Chemical reactions can be exothermic or endothermic. Exothermic reactions release energy, while endothermic reactions absorb energy. The formation of chemical bonds releases energy, but breaking bonds requires energy. The enthalpy change, represented by ΔH, indicates whether a reaction is exothermic or endothermic. Endothermic reactions can occur at room temperature if there is a large increase in entropy.
An endothermic process is one that absorbs energy from its surroundings. In a chemical reaction, two or more substances – the reactants – interact with each other to produce one or more new substances – the products. Where the energy contained in the products is less than that in the reactants, the energy is released and the reaction is said to be exothermic. In endothermic reactions, the products have more energy than the reactants, so the energy is absorbed from their environment. Thus, in exothermic reactions, the reactants lose heat to their surroundings, which gets hotter, while in endothermic reactions, the reactants gain heat from their surroundings, which gets 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 the combined energy of the atoms after bonding is lower than it was before bonding. Thus, the formation of chemical bonds releases energy. In chemical reactions, however, bonds must be broken before new compounds can be formed. Breaking a chemical bond requires energy, and if more energy is required to break the bonds within the reactants than is released by forming new bonds, the overall reaction is endothermic, because there is a net transfer of energy from the environment surrounding the reagents.
It is not necessarily true that a reaction that requires the application of heat is an endothermic reaction. Sometimes heat is needed to break the bonds in order to start the reaction, but more heat is released by the new bonds being 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 needed to break the bonds within the hydrogen and oxygen molecules, but much more heat is released by the formation of the new hydrogen-oxygen bonds. It is therefore an exothermic reaction.
Conversely, the combination 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 combination 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 products in a chemical reaction is known as the enthalpy. It is expressed in kilojoules (kJ) of energy and is represented by the symbol ΔH. A chemical reaction causes 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 water molecule formed. The endothermic reaction of nitrogen and oxygen to form nitric oxide results in a positive enthalpy change 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); H = +180.5 kJ
These equations include the states of reactants and products: s = solid, l = liquid and g = gas.
Endothermic chemical reactions can occur at room temperature if there is a large increase in entropy. An 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 since three molecules of solids react to produce 13 molecules of which 10 are liquids and two are gases, 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 will freeze as heat is absorbed from its surroundings. In fact, the temperature can drop between -4 and -22°F (-20 and -30°C).
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