Whenever a chemical reaction occurs, there is an exchange of heat amongst the reactants and their surroundings (which can either be the medium in which the reaction is occurring if the reaction is taking place in the liquid state, or directly the atmosphere in case the reaction is occurring in solid or vapour state). With this net transfer of energy either from the surrounding to the reactant or from the reactants to the surroundings, there is always a change in the average kinetic energy of the system that occurs. This change in the average kinetic energy is always proportional to the change in temperature of the medium in which the reaction is occurring and its surroundings.
If the flow of this net transfer of heat energy during the reaction is from the medium into its surroundings, then the reaction is known as an exothermic reaction.
Properties of Exothermic Reactions
• In Exothermic reactions, the average kinetic energy of the surroundings always increase as compared to the medium, resulting in an increase in the temperature of the reaction mixture, making it warm and even, sometimes, hot.
• In exothermic reactions, the reactants always possess more energy than the products and hence are less stable. For this reason, the exothermic reactions require very less amount of activation energy to initiate the reaction.
• Once the reaction is initiated, the extra amount of energy present in the reactants is released into the surroundings, resulting in the formation of less energized and more stable products.
• The ΔH (flow of heat) for an exothermic reaction is always less than zero and the enthalpy involved with the reaction is also negative.
Examples of Exothermic Reactions
Some classical examples of exothermic reactions encountered commonly in laboratory practices or everyday life is discussed below:
• Combustion Reactions:Combustion Reactions are the ones which involves oxidation of the one or more of the reactants involved in the reaction i.e., the reactants of this reaction lose their electrons. These types of reactions always involve molecular oxygen (which also acts as the oxidising agent for the reactant). It initiates the reaction by breaking the double bond present in the molecular oxygen and providing that energy for the formation of new bonds between the carbon (from the organic reactant source) and the oxygen atom. The combustion reaction of any organic molecule occurs as follows:
• Neutralization Reactions (Acid-Base Reactions): The neutralization reaction between a strong acid and a strong base is always exothermic. It is because strong acids and bases are almost completely ionized in solution phase. So, technically there is no bond that is being broken to initiate the reaction since the reactants dissociate into ions and rearrangement of ions occur to form new products (viz., salt and water). After the reaction is complete, however, there is a bond formation between the conjugate ions of acid and base with each other and a strong bond of O–H is also formed, releasing heat energy, thereby making the reaction exothermic. An example of acid-base neutralization reaction is shown below:
• Reaction ofAlkali oxides andhydroxideswith water:The reaction between alkali metal oxides and water and the reaction between alkali metal hydroxides and water is exothermic. When calcium oxide (also known as quick lime) is mixed with water, a vigorous reaction occurs with the liberation of a lot of heat to form slaked lime (calcium hydroxide). The heat produced is so much that after the formation of calcium hydroxide, the excess water present in the reaction mixture is liberated into the air in the form of steam. The reaction occurs as follows:
Sodium hydroxide also shows an exothermic reaction with water because it, being a strong base, dissociates completely into the solution shifting his highly stabilized –OH ions towards lower energy state, thus releasing a lot of energy in the form of heat.
• Respiration: Respiration process among all living organisms is an exothermic reaction. During respiration, the glucose molecules (which we get after the complete digestion and metabolism of the food that we eat) are oxidized and after undergoing a series of long-chain reactions get converted into carbon dioxide, water and a large amount of heat energy which is released into our body and utilized to perform various day to day tasks. Oxidation of one molecule of glucose results in the release of approximately 2800 kJ of heat energy. This energy is mainly produced due to the formation of carbon-oxygen double bonds in carbon dioxide molecule and oxygen-hydrogen single bonds in the water molecule which are much more stable than the bonds present in the glucose molecule. The reaction takes place as follows:
• Formation of metallic salt: All metals generally react with strong acids to form metallic salts and liberation of hydrogen gas. This reaction is exothermic because it involves the breakage of only one bond while making of two bonds. Since every bond breakage requires a certain amount of energy from outside and every bond making releases a certain amount of energy into the surroundings, there is more amount of energy released in this reaction than was utilized in the initial bond breaking of the reactants. This is because strong acids readily dissociated in the solution phase and do not hence require any energy for bond breaking. An example of the formation of metallic salt (magnesium chloride) is when magnesium metal reacts with a strong acid (hydrochloric acid) with the release of heat energy:
• Condensation Reactions:Condensation reaction involves a process by which vapours state gets turned into the liquid state. This reaction is exothermic in nature. This can be explained as a fact that the molecules in a liquid state, in general, have relatively lesser kinetic energy as compared to the molecules in the vapour state. Whenever we need to convert any liquid state into a gaseous state, we need to provide them energy from an external source to excite the molecules and increase their kinetic energy. So, the opposite of this happens when any molecule from the vapour state has to convert into the liquid state. It has to lose some amount of its kinetic energy into the surroundings in the form of heat so as to be able to get converted into a lower energy state of the liquid state of matter. This energy exchange is however not so noticeable.
• Solidification Reactions: Solidification reactions (or freezing reactions) are always exothermic in nature. The explanation of this is quite similar to one given for condensation reactions. Like condensation reactions, solidification (or freezing reactions) too, involves a change in the phase or state of the matter. In solidification or freezing reaction, the substance has to get converted from a liquid state to a solid state. Since the molecules in the solid state have a comparatively lower energy state as compared to the ones in the liquid state, they have to lose some amount of their energy in the form of heat into the surroundings to be able to shift to a lower energy state with lesser kinetic energy.
• Dissolution of metal chlorides and metal sulphates in water:The dissolution of metal chlorides (such as calcium chloride) in water releases heat energy into the surrounding, making it an exothermic reaction. This is because the crystal lattice of calcium chloride requires a certain amount of energy (known as the crystal lattice energy) during its formation. So when its crystalline structure dissociates into ions again, it loses that energy which is more than the energy it requires to form a new bond with water and hence leads to an exothermic reaction. This reaction occurs as follows:
The dissolution of metal sulphates (such as magnesium sulphate) in water is also exothermic due to the same reason (Lattice breaking energy released is more than bond making energy with water).This reaction occurs as follows: