Question

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

Short answer

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

Step-by-step solution

Understand the Process for (a)

The process described is \( \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{s}) \). This is a transition from liquid water to solid water, or freezing. Freezing releases energy into the surroundings, making it an exothermic process.

Understand the Process for (b)

The reaction \(2 \mathrm{H}_{2}(\mathrm{g}) + \mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})\) represents the formation of water from its gaseous elements. The formation of water releases heat, therefore it is an exothermic reaction.

Understand the Process for (c)

The process \( \mathrm{H}_{2} \mathrm{O}\left(\ell, 25^{\circ} \mathrm{C}\right) \rightarrow \mathrm{H}_{2} \mathrm{O}\left(\ell, 15^{\circ} \mathrm{C}\right) \) indicates cooling of liquid water from 25°C to 15°C. During cooling, water loses heat to the surroundings, so this is an exothermic process.

Understand the Process for (d)

The process \( \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \) involves the conversion of liquid water to water vapor, or evaporation. This requires energy input, as energy is absorbed during the phase change from liquid to gas, making it an endothermic process.

Key concepts

Exothermic Processes

Exothermic processes release energy into their surroundings. This usually happens in the form of heat, making the environment warmer.
In an exothermic reaction or process, energy is a by-product, typically because bonds are forming. Let's look at a few examples:

• Freezing of water: When liquid water freezes to become ice, it gives off heat to the surroundings, making the process exothermic.
• Formation of water from hydrogen and oxygen gas: This chemical reaction produces water vapor and releases energy, hence it is exothermic.

Exothermic processes are found everywhere in our daily lives, from the heat given off by burning candles to the warmth generated by your body's metabolism.

Endothermic Processes

Endothermic processes are those that absorb energy from their surroundings. This means they require energy input to proceed.
Energy absorption usually results in a decrease in temperature in the nearby environment. Here are some key examples:

• Evaporation of water: Turning liquid water into vapor absorbs energy, making it an endothermic process. This is why sweating cools you down.

Understanding endothermic processes helps explain why certain reactions require heat or other energy sources to take place.

Phase Transitions

Phase transitions are changes in the state of matter of a substance. These changes can either absorb or release energy depending on the direction of the transition.
Here are some common transitions:

• Freezing: Liquid to solid. This is exothermic as it releases energy.
• Melting: Solid to liquid. This is endothermic because it requires energy.
• Evaporation: Liquid to gas. Energy absorption makes this an endothermic process.

Each transition involves distinct energy dynamics which determine whether the process is exothermic or endothermic.

Chemical Reactions

Chemical reactions involve rearranging atoms to form new substances. This can involve breaking bonds, which absorbs energy, or forming new bonds, which releases energy.
Here are a few points to consider:

• Exothermic reactions: In these, more energy is released in forming product bonds than is absorbed in breaking reactant bonds. For example, burning wood releases heat.
• Endothermic reactions: These require more energy to break reactant bonds than is released in forming product bonds. Photosynthesis in plants is an example.

Understanding the energy dynamics helps in determining whether a reaction is exothermic or endothermic, which is key in fields like chemistry and engineering.