Endothermic and Exothermic Reactions

All molecules have a certain amount of energy. Sometimes the reactants have less energy than the products, in which case they will absorb energy in an endothermic reaction. In other reactions, the reactants have more energy than the products so release this excess energy in an exothermic reaction.

Energy transfer in endothermic and exothermic reactions

Energy is conserved in chemical reactions, which means the amount of energy present at the start of a chemical reaction is equal to the amount of energy at the end. If the products have a lower amount of energy than the reactant, this means that energy must have been released to the surroundings. If the products have more energy than the reactants, energy has been absorbed from its surroundings.

Reactions which release heat energy are known as exothermic reactions and those which absorb heat energy are known as endothermic reactions. You can tell whether a reaction is exothermic or endothermic by monitoring the temperature of the surroundings – exothermic reactions increase the temperature of the surroundings by releasing heat whereas endothermic reactions decrease the temperature of the surroundings by absorbing heat.

Examples of exothermic reactions are combustion, most oxidation reactions and neutralisation. Exothermic reactions are used in things like self-heating cans and hand warmers. Examples of endothermic reactions are thermal decomposition reactions and the reaction of citric acid with sodium hydrogencarbonate. Endothermic reactions are used in some sports injury packs to help muscles cool after injury.

Reaction profiles

Reaction profiles show the amount of energy that a substance has at the start and end of a reaction. If you look at the reaction profile below and to the left, you can see that during an exothermic reaction the reactants lose energy as they form products. The difference in energy between the reactants and products is the enthalpy change which is negative. The initial increase in energy is the activation energy. Activation energy is the minimum amount of energy needed to get a reaction going.

The graph on the right shows an endothermic reaction. You can see from the diagram that the products have more energy than the reactants. This is because energy has been absorbed from the surroundings so the enthalpy change is positive.

Bond energies (triple science)

During a chemical reaction:

• Energy is absorbed to break bonds in the reactants

• Energy is released to form bonds in the products

The change in heat energy that occurs during a reaction is referred to as the change in enthalpy. The overall enthalpy change for a reaction is the difference between the total energy needed to break the bonds in the reactants and the total energy released forming bonds in the products.

If a reaction is exothermic, more energy will be released forming the products than was used to break the bonds in the reactants. If a reaction is endothermic, more energy is required to break the bonds in the reactants than is released forming bonds in the products.

You may be asked to calculate the overall energy change of a reaction using bond energies. Bond energies are simply the amount of energy released / absorbed when breaking / forming the bond. Have a look at the worked example below to see how to do this.

Worked example: bond energies

Methane burns in oxygen in the following reaction:
CH4 + 2O2 --> CO2 + 2H2O
The bond energies are:
C-H: 413 kJ/mol
O=O: 498 kJ/mol
C=O: 745 kJ/mol
O-H: 467 kJ/mol

Calculate the enthalpy change for the reaction.

Answer:

With these questions you may find it useful to draw out the molecules to make sure you don’t miss out any bonds. Remember that when we have a big number in front of the molecule, we have to multiply the molecule by that number.

• Use the equation: enthalpy change = bonds broken - bonds formed

• Energy to break bonds in the reactants: (4 x 413) + (2 x 498) = 2648 kJ/mol

• Energy needed to form bonds in the products: (2 x 745) + (4 x 467) = 3358 kJ/mol

• Enthalpy change = 2648 - 3358 = -710 kJ/mol

• Enthalpy change is negative which means this is an exothermic reaction.

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