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Activation Energy

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Activation Energy
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How do fireworks light up the sky?

The sight of fireworks exploding across the night sky is always exciting. These materials, invented hundreds of years ago, can be dangerous if not handled properly. The chemicals do not react until the fuse burns down and heat is applied to the system. Then the rocket is launched and explodes high in the sky.

Activation Energy

Calcium metal stored in an argon atmosphere.

Why do some chemical reactions occur readily while others require input of heat in order to take place? If we mix metallic sodium with water, a reaction occurs immediately, releasing a great deal of heat (enough to ignite the hydrogen gas that is formed). Group II metals, such as calcium, react at a much slower rate. Unlike the extremely vigorous reaction with sodium, the reaction with calcium is slow enough that we can trap the hydrogen gas released.

Supplying reactant particles with energy causes the bonds between the atoms to vibrate with a greater frequency. This increase in vibrational energy makes a chemical bond more likely to break and a chemical reaction more likely to occur when those particles collide with other particles. The activation energy for a reaction is the minimum energy that colliding particles must have in order to undergo a reaction. Some reactions occur readily at room temperature because the reacting particles already have the requisite activation energy at that temperature. Other reactions only occur when heated because the particles do not have enough energy unless an external source of heat provides the particles with more kinetic energy.

Summary

  • Activation energy is defined for a chemical reaction.

Practice

Watch the video at the link below and answer the questions:

http://www.youtube.com/watch?v=u_1uLP30uxY

  1. Does magnesium react with water at room temperature?
  2. How did the speaker get magnesium to react with water?
  3. What is one product of this reaction?

Review

  1. Does sodium react faster or slower with water than calcium does?
  2. How does vibrational energy contribute to a reaction?
  3. Before some molecules react, what has to happen?

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