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 among the given options are b. \(H_2O(l) \longrightarrow H_2O(s)\) and d. \(2 H_2(g) + O_2(g) \longrightarrow 2 H_2O(g)\).

Step-by-step solution

a. N2(g) → 2 N(g)

In this process, we are breaking the triple bond between two nitrogen atoms in the nitrogen molecule to form two separate nitrogen atoms. Breaking a bond requires energy, so this process is endothermic, not exothermic.

b. H2O(l) → H2O(s)

In this process, liquid water is being converted to solid water (ice). As water freezes, it forms a lattice structure that leads to the formation of hydrogen bonds. The creation of hydrogen bonds releases energy, making this process exothermic.

c. Cl2(g) → 2 Cl(g)

In this process, we are breaking the single bond between two chlorine atoms in the chlorine molecule to form two separate chlorine atoms. Breaking a bond requires energy, so this process is endothermic, not exothermic.

d. 2 H2(g) + O2(g) → 2 H2O(g)

In this process, hydrogen and oxygen molecules are reacting to form water molecules. During this reaction, the bonds in the hydrogen and oxygen molecules are broken and new bonds are formed in the water molecules. The overall energy released in forming new bonds is more than the energy required to break the initial bonds, making this process exothermic.

e. O2(g) → 2 O(g)

In this process, we are breaking the double bond between two oxygen atoms in the oxygen molecule to form two separate oxygen atoms. Breaking a bond requires energy, so this process is endothermic, not exothermic. In conclusion, processes b and d are exothermic, while processes a, c, and e are endothermic.

Key concepts

Bond formation

Bond formation is a crucial concept in understanding chemical reactions, especially exothermic ones. In chemical reactions, atoms rearrange to form new substances. During this process, existing bonds between atoms break and new bonds form. Breaking bonds requires energy input, while forming bonds releases energy.

• When new bonds form, atoms attain a more stable electron configuration, which results in the release of excess energy as heat or light.
• The type of bonds formed affects the amount of energy released. For example, the formation of covalent bonds, found in many molecules, can release significant energy.
• Exothermic reactions are those where the energy released from bond formation exceeds the energy consumed for bond breakage.

A good example of bond formation in an exothermic reaction is the combustion of hydrogen and oxygen to form water, as shown in Step 4 of the exercise. The energy released during the new bond formation in water molecules compensates for the energy required to break the initial hydrogen and oxygen bonds.

Phase transition

Phase transitions describe the change of matter from one state to another, such as solid to liquid, liquid to gas, or vice versa. These transitions can either absorb or release energy, depending on the direction of the transition.

• In an exothermic phase transition, energy is released to the surroundings as the substance cools down and stabilizes at a lower energy state.
• Freezing or crystallization is a common exothermic phase transition. For example, when liquid water freezes into ice, it releases heat as hydrogen bonds form between water molecules.

In Step 2 from the exercise, water transitioning from liquid to solid is an exothermic process. This process illustrates how forming the rigid hydrogen bond network in ice releases energy, ultimately warming the surroundings. It's a perfect example of energy release during a phase transition.

Energy release

Energy release is a fundamental aspect of exothermic reactions. The amount of energy released often determines the practical application and safety considerations of a chemical reaction.

• The release occurs when the formation of new bonds in products creates more stability and energy efficiency than the original reactants.
• In practical terms, the energy released during an exothermic reaction can manifest as heat, light, or even sound.
• This release is harnessed in many applications, from heating homes to driving combustion engines.

In the exercise, the reaction of hydrogen and oxygen to create water vapour releases a considerable amount of energy. This energy not only propels the reaction forward but also underpins many industrial and natural processes. The comprehension of energy release from reactions helps in the efficient and safe design of chemical processes.