When physical or chemical changes occur, they are generally accompanied by a transfer of energy. The law of conservation of energy states that in any physical or chemical process, energy is neither created nor destroyed. In other words, the entire energy in the universe is conserved. In order to better understand the energy changes taking place during a reaction, we need to define two parts of the universe, called the system and the surroundings. The system is the specific portion of matter in a given space that is being studied during an experiment or an observation.
The surroundings is everything in the universe that is not part of the system. In practical terms for a laboratory chemist, the system is the particular chemicals being reacted, while the surroundings is the immediate vicinity within the room.
During most processes, energy is exchanged between the system and the surroundings. If the system loses a certain amount of energy, that same amount of energy is gained by the surroundings. If the system gains a certain amount of energy, that energy is supplied by the surroundings.
A chemical reaction or physical change is endothermic if heat is absorbed by the system from the surroundings. To understand these diagrams, compare the energy level of the reactants on one side with that of the products on the other side. Consider, for example, a diagram that charts the energy change when a candle burns. Because more energy is released when the products are formed than is used to break up the reactants, this reaction is exothermic. Brittny All of this stuff relates to thermodynamics—the study of heat and its relationship to energy and work.
Classifying a chemical reaction as exothermic or endothermic is simple. It comes down to weighing the energy needed to break bonds in the reactants with the energy released when the products are formed. If you do not respond, everything you entered on this page will be lost and you will have to login again. Skip Navigation. Vimeo ID: Video Transcript. Energy is released when bonds are formed. Download Video Transcript. These terms are usually applied to chemical reactions. A chemical reaction can only be one of these three terms at once.
A reaction that is exothermic will be endothermic if run backward and vice-versa. Exothermic chemical reactions liberate heat. A simple and familiar example is the combustion of methane gas CH 4. We could write "heat" as one of the products on the right products side of the reaction if we wished. The term enthalpy , H , is used by chemists to describe how heat flows into or out of a system.
It doesn't tell us whether the reaction will occur or how fast. Endothermic reactions are usually not a great safety hazard. However, because the reaction draws heat from its surroundings, the reaction container may become cold and cause condensation or ice to form. This can be a safety hazard if the materials involved react with water.
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