2.3 Threshold Energy

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Let's return to one of the key points of the collision theory - reacting particles must collide with sufficient force or energy.

Consider two cars about to become involved in an accident. If both cars are moving very slowly, say 2 km/hr when they collide, not much is going to happen. Perhaps the drivers don't even realize they've hit one another.

But if the cars are both travelling at 40 km/hr (the greater speed indicates the cars have more energy), then the collision is going to be effective and damage will be done.

In our scenario there is a certain speed at which damage will be done to the cars. In chemistry, we call this minimum amount of energy required for particles to collide successfully activation energy.

Activation energy
is the minimum amount of energy
required for a successful collision.

Symbol: E_a

Sometimes referred to as threshold energy.

We can graphically represent threshold energy on a kinetic energy diagram.
This graph illustrates that some particle have a low amount of kinetic energy (yellow region), some are very energetic and fast moving and have a lot of kinetic energy (blue), while most particles have some intermediate amount of kinetic energy (purple).
Threshold energy is the minimum amount of energy needed for particles to react. For a particular reaction, the threshold energy might be as shown here: Only particles that have at least as much energy as the required threshold energy will have enough energy to have a successful collision. Thus only particles in the green area of the graph will actually react because only they have more than the minimum amount of energy needed.
In the next graph representing a different reaction, the threshold energy is lower; more particles meet this minimum requirement, so more particles will successfully collide. This, in turn, means that this reaction will have a faster rate.