4.1 The Nature of the Reactants

We can now turn our attention to a question we asked earlier - how can the rate of a chemical reaction be changed? There are four main of factors to consider:

• Nature of the reactants
• Temperature
• Concentration and Pressure
• Catalysts

Nature of Reactants

There are several points to consider when we examine how the properties of the reactants affects reaction rate.

• During chemical reactions, chemical bonds are broken and new bonds are formed. The nature (or type) of these chemical bonds - and how readily they are broken and formed - plays a critical role in the rate of a reaction. When the reaction involves primarily the exchange of electrons (as occurs in Redox reactions, the topic of our last unit of study), reactions tend to be very rapid.

  For example, consider this very fast double displacement reaction that involves the formation of a yellow precipitate, barium chromate:

• \(\ce{Ba(NO_3)_2(aq) + Na_2CrO_4(aq) \rightarrow BaCrO_4(s) + 2NaNO_3(aq)}\)

We can write the net ionic equation for this reaction:

\(\ce{Ba^{2+} (aq) + CrO_4^{2-} \rightarrow BaCrO_4(s)}\)

Reactions such as this that involve ions in solution tend to be very rapid.

Closely related to phase is the amount of exposed surface area of the reacting particles. If you put a lit match to a large log you wouldn't get much of a fire started. However, if you took the same log and increased it's surface area by chopping it into kindling, you'd have much more success in starting your fire.

• As you might expect, however, if we had combined solid barium nitrate with solid sodium chromate, the reaction would be so slow that we would not be able to detect it. The phase of the reacting particles is important. Reactants in solution, liquids, and gases will react much faster than solids.

• Closely related to phase is the amount of exposed surface area of the reacting particles. If you put a lit match to a large log you wouldn't get much of a fire started. However, if you took the same log and increased it's surface area by chopping it into kindling, you'd have much more success in starting your fire.


• Closely related to phase is the amount of exposed surface area of the reacting particles. If you put a lit match to a large log you wouldn't get much of a fire started. However, if you took the same log and increased it's surface area by chopping it into kindling, you'd have much more success in starting your fire.
  
• Reactions involving covalently-bonded molecules tend to be slow unless highly exothermic. The decomposition of hydrogen peroxide into hydrogen gas and oxygen gas is a spontaneous reaction, but not a particularly fast one.

\(\ce{H_2O_2(l) \rightarrow H_2(g) +O_2(g)}\)

• Finally, we will include stirring as another factor that increases the rate of a reaction. Stirring causes the particles to collide more frequently since they will come into contact with one another more frequently.

One other thing - notice we are discussing the nature of the reactants here. Characteristics of the products - whether they are solids or gases, ionic or covalently-bonded, etc., are important in determining reaction rate.