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In chemical reactions, the threshold energy is the minimum amount of energy that molecules must have for a reaction to take place. Imagine molecules as tiny moving balls. When they collide with each other, they need a certain amount of energy to break the old bonds and form new ones. If the molecules don’t have enough energy, the collision won’t cause a reaction. But if the molecules have enough energy, at least the threshold energy, the reaction can happen.
Example of Threshold Energy:
For example, let’s look at the reaction between hydrogen (H₂) and oxygen (O₂) to form water (H₂O):
2H₂ + O₂ → 2H₂O
Hydrogen and oxygen gases can exist together, but they won’t react unless they have enough energy. At room temperature, the molecules of hydrogen and oxygen have too little energy to react. However, if you provide a spark or heat, this energy gives the molecules enough threshold energy to react. The spark provides the energy needed to start breaking the bonds of hydrogen and oxygen molecules, causing the reaction to occur and form water.
Difference between Threshold Energy and Activation Energy:
Threshold energy is often mixed up with activation energy, but they are different. Activation energy is the extra energy needed to push the reaction forward once the molecules have collided. Threshold energy, on the other hand, is the total amount of energy the molecules need to start the reaction. It includes the molecules’ own energy and the energy they need to get over the energy barrier for the reaction to occur.
Relationship between Thermal Energy, Kinetic Energy, and Activation Energy
The relationship between threshold energy and activation energy can be written like this:
Threshold Energy = Average Kinetic Energy of Reactants + Activation Energy
| Ethreshold = Einitial + Ea |
Where:
- Ethreshold is the threshold energy (the total energy needed for a reaction),
- Einitial is the energy the molecules already have when they start,
- Ea is the activation energy (the extra energy needed for the reaction to happen).
If the molecules already have enough energy at the start, then the threshold energy and activation energy are the same. This means they have just the right amount of energy to react.
Only the molecules that have energy equal to or more than the threshold energy will react. Molecules with less energy will just collide and bounce off, without causing a reaction.
It’s also important not to confuse threshold energy with internal energy. Internal energy is the energy molecules already have inside them. The difference between internal energy and threshold energy is what we call activation energy.
For a better understanding of how reactions happen, it is useful to study chemical kinetics. This helps explain how energy works in chemical reactions.
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