 
A chemical reaction is a process where one or more substances (the reactants) are converted to one or more different substances (the products). The substances can be either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products. All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. Chemical reactions that release energy are called exothermic. Exothermic reactions are accompanied by an increase in temperature of the reaction mixture. The term 'exo' refers to 'to release', and 'thermic' refers to 'heat'. Chemical reactions that absorb or use energy are called endothermic. Endothermic reactions are accompanied by a decrease in temperature of the reaction mixture. The term 'endo' means 'to absorb', and 'thermic' means 'heat'.  Exothermic ReactionsIn an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products. As a result of the reactions between molecules and compounds, as well as the breaking of bonds, a tremendous amount of energy is released. An exothermic reaction releases this difference in energy into the surroundings. All combustion reactions, for example, are exothermic reactions. During a combustion reaction, a substance burns as it combines with oxygen. When substances burn, they usually give off energy as heat and light. The combustion of wood is an exothermic reaction that releases a lot of energy.  Endothermic ReactionsIn an endothermic reaction, less energy is released when new bonds form in the products than is needed to break bonds in the reactants. Endothermic reactions absorb energy from the surrounding in the form of heat, in order for the reaction to occur. Heat is taken up from the surroundings, so the temperature of the system ends up being cooler. Endothermic reaction examples include frying eggs, melting ice, the evaporation or boiling of water, and photosynthesis. When water evaporates from your skin. for example, it feels cooler. In photosynthesis, plants absorb energy from the sun and use it to convert carbon dioxide and water into glucose and oxygen. Summary  
|