Question

For each of the following, tell whether the process is exothermic or endothermic. (No calculations are required.) (a) \(\mathrm{H}_{2} \mathrm{O}(\ell) \longrightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{s})\) (b) \(2 \mathrm{H}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})\) (c) \(\mathrm{H}_{2} \mathrm{O}\left(\ell, 25^{\circ} \mathrm{C}\right) \longrightarrow \mathrm{H}_{2} \mathrm{O}\left(\ell, 15^{\circ} \mathrm{C}\right)\) (d) \(\mathrm{H}_{2} \mathrm{O}(\ell) \longrightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g})\)

Short answer

(a) Exothermic, (b) Exothermic, (c) Exothermic, (d) Endothermic.

Step-by-step solution

Freezing of water

The process \( \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{s}) \) represents the freezing of water. Freezing is an exothermic process because it involves the release of heat as water molecules slow down and form a solid structure.

Formation of water from hydrogen and oxygen

The reaction \( 2 \mathrm{H}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \) is the formation of water from hydrogen and oxygen gas. This is a combustion reaction, which is typically exothermic as it releases energy in the form of heat.

Cooling of liquid water

In the process \( \mathrm{H}_{2} \mathrm{O}(\ell, 25^{\circ} \mathrm{C}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\ell, 15^{\circ} \mathrm{C}) \), water is being cooled. Cooling involves the release of heat energy to the surroundings, making it an exothermic process.

Evaporation of water

The process \( \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \) represents the evaporation of water, which involves absorbing heat to convert liquid into gas. Therefore, it is an endothermic process.

Key concepts

Exothermic Process

Exothermic processes are those that involve the release of energy, usually in the form of heat, to the surroundings. It's like a "giveaway" of energy. This happens when bonds are formed, molecules slow down, or substances cool down. Consider the process of freezing water: when liquid water freezes to become ice, energy is released. Here is why:

• Water molecules lose thermal energy as they transition from a higher energy liquid state to a lower energy solid state.
• The alignment of water molecules into a structured ice lattice releases energy, warming the surroundings slightly in the process.

Similarly, when hydrogen gas reacts with oxygen gas to form water, this chemical reaction also releases energy. Combustion reactions, like this one, are typically exothermic because they result in a change to a more stable, lower-energy state by releasing heat.
Another example is cooling liquid water from 25°C to 15°C. Cooling involves a reduction in molecular energy, resulting in the release of heat to the environment.

Endothermic Process

Endothermic processes require the absorption of energy, making them sound like energy "sponges." During these processes, energy is taken in from the surroundings to facilitate the reaction or process. Think of evaporation: when liquid water turns into vapor, it absorbs energy from its surroundings. This absorbed energy is necessary for the water molecules to overcome the intermolecular forces holding them together as a liquid.

• Heat energy is used to increase the potential energy of the molecules, allowing them to transition into the gaseous state.
• The absorption of heat results in a cooling effect on the surroundings as the energy is drawn into the process.

Energy Transfer

Energy transfer is an essential concept in thermochemistry, highlighting how energy moves from one part of a system to another or from a system to its environment. Whenever a process occurs, energy gets transferred.
During exothermic processes, energy is often transferred from a system to its surroundings. For example:

• In combustion reactions, the system releases energy, increasing the temperature of the surroundings.
• During freezing, the environment gains thermal energy from the water, which freezes into ice.

In endothermic processes, the opposite happens:

• The system absorbs energy, typically causing the surroundings to lose heat.
• Evaporation involves energy being transferred to water molecules, resulting in vapor formation and cooling the environment.

Understanding these dynamics provides insights into how temperature changes, phases shift, and reactions occur across different contexts.