Nature of reactants

The nature of reactants is different from the other factors that affect rates because it explains the difference in rates of two different reactions but cannot be used to make one reaction go faster or slower. A simple example would be dropping a piece of zinc into water and comparing the reaction to what happens when a piece of sodium is dropped into water.

When the zinc is dropped into water a single displacement reaction occurs that produces hydrogen gas zinc hydroxide and heat. If you watch the reaction carefully you may be able to see the bubbles of hydrogen forming on the zinc, and if you given time you may be able to sense the increase in temperature.

When sodium is dropped into water a similar reaction takes place producing hydrogen gas, sodium hydroxide and heat. However this reaction happens fast enough that you can see fizzing on the surface of the sodium and can hear the hydrogen bubbles being released. If the piece of sodium is of a decent size (about that of a pea or larger) so much heat will be generated, so quickly, that the hydrogen will be ignited resulting in an explosion.

In both cases, a single displacement reaction occurs between a metal and water. Both reaction produce hydrogen gas and heat, but the result appears very different due to the rate at which the hydrogen and heat are produced. So, why the difference?

In each reaction, the metal is losing an electron or two. Some energy must be absorbed by the metal to release the electron and then energy is given off when the electron is added to a hydrogen atom. Since both reactions include adding the electron to hydrogen, the difference, in these two reactions, must be the amount of energy required to remove the electron(s) from the metal.

The amount of energy required to remove an electron from a neutral atom is called ionization energy. Sodium, as an alkali metal, has a very low ionization energy while zincs (although not really high) is much higher. This means that more collisions between water molecules and the sodium will have enough energy to steal an electron, than between water and zinc even if all things start at the same temperature. Since more collisions have enough energy, more collisions will result in reactions. Thus, the reaction will go faster.

Of course not all reactions involve ionization energy. But, in every reaction there is an amount of energy required for a collision to result in a reaction and the lower that amount, the more collisions will be successful and the faster the reaction will occur.
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