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 of how the nature of the reactants influences the rates of reactions 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 place a thermometer in the water, you may be able to measure an increase in temperature.

On the other hand, 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, and so quickly, that the hydrogen will be ignited resulting in an small explosion.

In both cases, a single displacement reaction occurs between a metal and water. Both reaction produce a metal hydroxide, 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.

In simple terms, the energy to remove the electron from the metal atom is the activation energy. 

To be (much) more specific, 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 zinc's ionization energy (although not really high) is much higher.

Because the ionization energy of sodium is small, many of the collisions between water and sodium will already have enough energy for the electron to be transferred. Since the ionization energy of zinc is much higher, a smaller number of the water/zinc collisions will have the energy needed for the reaction. Since more sodium/water collisions have enough energy, more of those collisions will result in reactions. Thus, the sodium 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 (activation energy) and the lower that amount, the more collisions will be successful and the faster the reaction will occur.