When a chemical reactionA process in which one or more substances, the reactant or reactants, change into one or more different substances, the products; chemical change involves rearrangement, combination, or separation of atoms. Also called chemical change. occurs, there is usually a change in temperatureA physical property that indicates whether one object can transfer thermal energy to another object. of the chemicals themselves and of the beaker or flask in which the reaction is carried out. If the temperature increases, the reaction is exothermicDescribes a process in which energy is transferred to the surroundings as a result of a temperature difference.—energy is given off as heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. when the container and its contents cool back to room temperature. (Heat is energy transferred from one place to another solely because of a difference in temperature.) An endothermicIn chemical thermodynamics, describes a process in which energy is transferred from the surroundings to the system as a result of a temperature difference. reaction produces a decrease in temperature. In this case heat is absorbed from the surroundings to return the reaction products to room temperature. ThermochemistryThe study of the quantity of heat transferred into or out of a chemical system during a chemical reaction., a word derived from the Greek thermé, “heat,” is the measurement and study of energy transferred as heat when chemical reactions take place. It is extremely important in a technological world where a great deal of workA mechanical process in which energy is transferred to or from an object, changing the state of motion of the object. is accomplished by transforming and harnessing heat given off during combustionVigorous combination of a material with oxygen gas, usually resulting in a flame. of coal, oil, and natural gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container..
If your workstation is authorized to view JCE Software, you will see a video below which shows an example of an endothermic reaction. Ammonium thiocyanate is mixed with barium hydroxide, and the reaction takes in enough heat to freeze water.