Which of the following processes are exothermic? a. \(\mathrm{N}_{2}(g) \longrightarrow 2 \mathrm{N}(g)\) b. \(\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2} \mathrm{O}(s)\) c. \(\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{Cl}(g)\) d. $2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g)$ e. \(\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{O}(g)\)

Exothermic reactions release energy to the surroundings because the products have lower energy than the reactants. In contrast, endothermic reactions absorb energy because the products have higher energy than the reactants. To identify whether a process is exothermic, we need to observe if the products have lower energy than the reactants. Processes where bonds are formed are often exothermic, while processes where bonds are broken are usually endothermic.

a. \(\mathrm{N}_{2}(g) \longrightarrow 2 \mathrm{N}(g)\) In this process, a nitrogen molecule (N2) is broken into two nitrogen atoms. Since a bond is being broken, it will absorb energy, making this process endothermic. b. \(\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2} \mathrm{O}(s)\) In this process, liquid water (H2O) changes to solid water through freezing. During freezing, bonds between water molecules are formed, making the process exothermic. c. \(\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{Cl}(g)\) This process involves breaking the bond between two chlorine atoms. Since a bond is being broken, it will absorb energy, making this process endothermic. d. $2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g)$ In this process, hydrogen and oxygen molecules react to form water molecules. Bonds are being formed between the hydrogen and oxygen atoms, which releases energy, making this process exothermic. e. \(\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{O}(g)\) This process involves breaking the bond between two oxygen atoms. Since a bond is being broken, it will absorb energy, making this process endothermic.

According to the analysis, the following processes are exothermic: b. \(\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2} \mathrm{O}(s)\) d. $2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g)$