Energy level diagrams and bond energyIB MYP Chemistry: Revision notes

A reaction profile (energy level diagram) shows how the energy of the substances changes during a reaction. Energy is on the vertical axis and the progress of the reaction is along the horizontal axis.

To draw one:

• Draw a horizontal line at the energy level of the reactants on the left.
• Draw a horizontal line for the products on the right: lower than the reactants for an exothermic reaction, higher for an endothermic one.
• Join them with a curve that rises to a peak and then falls to the products.
• Label the axes, the reactants, the products, the activation energy and the overall energy change.

The overall energy change is the difference between the energy of the products and reactants. It is negative for exothermic reactions and positive for endothermic ones.

Even exothermic reactions need a push to start. The activation energy is the minimum energy the reactants need for the reaction to take place. On a reaction profile it is the height from the reactants' level to the top of the peak.

Example: methane does not burn on its own at room temperature. A spark or flame supplies the activation energy. Once the reaction starts, the energy it releases keeps it going.

A reaction with a high activation energy is slow at room temperature because few particles have enough energy to react.

Chemical reactions involve breaking the bonds in the reactants and making new bonds in the products.

• Breaking bonds takes in energy (endothermic).
• Making bonds releases energy (exothermic).

Whether the whole reaction is exothermic or endothermic depends on which is bigger:

• If more energy is released making bonds than is taken in breaking bonds, the reaction is exothermic.
• If more energy is taken in breaking bonds than is released making bonds, the reaction is endothermic.

Worked example: H₂ + Cl₂ → 2HCl. Breaking the bonds in the reactants takes in 679 kJ. Making the bonds in the products releases 864 kJ.

Energy released − energy taken in = 864 − 679 = 185 kJ released overall.

The overall energy change is −185 kJ, so the reaction is exothermic. On its reaction profile the products are 185 kJ below the reactants, and the peak shows the activation energy.

For an endothermic reaction, such as citric acid reacting with sodium hydrogencarbonate, more energy is taken in than released, the temperature falls and the products are above the reactants.