The First Law of Thermodynamics states that energy cannot be created or destroyed but can be converted from one form into another. This law is directly applicable to energetics. Reactions involve the breaking and forming of chemical bonds and the excess energy cannot just disappear, but instead changes from bond energy to heat energy. Consider, for example, the reaction between carbon and oxygen. At 25 degrees C (77 degrees F) one mole of carbon (12 g) reacts with oxygen to liberate 393.5 kJ of energy. This liberated energy, called the enthalpy change of reaction, is liberated as heat. Enthalpy changes of reactions conducted at 77 degrees F and one atmosphere pressure are called standard enthalpy changes.
Chemical reactions will either liberate excess heat or draw in heat from the surroundings. The first type of reaction is called exothermic; the second is called endothermic. The addition of an alkali metal to water to form a metal hydroxide is an example of an exothermic reaction. Lithium, sodium, potassium, rubidium and cesium all react vigorously with water to form a hydroxide with the liberation of hydrogen gas. The heat generated in this reaction is sufficient to ignite this hydrogen, and the reaction with cesium is so strong that, depending on the amount of metal added, the explosive force generated is enough to shatter the glass vessel in which the reaction takes place. Exothermic reactions have negative changes of enthalpy, as there is a net release of energy during these reactions.
Endothermic reactions occur when the sum of the bonding energies of the end products is greater than of the reactants. Extra energy is required to form these products which is absorbed as heat from the surroundings. An example of an endothermic reaction is the addition of sodium carbonate to acetic acid to form sodium acetate, which has a positive enthalpy change. The addition of ammonium nitrate to water is another example of an endothermic reaction: adding the nitrate to cold water will lower its temperature enough to cause it to freeze.
In energetics, there are several important types of standard enthalpy change. The standard enthalpy change of formation of a compound is the enthalpy change when a compound is formed from its constituent elements. The standard enthalpy of combustion is the enthalpy change when an element or compound undergoes complete reaction with oxygen. Standard enthalpy of neutralization is the enthalpy change when one mole of acid is neutralized by one mole of a base. The reaction between acetic acid and sodium carbonate is an example of neutralization. Finally, the standard enthalpy change of solution is when one mole of a substance is dissolved in a solvent. This can be surprisingly violent, for example adding sulfuric acid to water is strongly exothermic.