Topic 5: Chemical energetics

Every chemical reaction involves a change in energy, usually seen as a change in temperature. Some reactions give out heat to their surroundings, while others take heat in. Understanding why means looking at the energy stored in chemical bonds.

Exothermic and endothermic reactions

An exothermic reaction transfers energy out to the surroundings, almost always as heat. Because energy leaves the reaction mixture, the temperature of the surroundings rises. Common examples include the combustion (burning) of fuels, the neutralisation of an acid with an alkali, the reaction of reactive metals with acids, and respiration in living cells. An endothermic reaction is the opposite: it takes in energy from the surroundings, so the temperature of the surroundings falls. Endothermic changes are less common but include the thermal decomposition of calcium carbonate, photosynthesis, and the dissolving of certain salts such as ammonium nitrate in water. A simple way to remember it: EXO means EXit, energy leaves and things warm up; ENDO means energy goes INto the reaction and things cool down.

Observing the temperature change

You can tell which type of change has happened by measuring temperature with a thermometer. In an exothermic reaction the reading goes up; in an endothermic reaction the reading goes down. For example, mixing dilute hydrochloric acid with sodium hydroxide solution causes the temperature to rise by several degrees because neutralisation is exothermic. In contrast, stirring ammonium nitrate crystals into water makes the beaker feel cold to touch as the temperature drops. When describing experiments, always refer to the change in the surroundings (the temperature you actually measure) and link it to whether energy was released or absorbed by the reaction itself.

Reaction pathway diagrams

A reaction pathway diagram (energy level diagram) shows the energy of the reactants and products on a vertical scale. In an exothermic reaction the products are at a lower energy level than the reactants, so a downward step represents energy lost to the surroundings. In an endothermic reaction the products are at a higher energy level than the reactants, shown as an upward step, because energy has been taken in. The overall change in energy between reactants and products is sometimes labelled as the energy change of the reaction.

Activation energy

Even an exothermic reaction usually needs a small push of energy to get started. This minimum amount of energy needed for colliding particles to react is called the activation energy. On a reaction pathway diagram it appears as a hump or hill that the reactants must climb over before falling to the product level. The size of this hump is the activation energy. A reaction with a large activation energy is harder to start, which is why a fuel often needs a spark or flame even though burning then releases far more energy than was supplied.

Bond breaking and bond making

Chemical reactions are really a process of breaking old bonds in the reactants and making new bonds in the products. Breaking a chemical bond always requires energy to pull the atoms apart, so bond breaking is an endothermic process. Making a new bond releases energy as atoms come together, so bond making is an exothermic process. Whether the overall reaction is exothermic or endothermic depends on the balance between these two. If more energy is released when new bonds form than is taken in to break the old bonds, the reaction is exothermic overall. If breaking bonds takes in more energy than is given out by forming bonds, the reaction is endothermic overall.

Bond energy calculations

A bond energy is the amount of energy needed to break one mole of a particular bond, measured in kilojoules per mole (kJ/mol). The same value is the energy released when that bond forms. To find the overall energy change of a reaction, add up the bond energies of all bonds broken in the reactants (energy in, taken as positive) and subtract the total of all bonds made in the products (energy out). The rule is: energy change = (total energy to break bonds) - (total energy released making bonds). A negative answer means the reaction is exothermic; a positive answer means it is endothermic.

Worked example

Consider hydrogen burning: H-H + (1/2) O=O gives H-O-H. For one mole of water, bonds broken are one H-H (436 kJ/mol) and half an O=O bond ((1/2) x 498 = 249 kJ/mol), giving 436 + 249 = 685 kJ taken in. Bonds made are two O-H bonds (2 x 464 = 928 kJ/mol) given out. Energy change = 685 - 928 = -243 kJ/mol. The answer is negative, so the reaction is exothermic, which agrees with the fact that burning hydrogen releases heat. Always show the breaking total and the making total clearly so the marker can follow your method.