What Is Reaction Rate?
Every chemical reaction takes time — but how much time varies enormously. A stick of dynamite explodes in milliseconds. Iron rusts over years. Milk goes sour in a few days. Reaction rate describes how quickly a reaction produces its products or uses up its reactants.
More precisely, reaction rate is the amount of product formed (or reactant used up) per unit of time.
- A fast reaction has a high reaction rate — products are formed quickly
- A slow reaction has a low reaction rate — products are formed slowly
Reaction rate is not about whether a reaction will happen — it is about how quickly it happens. Even a very slow reaction will eventually complete if given enough time and the right conditions.
Fast and Slow Reactions: Examples
The range of reaction speeds in chemistry is astonishing:
| Reaction | Approximate Speed |
|---|---|
| Combustion of petrol (engine) | Milliseconds |
| Neutralisation (acid + alkali) | Less than 1 second |
| Precipitation reactions | Seconds |
| Cooking an egg | Minutes |
| Bread going stale | Days |
| Iron rusting | Weeks to months |
| Formation of limestone caves | Thousands of years |
Some reactions, like explosions, release energy so fast that they are described as almost instantaneous. Others, like the weathering of rock, operate on geological timescales.
How Do We Measure Reaction Rate?
To measure how fast a reaction is going, we track how much of something changes over time. Common measurement methods include:
1. Volume of gas produced per unit of time If a reaction produces a gas, we can collect the gas and measure its volume at regular intervals. For example, when marble chips (calcium carbonate) react with hydrochloric acid, carbon dioxide gas is produced. By measuring how much CO₂ is collected each minute, we can calculate the rate.
2. Mass change per unit of time If a gas escapes from an open container, the mass of the container decreases. Placing the reaction on a balance and recording the mass every 30 seconds gives a rate in grams per second (g/s).
3. Change in colour or light transmission For reactions involving coloured substances, measuring how the colour changes over time gives the rate.
A simple way to express rate:
Rate = quantity of product formed ÷ time taken
or
Rate = quantity of reactant used up ÷ time taken
What Is Activation Energy?
For a chemical reaction to start, the reacting particles must have enough energy to break the existing chemical bonds. This minimum energy needed to start a reaction is called the activation energy.
Think of it like pushing a ball up a hill before it rolls down the other side. The ball can reach a lower energy state on the other side, but first it needs an energy push to get over the top of the hill.
- Reactions with low activation energy start easily and are often fast (e.g. neutralisation)
- Reactions with high activation energy are difficult to start and are often slow or need a high temperature (e.g. burning coal — you need to heat it first before it will burn on its own)
Activation energy explains why petrol does not simply burst into flames on its own even though the combustion reaction releases huge amounts of energy. A spark or flame is needed to provide that initial activation energy.
Collision Theory: Why Particles Must Collide
Chemical reactions happen when particles collide with each other. But not just any collision will do — the collision theory states that for a reaction to occur:
- Particles must collide with each other
- The collision must have enough energy (at least equal to the activation energy)
- The particles must collide with the correct orientation (they must hit each other in the right direction)
Imagine trying to unlock a door — you need to be in the right place (collision), press the card firmly enough (energy), and hold it the right way round (orientation). Only then does the door open (reaction occurs).
Most collisions between particles in a mixture do not lead to a reaction — the particles simply bounce off each other. Only the higher-energy collisions succeed.
Real world: Why food rots faster in warm weather
Food spoilage is a chemical reaction driven by bacteria and enzymes. At room temperature, the particles involved in these chemical processes collide frequently and with enough energy to react — so food goes off quickly. In a refrigerator, the lower temperature means particles move more slowly and collide less often and with less energy. Fewer successful collisions mean a slower reaction rate, so food stays fresh much longer.
Fast Reactions: Explosions
Explosive reactions are among the fastest in chemistry. They involve a very large number of reactive particles in close proximity, with enough energy to start a chain reaction — where the energy released by the first reaction immediately triggers more reactions.
Examples include:
- TNT and other explosives in mining and demolition
- The airbag in your car, which inflates in about 30 milliseconds using a rapid gas-producing reaction
- Fireworks, where metal compounds react rapidly to produce coloured light
Real world: Car airbags and reaction rate
When a car crashes, a sensor triggers a rapid chemical reaction inside the airbag — sodium azide (NaN₃) decomposes to produce nitrogen gas almost instantly. The entire airbag inflates in around 30 milliseconds — far faster than a human can blink. This incredibly high reaction rate is what makes airbags effective safety devices.
Slow Reactions: Rusting
The rusting of iron is a very slow reaction between iron, oxygen, and water to form iron oxide (rust). Under normal conditions, this reaction has a relatively low rate — a piece of iron left outdoors might show visible rust after several weeks.
Slow reactions are not always unwanted. The slow decomposition of organic matter in soil returns nutrients to the ecosystem. The slow reactions in batteries allow them to release energy steadily over hours or days rather than all at once.
Practice Problems
A student collects gas from a reaction every 30 seconds and records the total volume. At 30 s she has 15 cm³, at 60 s she has 24 cm³, at 90 s she has 30 cm³. Is the reaction speeding up or slowing down over time? Explain.
Show answer
Explain using collision theory why a reaction that produces a gas slows down as the reaction proceeds.