EXOTHERMIC AND ENDOTHERMIC REACTIONS

1. Introduction

Molecules have energy, and when they react, changes in the total energy of the reacting molecules take place. Some of this energy is available to do work, either directly or indirectly.

It can be seen from the above that energy changes during chemical reactions are of considerable practical importance, and so, the theory behind such changes is of great interest. At present, most of our energy needs rely on the availability of suitable sources of chemical energy. Note that nuclear energy, such is utilized in nuclear power stations to provide energy, and solar energy, as used in solar heaters, are NOT forms of chemical energy.

2. The energy of molecules

The total energy, E, of a molecule is made up of various energies:

The total energy of a molecule will be the sum of these energies:

E = Eb + Et + Ev + Er + En

The change in energy ΔE that occurs during a reaction will be therefore:

ΔE = ΔEb + ΔEt + ΔEv + ΔEr + ΔEn

for each molecule that takes part in the reaction. It turns out that at ordinary temperatures, the changes in bonding energies is the dominant term, so we can write

ΔE ≈ ΔEb

Since a reaction is normally carried out at constant pressure or constant volume, we define a quantity ΔH, the HEAT OF REACTION, () which is the change in energy of a reaction occurring at constant pressure. We unfortunately tend to be sloppy and talk about the "energy change" in a reaction. Further, all such changes have values that are affected by temperature and pressure, so that one should mention the values of these two variables. The STANDARD HEAT OF REACTION is the value for the reaction taking place at 298 K and 103 kPa.

3. Calculation of ΔH from bond energies

The Law of Conservation of Energy, also known as the First Law of Thermodynamics, states that

Law of Conservation of Energy:
"In a system that is completely isolated from its surroundings, the total energy remains constant."

This means that we can calculate the heat of a reaction, given the bond energies of the participating molecules.

4. Endothermic and exothermic reactions

The heat of reaction is frequently discerned the senses as heat - in the laboratory, one frequently can feel a test tube getting warmer or colder as a reaction takes place.

ΔH is the DIFFERENCE between the energy of the products and the energy of the reactants: ΔH = Eproducts - Ereactants

This is shown in the diagram on the right. In this case, the reactants have a higher energy than the products, and the numerical value of ΔH will therefore be negative., that is, (ΔH < 0)

Reactions which have a negative value of ΔH, i.e., ΔH < 0, are said to be EXOTHERMIC, and energy is released (usually as heat) during the conversion of the reactants into products.

On the other hand, reactions which have a positive value of ΔH, i.e., ΔH > 0, are said to be ENDOTHERMIC, and energy is taken up (usually as heat) during the conversion of the reactants into products.


5. The energy of activation

When a reaction proceeds from reactants to products, the change takes place by a complicated route. In the diagram on the right, we will plot the energy change as the reaction proceeds. Before you read on, click here to see how the energy changes as the reaction proceeds.

The overall change in energy is ΔH = P - R, but as the reaction progresses, the energy, H, follows the curve in red. It first rises well above the energy value of the reactants, reaches a maximum, then drops to the energy value for the products.

The "hump" in the curve represents an "energy barrier" of value Ea which is called the ENERGY OF ACTIVATION for the reaction. It may be defined as the minimum energy required to start the chemical reaction. Reactions with high values of Ea take place more slowly than those with small Ea values.

At the peak of the curve, the molecules that have acquired the activation energy have been converted to an ACTIVATED COMPLEX. This is neither a reactant nor a product, and can be converted to either. We can therefore also define the activation energy as the energy required to convert a reactant molecule to its activated complex. The activation energy is measured in joules per mole (J·mol-1).

A CATALYST increases the rate of a reaction by reducing the energy of activation of that reaction.

6. Additional questions