Causes of Energy Changes in Chemical Reactions

What causes the energy changes in chemical reactions?

When a chemical reaction occurs, bonds in the reactants break, while new bonds form in the product. The following example explains this. Hydrogen reacts with oxygen to form water, according to the following equation:

\[2\text{H}_{2}\text{(g)} + \text{O}_{2}\text{(g)} → 2\text{H}_{2}\text{O}\text{(g)}\]

In this reaction, the bond between the two hydrogen atoms in the \(\text{H}_{2}\) molecule will break, as will the bond between the oxygen atoms in the \(\text{O}_{2}\) molecule. New bonds will form between the two hydrogen atoms and the single oxygen atom in the water molecule that is formed as the product.

For bonds to break, energy must be absorbed. When new bonds form, energy is released. The energy that is needed to break a bond is called the bond energy or bond dissociation energy. Bond energies are measured in units of \(\text{kJ·mol$^{-1}$}\).

Remember when we discussed bonding (tutorial 3) we used the following energy diagram:

Graph showing the changes in energy that take place as the distance between two atoms changes.

We can use this diagram to understand why bond breaking requires energy and bond making releases energy. Point X on the diagram is at the lowest energy. When a bond breaks, the atoms move apart and the distance between them increases (i.e. the atom moves to the right on the \(x\)-axis or from point X to point A). Looking at the diagram we see that when this happens, the energy increases (i.e. the energy at point A is greater than the energy at point X). So when a bond breaks energy is needed.

When a bond forms the atoms move closer together and the distance between them decreases (i.e. the atom moves to the left on the \(x\)-axis or from point A to point X). Looking at the diagram we see that when this happens, the energy decreases (i.e. the energy at point X is less than the energy at point A). So when a bond forms energy is released.

Looking at the example of hydrogen reacting with oxygen to form water:

\[2\text{H}_{2}\text{(g)} + \text{O}_{2}\text{(g)} → 2\text{H}_{2}\text{O}\text{(g)}\]

We see that energy is needed to break the bonds in the hydrogen molecule and to break the bonds in the oxygen molecule). And we also see that energy is released when hydrogen and oxygen bond to form water). When we look at the entire reaction and consider both bond breaking and bond forming we need to look at the enthalpy of the system.

As we learn about exothermic and endothermic reactions we will see more on the concept of enthalpy.