Temperature Dependence

The measure of how fast a reaction proceeds is called the reaction rate. The rate of a chemical reaction is defined by the rate law, which describes the relationship between the speed of the reaction and the reactant concentrations. In this equation, k is the rate constant, A and B are the two reactants, and m and n are their respective reaction orders.

The rate constant converts the relationship to the proper units of rate, moles per liter per second. Thus, the rate constant has different units, depending on the overall order of the reaction. However, the rate constant holds more significance than simply unit conversion. The rate constant is related to the minimum amount of energy required for a chemical reaction to occur - called the activation energy.

In a reaction, the reactants are at an initial state of potential energy. As the reaction proceeds, it must overcome a certain potential energy, the activation energy, before reaching its final state. The net energy of the reaction is the difference between the initial and final states. This difference can be negative, meaning that the reaction releases energy, or positive, meaning that it absorbs energy.

If there is not enough energy available to overcome the activation energy, the reaction will not proceed. In some cases, energy can be supplied in the form of heat. This provides additional energy to overcome the barrier to activation, and the reaction can proceed. A catalyst may also be added, which provides an alternative lower activation energy pathway between the reactants and products.

Catalysts are not consumed in the reaction and, therefore, do not affect the net energy of the reaction. The activation energy is determined experimentally, and it is related to the reaction constant k by the Arrhenius equation where A is the pre-exponential or frequency factor, R is the universal gas constant, and T is the absolute temperature at which the reaction occurs.

From this equation, we know that increasing the reaction temperature or decreasing the activation energy increases the rate constant. Going back to the rate law equation, it follows that a higher rate constant results in a higher reaction rate. This makes sense because as temperature increases, molecules move faster and collide more frequently, resulting in an increased fraction of molecules with higher energy than the activation energy.

In this lab, you will learn how to measure the activation energy of a reaction experimentally using the decomposition of hydrogen peroxide as the model reaction.