Subsection 8.4.1 Endothermic and Exothermic Reactions
Endothermic and exothermic reactions are the chemical reactions based on the heat energy released or absorbed during the reaction. An endothermic reaction is a chemical reaction that absorbs heat energy from its surroundings. In other words, it requires an input of energy to occur. During an endothermic reaction , the temperature of the system decreases as the reaction proceeds because the heat is being absorbed into the reaction. Examples of endothermic reactions include melting of ice, the reaction of baking soda and vinegar, and photosynthesis. The decomposition of water into hydrogen and oxygen
\begin{equation*}
2H_2O + \text{energy absorbed} \longrightarrow 2H_2 + O_2
\end{equation*}
The formation of nitric oxide from nitrogen and oxygen
\begin{equation*}
N_2 + O_2 + \text{energy absorbed} \longrightarrow 2NO
\end{equation*}
When chocolate melts in your mouth, it absorbs heat energy from your mouth and the surrounding environment, causing the temperature of the chocolate to increase to melt it into a liquid form. This is an example of an endothermic reaction because it requires an input of energy (in the form of heat) to occur. As a result, the process of chocolate melting in your mouth can give a cooling sensation. When sugar is added to water, the water molecules surround the individual sugar molecules and break apart the sugar crystals through a process called hydration. This requires energy to overcome the intermolecular forces holding the sugar molecules together. As a result, the temperature of the mixture may actually decrease slightly due to the energy absorbed during the process.
On the other hand, an exothermic reaction is a chemical reaction that releases heat energy into its surroundings. In other words, it gives off energy during the reaction. During an exothermic reaction, the temperature of the system increases as the reaction proceeds because heat is being released from the reaction. Examples of exothermic reactions include burning of wood, combustion reactions, and neutralization reactions. The burning of coal (carbon)
\begin{equation*}
C + O_2 \longrightarrow CO_2 + \text{energy released}
\end{equation*}
The burning of hydrogen
\begin{equation*}
2H_2 + O_2 \longrightarrow 2H_2O + \text{energy released}
\end{equation*}
The dissolution of sodium hydroxide (NaOH) in water is an exothermic process, which means that it releases heat. When NaOH dissolves in water, it undergoes a chemical reaction that releases energy in the form of heat. This energy is then transferred to the surrounding environment, causing the temperature of the solution to increase.
