Scores of chemical reactions discharge energy in the form of heat, light, or sound. These types of reactions are termed exothermic reactions. Exothermic reactions may occur instinctively and lead to an increased randomness or entropy (ΔS > 0) of the reacting system. They are designated by a negative heat flow meaning that heat is expelled to the surroundings and decrease in enthalpy (ΔH < 0). Exothermic reactions generate heat and may be explosive when performed in the lab.
The second group of chemical reaction rather than give out heat to the surroundings absorbs heat from the surrounding in order to occur. These types of chemical reactions are referred to as endothermic reactions. Endothermic reactions are not spontaneous reactions. Work must be performed to be able to cause the reaction to take place. When endothermic reactions take up energy, a drop in temperature is calculated and noted on the course of the reaction. Endothermic reactions are denoted by positive heat flow and a rise in enthalpy (+ΔH).
Examples of Endothermic and Exothermic Processes
Photosynthesis is one example of an endothermic chemical reaction. During photosynthesis, plants make use of the energy obtained from the sun in the conversion of carbon dioxide and water into glucose and oxygen. The process of photosynthesis takes up 15MJ of energy (sunlight) in order to produce one kilogram of glucose as exemplified in the equation below:
Sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)
One example of an exothermic reaction is the reaction between sodium and chlorine to form table salt. The formation of the table salt reaction gives off 411 kJ of energy for one mole of salt formed as exemplified below:
Na(s) + 0.5Cl2(s) = NaCl(s)
Exothermic- The term exothermic explains the process that gives out energy in the form of heat.
Formation of a chemical bond usually leads to the release of an energy to the surrounding and can therefore be termed an exothermic process. Exothermic reactions frequently feel hot due to the fact that it is releasing energy to the surrounding.
Endothermic – is used to denote a process or reaction that absorbs energy in the form of heat before it could occur.
Breaking a chemical bond needs energy and is consequently regarded as Endothermic. Endothermic reactions frequently feel cold because they absorb heat from the surrounding.
Examples of exothermic Processes Examples of endothermic Processes
• freezing of water
• solidification of solid salts
• condensation of water vapor
• formation of a hydrate from an anhydrous salt
• formation of an anion from an atom in gaseous state
• Total destruction of matter E=mc2
• division of an atom
• melting of ice cubes
• melting of solid salts
• evaporation of liquid water
• production of an anhydrous salt from a hydrate
• producing a cation from an atom in the gaseous state
• breaking up of a gas molecule
• separation of ion pairs
• boiling of an egg
• baking of bread
Examples of Exothermic Reactions Examples of Endothermic Reactions
• burning of hydrogen
• liquefaction of lithium chloride in water
• combustion of propane
• drying out the moisture of sugar with sulfuric acid
• disintegration of hydrogen peroxide
• disintegration of ammonium dichromate
• halogenation of acetylene
• Reaction of barium hydroxide octahydrate crystals with dry ammonium chloride
• melting ammonium chloride in water
• reacting thionyl chloride (SOCl2) with cobalt(II) sulfate heptahydrate
• mixture of water and ammonium nitrate
• mixture of water and potassium chloride
• reaction between ethanoic acid and sodium carbonate
• Photosynthesis (chlorophyll is used in the reaction of carbon dioxide, water and energy to produce glucose and oxygen.