3.5C-3.7C - Energy profiles, bonds and enthalpy from bond energies

Reading energy level diagrams

An energy level diagram shows how the energy of the reacting substances changes as a reaction happens. The vertical axis is energy. The horizontal axis is the progress of the reaction; it is not time and it normally has no numerical scale.

The reactants and products are drawn as horizontal levels. The difference between those levels is the enthalpy change, ΔH.

For this course:

Some exam energy profiles also show a hump between reactants and products. The height from the reactant level to the top of the hump is the activation energy, E_a. In this lesson you only need it as a labelled energy barrier on the diagram; catalysts and alternative pathways are taught later.

Exothermic energy profiles

In an exothermic reaction, heat energy is given out to the surroundings. The products are at a lower energy level than the reactants, so the enthalpy change is negative.

[DIAGRAM: asset_name: Energy Profiles, Bonds and Enthalpy from Bond Energies - diagram 01; asset_slug: c41_energy_profiles_bonds_and_enthalpy_from_bond_energies__diagram_01; recommended_method: deterministic_drawing; description: Monochrome energy profile for an exothermic reaction such as H2 + Cl2 -> 2HCl, with energy on the vertical axis, reaction progress on the horizontal axis, reactants higher than products, a smooth activation-energy hump, an upward activation energy arrow from reactants to the peak, and a downward ΔH arrow from reactants to products labelled ΔH negative.]

For H2 + Cl2 -> 2HCl, the products have less energy than the reactants. The reaction releases energy overall, so ΔH is negative. If you are asked to draw the diagram, make the product line clearly lower than the reactant line and make the ΔH arrow run from the reactant level down to the product level.

Common Pearson mark-scheme details are:

• horizontal reactant and product energy levels in the correct positions
• correct formulae or names on the correct levels
• an activation-energy hump if the question asks for a profile
• ΔH shown as a vertical arrow or line between the two levels
• E_a or activation energy shown from the reactant level to the top of the hump

Endothermic energy profiles

In an endothermic reaction, heat energy is taken in from the surroundings. The products are at a higher energy level than the reactants, so the enthalpy change is positive.

[DIAGRAM: asset_name: Energy Profiles, Bonds and Enthalpy from Bond Energies - diagram 02; asset_slug: c41_energy_profiles_bonds_and_enthalpy_from_bond_energies__diagram_02; recommended_method: deterministic_drawing; description: Monochrome energy profile for an endothermic reaction, with energy on the vertical axis, reaction progress on the horizontal axis, reactants lower than products, a smooth activation-energy hump, an upward activation energy arrow from reactants to the peak, and an upward ΔH arrow from reactants to products labelled ΔH positive.]

The same diagram rules apply, but the product level is now above the reactant level. A correct endothermic diagram should make it visually obvious that energy has been taken in overall.

Do not decide the type of reaction from the size of the hump. The hump shows activation energy. The exothermic or endothermic label comes from comparing the energy levels of the reactants and products.