An endothermic process is one that absorbs energy from its surroundings. In a chemical reaction, two or more substances — the reactants — interact with one another to produce one or more new substances — the products. Where the energy contained in the products is less than that in the reactants, energy is released, and the reaction is said to be exothermic. In endothermic reactions, the products have more energy than the reactants, so energy is absorbed from their environment. Thus in exothermic reactions, the reactants lose heat to their surroundings, which grow hotter, while in endothermic reactions the reactants gain heat from their surroundings, which are cooled.
A chemical reaction involves the formation of bonds between atoms. Since a system will always try to reach its lowest energy state, bonds will only form if they result in the overall energy of the atoms after bonding being lower than it was before bonding. Thus, the formation of chemical bonds releases energy. In chemical reactions, however, bonds have to be broken before new compounds can form. Breaking a chemical bond requires energy and if more energy is required to break bonds within the reactants than is released by the formation of new bonds, the overall reaction is endothermic, because there is a net transfer of energy from the surroundings to the reactants.
It is not necessarily the case that a reaction that requires heat to be applied is an endothermic reaction. Sometimes heat is required to break bonds in order to start the reaction, but more heat is released by the new bonds that are formed, so the reaction is exothermic. For example, hydrogen (H2) will not react with oxygen (O2) at room temperature; however, lighting a hydrogen/oxygen mixture with a match causes the gases to combine explosively in a highly exothermic reaction: 2H2 + O2 → 2H2O. Heat is required to break the bonds within the hydrogen and oxygen molecules, but considerably more heat is released by the formation of the new hydrogen-oxygen bonds. It is therefore an exothermic reaction.
In contrast, the combining of oxygen with nitrogen (N2) to form nitric oxide (NO) is an endothermic reaction. In a nitrogen molecule, the atoms are held together by a very strong triple bond. The energy required to break this bond is greater than the energy released by the formation of nitric oxide, so the reaction is endothermic. Other endothermic reactions include the combining of water and carbon dioxide to form glucose in photosynthesis, where the required energy comes from sunlight.
The total amount of energy of the reactants or the products in a chemical reaction is known as the enthalpy. It is expressed in kilojoules (kJ) of energy and represented by the symbol ΔH. A chemical reaction results in a change in enthalpy. In exothermic reactions, the products have less energy than the reactants, so the change is negative. In endothermic reactions, the products have more energy than the reactants, so the change is positive.
The exothermic reaction of hydrogen and oxygen to form water results in a negative enthalpy change of -285.8 kJ for each molecule of water formed. The endothermic reaction of nitrogen and oxygen to form nitric oxide results in a positive change of enthalpy of +180.5 kJ. Chemical equations can be written to include the enthalpy change, thus indicating whether the reaction is exothermic or endothermic, for example:
N2(g) + O2(g) → 2NO(g); ΔH = +180.5 kJ
These equations include the states of the reactants and products: s = solid, l = liquid and g = gas.
Endothermic chemical reactions can take place at room temperature if there is a large increase in entropy. One example is the reaction of barium hydroxide octahydrate and ammonium thiocyanate:
Ba(OH)2·8H2O(s) + 2NH4SCN(s) → Ba(SCN)2(s) + 10H2O(l) + 2NH3(g)
This is a highly endothermic reaction, and because three molecules of solids are reacting to produce 13 molecules of which 10 are liquid and two are gas, there is a large increase in entropy. If the reactants are mixed in a beaker and the beaker is placed on top of a block with a few drops of water on it, the water freezes as heat is absorbed from the surroundings. In fact, the temperature can drop to between -4 and -22 °F (-20 and -30 °C).