Reaction Rate Influences

Core Concepts

In this article, you will learn about the different factors that can impact the rate of a reaction, such as temperature, concentration, and surface area of reactants. You will also learn about the collision model, and how it can be used to explain how temperature and concentration affect reaction rate. Finally, you will learn about catalysts and how they can speed up reactions while learning how to read reaction coordinate diagrams.

Topics Covered in Other Articles

The Collision Model — Temperature, concentration, and surface area

The collision model presents reactions as collisions between reactant molecules and categorizes each collision as either effective or ineffective. For a collision to be effective, meaning a reaction takes place, the collisions must have enough energy, known as the activation energy, and the molecules must have the proper orientation. Activation energy is the energy needed to start a reaction.

The image above shows orientation in the collision model. The first two collisions are ineffective because the molecules aren’t facing the correct way, so they bounce off of each other and no reaction occurs. The orientation of the molecules to each other when they collide allows a reaction to occur in the third example.

More collisions means that there will be more successful collisions, and the reaction will progress faster, or have an increased reaction rate. A reaction with increased temperature will result in more successful collisions, due to an increase in the kinetic energy of molecules. This heightened kinetic energy causes molecules to move faster and collide with greater force and frequency. An increased concentration of reactants also increases the number of successful collisions, as there are more reactant molecule collisions. For reactants which are gases, increased pressure will also result in more successful collisions. The reaction rate increases in this case because there are more molecules per a unit area.

Collision Model – Surface Area

Following the collision model, a larger surface area can also speed up a reaction. This is because a larger surface area provides more sites for collisions to occur. As a result, there is a higher probability of reactant molecules coming into contact with the active sites on the surface, leading to a higher reaction rate. This is one reason why for certain reactions chemists use reactants that are ground up into powder — to help the reaction progress faster. A powder will react faster than a reactant that is a large solid block for example.

Catalysts — Reaction rate diagrams

A catalyst is a substance that speeds up a chemical reaction without itself being consumed in the process. There are two types of catalysts: homogenous, which is in the same phase as the reactants, and heterogenous, which is in a different phase than the reactants. A catalyst speeds up the reaction by altering its mechanism to have lower activation energy, which increases the rate of the reaction. This can be shown with reaction rate diagrams, which are also sometimes called reaction coordinate diagrams.

A reaction coordinate diagram showing how the reaction rate is affected by the presence of a catalyst by it lowering the activation energy.

At the beginning (left side) of the diagram, you have the reactants. The height of the first “hill” in the graph is the activation energy. This is the energy the reactants need to reach to get over the hill and turn into products. Activation energy can be thought of as the energy “hurdle” that reactant molecules need to overcome to turn into products. With enough energy, the reactants form what is called an activated complex. The activated complex, represented by the top of the “hill”, also called the transition state, is the high-energy state which the molecules must go through to “get to the other side” and form product. Notice how the hill for the reaction with a catalyst is lower, meaning it has lower activation energy. That’s the power of catalysts!

Can you get any more information from reaction coordinate diagrams?

Yes! The distance between the reactant line and the product line is the \Delta E of the reaction. Some graphs will also have flat lines in the middle, with intermediates. This can tip you off that the reaction represented by the diagram you’re looking at may be catalyzed.

A more complicated example of a reaction coordinate diagram. This one represents a reaction that has two transition states, and an intermediate that is formed and then used up. It also marks the distance between reactants and products as \Delta G.

Enzymes are a type of catalyst that increase the rate of biochemical reactions in living organisms. Enzymes function by providing an alternative reaction mechanism that lowers the activation energy required for a reaction to occur. But enzymes are unique in that each is designed to work only with extremely particular substrates, unlike inorganic catalysts, which can be used to speed up many different reactions. Some enzymes decrease activation energy of reactions necessary for life by a small but significant amount that allows the reaction to occur. Without enzymes, biochemical reactions necessary for life wouldn’t be able to occur! Learn more about enzymes here.

The Arrhenius Equation — Quantifying reaction rate

The Arrhenius equation is used to determine the activation energy or rate constant of a chemical reaction as the temperature changes.

k = A \cdot e^{-\frac{E_a}{RT}}

The units are as follows:

  • k represents the rate constant of the reaction. The units are dependent on the reaction order.
  • A is the frequency factor. Each reaction has a unique A value.
  • e is a unitless mathematical constant that approximates to 2.718.
  • E_{a} represents activation energy, usually in J or kJ.
  • R is the ideal gas constant (8.314 \frac{\mathrm{J}}{\mathrm{mol\cdot K}})
  • T is the absolute temperature in Kelvin.

The Arrhenius Equation can also be expressed as the following:

\ln\left(\frac{k_1}{k_2}\right) = -RE_a\left(\frac{1}{T_1} - \frac{1}{T_2}\right)

This version allows you to see how the rate constant of an elementary reaction changes when temperature is changed by using the activation energy. Learn more about how to derive, graph, and visualize the Arrhenius equation here.

For More Help, Watch our Interactive Video on the Influences on Reaction Rate