An energy level diagram will show you the energy changes that take place during a chemical reaction and will immediately show if the reaction is an exothermic or endothermic one. As an example consider the reaction between hydrogen and oxygen to make water (hydrogen oxide), equations for this reaction are shown below:

In this combustion reaction two molecules/moles of hydrogen react with one molecule/mole of oxygen to make two molecules/moles of water. If you study the image carefully you should notice that the hydrogen atoms; which were once part of a hydrogen molecule in the reactants are now separated and joined to an atom of oxygen in the products. Similarly the two oxygen atoms which were joined together in a molecule of oxygen are now separated from each other and are now combined to atoms of hydrogen in the water molecules.

Bond breaking- an endothermic process

This tells us that before any reaction can take place all the covalent bonds holding the atoms together in the reactants must be broken. However the breaking of covalent bonds is an endothermic process; it will require a large input of energy since covalent bonds are strong bonds. You can imagine that dismantling and breaking apart molecules consisting of strong covalent bonds requires a lot of energy.

Bond energies or bond enthalpies

You can see from the table that you need 498 kilojoules of energy to break 1 mole of O=O bonds and separate the oxygen molecules into two individual oxygen atoms and that 436 kilojoules of energy are required to break 1 mole of hydrogen molecules into two moles of hydrogen atoms:

Bond dissociation and bond formation

Remember the law of conservation of energy. Energy cannot be created or destroyed; it can only change from one form to another. If it takes 498 kJ/mol of energy to break the covalent bonds holding the oxygen molecules together then what do you think will happen if you reverse the above equation and join the two moles of oxygen atoms together to form 1 mole of oxygen molecules?

While bond breaking is an endothermic process that requires an energy input, bond formation is an exothermic process that releases heat energy to the surroundings; it is simply the opposite of bond breaking in terms of energy change. If a chemical bond has a bond energy of 100 kJ/mol then it needs 100 kJ/mol to break the covalent bonds and 100 kJ/mol of heat energy will be released if you form these same covalent bonds.

Energy profile diagrams

We can draw an energy profile diagram for the reaction of hydrogen with oxygen to form water. These energy profile diagrams outline the energy changes taking place during a chemical reaction in terms of bonds being broken and bonds being formed and perhaps most importantly they will show immediately whether the reaction is an exothermic or an endothermic reaction. So let's look at the energy changes that take place when hydrogen and oxygen react to form water vapour. The word and symbolic equation for this reaction is shown below:

hydrogen_(g) + oxygen_(g) → hydrogen oxide_(g)

2H_2(g) + O_2(g) → 2H₂O_(g)

An outline of the covalent bonds being broken and formed as this reaction takes place are outlined in the diagram below:

• Step 1. Start with the reactants. The horizontal light blue line under the reactants hydrogen and oxygen molecules in the image above represents the amount of energy stored in the bonds of the reactants. The first step in any chemical reaction is breaking the covalent bonds in the reactant molecules to form individual atoms. This as we have said is an endothermic process and requires an input of energy. It is the energy needed to start or activate the reaction and it is called the activation energy (see step 2 in image above). Here the covalent bonds between the hydrogen atoms in the hydrogen molecules are broken to form two hydrogen atoms. Similarly the bonds holding the oxygen molecules together are broken to form individual oxygen atoms.


• Step3. The individual separate atoms now form new covalent bonds. The bonds formed will be the O-H bonds in the water molecules (see step 4 in image). Four O-H covalent bonds will form, two in each molecule of water. This bond formation step is an exothermic process (see step 4 in image) and energy will be released by this step.

This means that there are two basic steps in this chemical reaction:

• First the covalent bonds holding the molecules together in the reactants are broken and this is an endothermic step requiring an input of energy to break up the reactant molecules
• Secondly a step which involves the formation of new covalent bonds in the product molecules. This bond formation is an exothermic step and will release energy.

Enthalpy change

The overall amount of energy released or taken in during a chemical reaction, that is whether it is an exothermic reaction releasing energy to the surroundings or an endothermic reaction absorbing energy from the surroundings is called the enthalpy change for the reaction and it is given the symbol ΔH (pronounced delta H), where Δ is the Greek symbol delta which is often used in chemistry to represent the difference between two quantities and H is the symbol used for enthalpy. You can simply think of enthalpy change as the amount of heat energy taken in or given out during a chemical reaction. The enthalpy change ΔH for a chemical reaction is calculated using the formula below:

ΔH = Σ(energy required to break the reactants bonds ) - Σ( energy released by bond formation in the products)

The Greek symbol Σ (sigma) means sum. For example on how to calculate the enthalpy changes in a chemical reaction click the bond enthalpy and energies link below or click here.

Exothermic or endothermic?

We can simplify the diagram above to give two simple graphs to show the difference between exothermic and endothermic reactions in terms of the enthalpy of reaction (that is the amount of heat energy released or taken in), see image below:

Energy profile diagrams

These energy profile diagrams show how the energy stored in the reactants and products chemical bonds changes as the reaction takes place. For all chemical reactions, both exothermic and endothermic the reactants need to be supplied with energy to break the bonds in the reactants, this is the activation energy. Once all the bonds in the reactants are broken new bonds can form in the products; remember bond formation releases energy and the stronger the bonds formed in the products the more energy will be released.

Energy profile diagrams and catalysts

So far we have looked at energy profile diagrams without thinking about catalysts, but in real chemical reactions we often add a catalyst to speed things up. However there are a few important facts you need to be aware of; a catalyst does not change the overall energy of the reactants or products and it does not change the enthalpy change ΔH . What it does change is the activation energy (E_a).

A catalyst provides an alternative reaction pathway with a lower activation energy. That means fewer collisions need enough energy to be successful, so the reaction happens faster. The starting and finishing energy levels stay exactly the same, but the “hump” in the energy profile diagram is lower when a catalyst is present. The diagrams below show this clearly for both exothermic and endothermic reactions, you should notice that the only thing that changes is the height of the activation energy barrier in both the reaction when a catalyst is used.

In the diagrams below the green line represents the reaction profile for a catalysed reaction while the blue line is for an uncatalysed reaction. In both the exothermic and endothermic reactions you can see that the activation energy is much less when a catalyst is used.

Build your own energy profile diagram

In the activity below there are three energy profile diagrams that you can draw, simply click the challenge button to select the energy profile diagram to draw and then click the remaining options to complete the diagram. Draw or sketch what you think the energy profile diagram should look like then compare your sketch to the one in the activity.

Summary table

Why not use the summary table below to make a few flashcards to highlight the main points on energy profile diagrams.

Self-check quiz

Try the quick quiz below to test your understanding- click the button to start the quiz.

Key points

• An energy profile (energy level) diagram shows how the energy changes as reactants turn into products.
• Bond breaking always needs an input of energy, so it is endothermic.
• Bond formation releases energy to the surroundings, so it is exothermic.
• The activation energy is the minimum energy needed to start the reaction (to get bonds breaking).
• Exothermic reactions release energy overall because more energy is released when new bonds form than is needed to break the original bonds.
• Endothermic reactions take in energy overall because more energy is needed to break bonds than is released when new bonds form.
• The enthalpy change for a reaction is the overall energy change, and it depends on the balance between energy needed for bond breaking and energy released by bond formation.

Practice questions and self-check quiz

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