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

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.

The table below list the bond energies or bond enthalpies as they are often called for the H-H, O=O and O-H bonds. The bond energy or bond enthalpy is the amount of energy needed to break 1 mole of bonds in a molecule to form individual atoms. The higher the bond energy the stronger the covalent bond and the greater amount of energy needed to break it and separate the atoms.

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 atoms and 436 kilojoules of energy are required to break 1 mole of hydrogen molecules into two moles of hydrogen atoms.

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?

Well bond breaking is an endothermic process that requires energy but bond formation is exothermic, it 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 broken and bonds 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_(l)

2H_2(g) + O_2(g) → H₂O_(l)

An outline of the chemical 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 below). Here the 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 bonds. The bonds formed will be the O-H bonds in the water molecules (see step 4 in image). Four O-H 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 bonds holding the molecules together in the reactants are broken and this is an endothermic step requiring an input of heat energy to break up the reactant molecules
• Secondly a step which involves the formation of new chemical bonds in the product molecules. This bond formation is an exothermic step and will release heat energy.

The overall amount of heat energy released or taken in during a chemical reaction, that is whether it is an exothermic reaction releasing heat energy to the surroundings or an endothermic reaction absorbing heat 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 as the amount of heat energy stored in the reacting chemicals, it is given the symbol H. So the amount of heat energy released or taken in by 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 examples 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 release or taken in), see image below:

The 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.

Practice questions

Check your understanding - Questions on energy profile diagrams

Check your understanding - Additional questions on energy profile diagrams

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