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) \(\mathrm{H}_{2}\mathrm{O}(l) \longrightarrow \mathrm{H}_{2}\mathrm{O}(s)\) and (d) \(2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2}\mathrm{O}(g)\).

Step-by-step solution

Analyze Reaction (a)

: In Reaction (a), we have the process: \(\mathrm{N}_{2}(g) \longrightarrow 2 \mathrm{N}(g)\) Here, the bond within the nitrogen molecule, which is a triple bond, is broken. Breaking a bond requires energy; hence this process is endothermic, not exothermic.

Analyze Reaction (b)

: In Reaction (b), we have the process: \(\mathrm{H}_{2}\mathrm{O}(l) \longrightarrow \mathrm{H}_{2}\mathrm{O}(s)\) Here, we are converting liquid water to solid water (ice). This process involves the release of energy due to the formation of a more stable, solid structure. The hydrogen bonds in ice are stronger and more stable than those in liquid water. Thus, this process is exothermic.

Analyze Reaction (c)

: In Reaction (c), we have the process: \(\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{Cl}(g)\) Here, the bond within the chlorine molecule is broken. Breaking a bond requires energy; hence this process is endothermic, not exothermic.

Analyze Reaction (d)

: In Reaction (d), we have the process: \(2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2}\mathrm{O}(g)\) This reaction is a combustion reaction where hydrogen gas reacts with oxygen gas to form water vapor. In this process, the energy released upon the formation of two O-H bonds in the water molecule is greater than the energy required to break the initial H-H and O=O bonds. Thus, this process is exothermic.

Analyze Reaction (e)

: In Reaction (e), we have the process: \(\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{O}(g)\) Here, the bond within the oxygen molecule is broken. Breaking a bond requires energy; hence this process is endothermic, not exothermic.

Answer:

: Based on the analysis of the given processes, the exothermic processes are (b) and (d).

Key concepts

Endothermic Reactions

Endothermic reactions involve the absorption of energy from their surroundings. In these processes, energy is needed to break the chemical bonds in the reactants.
This absorption of energy makes the surroundings feel colder.Examples include:

• Melting of ice into water
• Evaporation of water

When you see a reaction where bonds are broken, like the breaking of the \((\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{Cl}(g))\) molecule, it typically requires energy input. Thus, such reactions are classified as endothermic.

Chemical Bonding

Chemical bonding is the process of atoms joining together to form compounds. This can occur through sharing electrons (covalent bonding), transferring electrons (ionic bonding), or through metallic bonds.**Types of Bonds**

• Covalent Bonds: Form when atoms share electron pairs. An example is the bond in \(\mathrm{N}_{2}\), which has a strong triple bond.
• Ionic Bonds: Occur when electrons are transferred from one atom to another, resulting in attraction between oppositely charged ions.
• Metallic Bonds: Found in metals, where electrons move freely, providing conductivity and malleability.

In reactions where bonds are formed, such as when \(2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2}\mathrm{O}(g)\), energy is often released, leading to exothermic reactions.

Energy Changes in Reactions

Energy changes are at the heart of chemical reactions. These changes determine whether a reaction is exothermic or endothermic.**Exothermic Reactions:**

• Release energy, usually in the form of heat, making the environment warmer.
• An example is the formation of ice from water \(\mathrm{H}_{2}\mathrm{O}(l) \longrightarrow \mathrm{H}_{2}\mathrm{O}(s)\), where energy is released as heat.

**Endothermic Reactions:**

• Absorb energy from the surroundings, leading to a cooling effect.
• Such as the dissociation of \(2\mathrm{O}(g)\) from \(\mathrm{O}_{2}(g)\).

The key to understanding these changes is recognizing whether bond formation releases more energy than is used to break the original bonds.