you hold HEAT and warm the mixture from 20 °C to 50 °C, every particle speeds up, including yours. The collisions become more frequent and more energetic, so a much bigger share of them have at least the activation energy and far more particles get a gold ring. That is why a rise in temperature has such a big effect on rate. The activation energy itself does not change: heating gives the particles more energy, so more collisions reach it.

You then build a four-mark exam answer from the right links: the particles move faster, so collisions are more frequent, and the collisions are more energetic so more of them have at least the activation energy, so there are more successful collisions per second and the rate increases. Writing “more collisions” on its own does not earn the mark, so the game teaches you to say more frequent collisions or more collisions per second.

How to play

Move the mouse, drag your finger or use the arrow keys to steer; aim further ahead to go faster. Kai, who runs the Collision Lab, explains each new idea, and before each change you predict whether the collisions will be more frequent, more energetic or both. The game ends with a 50-second rush where the conditions keep changing and you call each one while you keep reacting. A game takes about 8 minutes, works on a computer, tablet or phone, and your best score is saved so you can play again.