Question

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. \(\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{O}(g)\)

Short answer

The exothermic processes are: b. \(\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2} \mathrm{O}(s)\) d. \(2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2}\mathrm{O}(g)\)

Step-by-step solution

Understand the meaning of exothermic

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.

Analyze each process

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.

Identify exothermic processes

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

Key concepts

Bond Formation

When bonds are created between atoms, energy is released into the surroundings. This process is known as bond formation. This is because the formation of bonds results in a more stable arrangement of electrons, which generally corresponds to a lower energy state. For example, when hydrogen gas (\(\text{H}_2\)) and oxygen gas (\(\text{O}_2\)) come together to form water (\(\text{H}_2\text{O}\)), the new chemical bonds created between hydrogen and oxygen release a considerable amount of energy. To better understand this, consider that new bonds make the system more stable, while breaking bonds requires energy because atoms go back to a less stable, higher-energy state. Bond formation is a key indicator of an exothermic reaction because energy is released in the process, helping to identify reactions that give off heat.

Energy Release

Exothermic processes are characterized by energy release into the surroundings. When energy is released, it often manifests as heat, which people can feel. In chemical reactions, energy release occurs because the chemical bonds in the products contain less energy than those in the reactants.

• Energy is released because the stable arrangement of products requires less energy to maintain than the unstable arrangement of reactants.
• The temperature usually increases, or heat is felt, which is a clear indicator of an exothermic process.

A practical instance of energy release is when water freezes to form ice, bonding molecules more securely together and releasing energy in the form of heat. In daily life, many exothermic reactions provide heat and energy necessary for cooking, heating, and various industrial processes.

Chemical Reactions

Chemical reactions involve the rearrangement of atoms to create new substances. Whether a reaction is exothermic or endothermic depends on the energy changes involved during this rearrangement. Understanding the nature of the chemical bonds that are broken and formed is crucial.

• Exothermic chemical reactions, like the burning of fuels or the formation of rust, involve the release of heat.
• Endothermic reactions, by contrast, require the absorption of energy, like the melting of ice.

For instance, when hydrogen and oxygen gases react to form water, it is a chemical reaction where the resultant bond formation releases energy. Recognizing the flow of energy helps us predict whether a process is generally exothermic or endothermic, thus aiding in our understanding and control of various reactions in real-world applications.