What is a reaction rate?
The rate of reaction is how quickly reactants are used up or products are formed. It can be measured as the amount of product formed per second, the volume of gas given off per minute, or the time taken for a colour change or for a solid to disappear.
Collision theory
Particles must collide to react. But most collisions just bounce off: to react, particles must hit with at least a minimum amount of energy, called the activation energy (Eₐ), and in a suitable orientation. The activation energy is like a hill the particles must climb before the reaction can “roll” on to products. Anything that makes successful collisions more frequent increases the rate.
Four factors that change the rate
| Factor | Effect | Why |
|---|---|---|
| Temperature ↑ | Rate ↑ (often strongly) | Particles move faster, collide more often, and a much larger fraction has energy ≥ Eₐ |
| Concentration (or gas pressure) ↑ | Rate ↑ | More particles in the same space collide more often |
| Surface area ↑ | Rate ↑ | More particles of a solid are exposed and available for collisions |
| Catalyst | Rate ↑ | Provides a pathway with a lower Eₐ, so more collisions succeed |
Reading the rate graph
The graph shows the amount of product against time. Its slope is the rate: steepest at the start, when reactants are plentiful, then flattening as they are used up. A steeper curve means a faster reaction. If the same amount of reactant is used, every curve eventually reaches the same final amount of product — the factors changehow fast the reaction goes, not how much product it can make.
Formulas
Rate = change in amount of product ÷ time takenThe fraction of collisions with enough energy depends strongly on temperature (the Arrhenius relationship):
k = A · e^(−Eₐ / RT)Here k is the rate constant, A depends on collision frequency and orientation, R is the gas constant and T is the absolute temperature. Lowering Eₐ (a catalyst) or raising T both increase k.
Interesting facts
- A fridge keeps food fresh by slowing the reactions of decay: as a rough rule, many reactions slow to about half their rate for every 10 °C drop in temperature.
- The catalytic converter in a car uses metals such as platinum to speed up reactions that turn harmful exhaust gases into less harmful ones.
- Enzymes are biological catalysts. Catalase, found in liver and potatoes, breaks down hydrogen peroxide millions of times faster than it would decompose on its own.
Key terms
- Activation energy (Eₐ)
- The minimum energy colliding particles need to react.
- Catalyst
- A substance that speeds up a reaction without being used up. Biological catalysts are called enzymes.
- Successful collision
- A collision with enough energy and the right orientation to produce a reaction.
- Surface area to volume ratio
- How much surface a solid has relative to its size; it increases as a solid is broken into smaller pieces.
Real-world applications
- Food storage: refrigerators slow the reactions that spoil food.
- Catalytic converters in cars use platinum-group metals to convert toxic gases quickly.
- Industry: the Haber process uses an iron catalyst, high pressure and a carefully chosen temperature to make ammonia fast enough to be economical.
- Safety: fine dust (flour, coal, sawdust) has so much surface area that it can explode — a serious hazard in mills and mines.
- Your body: enzymes speed up digestion and metabolism enormously at body temperature.
Common misconceptions
- “Every collision causes a reaction.” Only a small fraction succeed.
- “A catalyst gives more product.” It only gives the same product faster.
- “Temperature only makes particles collide more often.” The main effect is more collisions with enough energy.
What the model simplifies
The simulator is two-dimensional, ignores orientation, and treats the solid as a grid of identical atoms. Particle speeds are randomly refreshed to mimic collisions with water molecules, keeping a temperature-dependent spread of energies.
Key takeaways
- Reactions need collisions with energy at least equal to the activation energy.
- Temperature, concentration, surface area and catalysts all increase the rate.
- The slope of a product–time graph is the rate.
- Catalysts lower the activation energy and are not used up.