Some reactions are over in a flash; others take years. Rate puts a number on that speed, and collision theory explains why concentration, pressure and surface area change it.
The rate of reaction is the change in amount or concentration of a reactant or product per unit time, often in mol dm−3 s−1. It is found from the gradient of a concentration-time (or volume-time) graph; the gradient of the tangent at t = 0 is the initial rate. By collision theory, particles react only if they collide with at least the activation energy, Ea, and in the right orientation. Raising the concentration, the pressure of a gas or the surface area of a solid makes collisions more frequent, so successful collisions happen more often.
In the collision simulator, count the collisions and the successful ones. Then raise the concentration and watch the number of collisions per second rise while the fraction that succeed stays the same.
Choose a method in the simulator that suits the reaction: something you can measure changing with time.
| Method | Suits | Example |
|---|---|---|
| gas volume (syringe) | a gas is produced | Mg + HCl, CaCO3 + HCl |
| mass loss on a balance | a dense gas escapes | CaCO3 + HCl losing CO2 |
| colour (colorimeter) | a coloured species appears or fades | iodine formed or used up |
| turbidity (disappearing cross) | a precipitate forms | Na2S2O3 + HCl → S; rate ∝ 1/time |
| titration of quenched samples | an acid, base or iodine changes | hydrolysis of an ester |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which is a correct unit for rate of reaction?
According to collision theory, particles react only when they collide:
Why does increasing the concentration of an acid increase the rate of its reaction with magnesium?
How is the initial rate found from a volume-time graph?
The rate at any moment is the gradient of the curve at that moment. Draw a tangent, pick two well-separated points on it, and divide the change in y by the change in x.
Increasing the concentration does not give the particles more energy; it only increases the frequency of collisions, so the number of successful collisions per second rises. Say “more frequent successful collisions”, not just “more collisions”. Temperature is the factor that changes the energy of the particles, covered in the next lesson.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
48.0 cm3 of gas is collected in the first 2.00 minutes. What is the mean rate?
Sodium thiosulfate reacts with hydrochloric acid to form a sulfur precipitate. The cross disappears after 40 s. What is a suitable measure of the rate?
Why does powdered calcium carbonate react faster with acid than the same mass of marble chips?
Why does the gradient of a volume-time graph decrease as the reaction proceeds?
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