Chain Reaction
Definition and meaning of Chain Reaction in chemistry.
A chain reaction is a series of chemical steps that keeps itself going. It starts when a highly reactive molecule creates another reactive molecule as it reacts. This new molecule then causes the next step to happen. The cycle repeats over and over until the reactive molecules are destroyed.
In more detail
A chain reaction always happens in three distinct stages. The first stage is called initiation. This is when heat or light breaks a stable bond to create the first reactive particles.
These particles are usually free radicals with an unpaired electron. The second stage is called propagation. The free radical bumps into a stable molecule and reacts with it.
This forms a new product, but it also creates a brand new free radical. This new radical goes on to react with another stable molecule. This single cycle can repeat thousands of times in a fraction of a second.
The final stage is called termination. This happens when two free radicals bump into each other and stick together. They form a stable molecule, which removes the reactive particles from the mix.
When enough radicals are destroyed, the chain reaction finally stops. Sometimes, one propagation step produces two or more new radicals instead of just one. This is called a branched chain reaction. It causes the reaction to speed up rapidly and often leads to an explosion.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key stages | Initiation, propagation, termination |
| Chain carriers | Free radicals (e.g., Cl•, CH3•) |
| Related phenomenon | Branched chain reactions cause explosions |
| Initiation trigger | Often heat or ultraviolet light |
| Termination cause | Two radicals colliding to form a stable bond |
A classic example is the reaction between methane gas and chlorine gas. First, ultraviolet light hits a chlorine molecule and splits it into two chlorine radicals. Next, a chlorine radical steals a hydrogen atom from methane. This creates a methyl radical and a molecule of hydrogen chloride. The methyl radical then attacks a new chlorine molecule. This forms chloromethane and a brand new chlorine radical. This new chlorine radical keeps the chain going until two radicals finally hit each other.
Frequently asked questions
What is the difference between a chain reaction and a branched chain reaction?
In a simple chain reaction, each step uses one radical and makes exactly one new radical. In a branched chain reaction, one step creates multiple new radicals. This causes the number of reactions to grow very fast and can cause an explosion.
Why do chain reactions eventually stop?
They stop because of termination steps. This happens when two reactive free radicals bump into each other and form a stable molecule. This removes the reactive pieces needed to keep the cycle going.
What makes the first free radicals in a chain reaction?
The first radicals are usually made by adding energy to the system. This energy often comes from intense heat or ultraviolet light. The energy breaks a weak chemical bond evenly to create two reactive pieces.