Surface Area of Reactants The greater the surface area of the reactants, the
greater the number of collisions; hence the faster reaction rate. For instance, a
cube of sugar will dissolve less quickly in water than the same amount of sugar
in loose form. Only the surface areas of solids and liquids can be changed: We
can increase the surface area of a solid by breaking it up or grinding it into a
powder, and a liquid’s surface area can be increased by spraying it out as a mist
of fine droplets.
Temperature The factor that has perhaps the most profound effect on reaction
rate is temperature. This is because a temperature change affects both the rate of
reactant collisions and the energy involved in the collisions. Remember that
temperature is a measure of the average kinetic energy of molecules. As the
temperature of the reactants is increased, the molecules move around faster; this
results in more frequent and energetic collisions and increases the likelihood that
a given collision will have sufficient energy to break bonds. A good rule of
thumb says that for every 10°C increase in temperature, the reaction rate will
double.
Nature of Reactants Since bond breaking is part of the reaction process, it
makes sense that reactant molecules composed of weaker bonds will react more
quickly than will reactant molecules held together by stronger bonds. Reactions
between dissolved ions tend to be rapid, since bond breaking has already
occurred, with the dissolution of the ionic substance.
Catalysts A catalyst increases the rate of a chemical reaction without being
consumed by it. Enzymes are examples of catalysts. They are involved in many
important biological reactions. How do catalysts accelerate reaction rates?
Before we get to that, let’s first consider the energy changes that occur in the
course of a reaction.