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Exothermic reactions are reactions that release heat energy to their surroundings. You can identify an exothermic reaction by looking at its chemical equation and noting where the heat term is located. If the heat is listed among the products, then the reaction is exothermic.

For example, in the reaction:
\(\text{C}_{3} \text{H}_{8}(g) + 5 \text{O}_{2}(g) \rightarrow 3 \text{CO}_{2}(g) + 4 \text{H}_{2} \text{O}(g) + 2220 \text{ kJ}\)
The 2220 kJ of heat is on the product side, meaning the reaction releases heat. This is an exothermic reaction.

Similarly, in the reaction:
\(\text{2 Na}(s) + \text{Cl}_{2}(g) \rightarrow 2 \text{NaCl}(s) + 819 \text{ kJ}\)
The 819 kJ of heat is on the product side, indicating that the reaction also releases heat, and is therefore exothermic.

Endothermic reactions are the opposite of exothermic reactions. They absorb heat from their surroundings. You can identify an endothermic reaction by finding the heat term among the reactants of the chemical equation.

For instance, in the reaction:
\(\text{PCl}_{5}(g) + 67 \text{ kJ} \rightarrow \text{PCl}_{3}(g) + \text{Cl}_{2}(g)\)
The 67 kJ of energy is on the reactant side, meaning that the reaction absorbs heat. This is an endothermic reaction.

In summary, if you see that heat is required for the reaction to occur and it appears on the reactants' side, you are dealing with an endothermic process.

Heat plays a crucial role in chemical reactions. Whether a reaction releases or absorbs heat can help understand the nature and behavior of the reaction.

In exothermic reactions:

• Heat is released to the surroundings.
• The temperature of the surroundings increases.
• Common examples include combustion reactions and many oxidation processes.

In endothermic reactions:

• Heat is absorbed from the surroundings.
• The temperature of the surroundings decreases.
• Common examples include photosynthesis and the melting of ice.

Understanding the heat dynamics in these reactions is essential for applications in everyday life, such as chemical manufacturing, energy production, and biological processes.