Problem 58 - Chemistry and Chemical Reactivity

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) \(\mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g})\)

Step-by-Step Solution

Answer

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

Step 1: Analyze the change of state in (a)

For the process \( \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{s}) \), water is freezing, which involves releasing heat. Therefore, it is an exothermic process.

Step 2: Analyze the chemical reaction in (b)

In the reaction \( 2 \mathrm{H}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \), gases combine to form water vapor, releasing energy as heat. This reaction is exothermic.

Step 3: Examine the temperature change in (c)

In the process \( \mathrm{H}_{2} \mathrm{O}\left(\ell, 25^{\circ} \mathrm{C}\right) \rightarrow \mathrm{H}_{2} \mathrm{O}\left(\ell, 15^{\circ} \mathrm{C}\right) \), the temperature of the liquid water decreases, indicating heat is released, making it exothermic.

Step 4: Consider the phase change in (d)

For \( \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \), water is undergoing vaporization, which requires absorption of heat. Thus, this process is endothermic.

Key Concepts

Understanding Phase Changes

Phase changes involve the transition of a substance from one state of matter to another, such as solid to liquid, liquid to gas, and vice versa. These changes require an energy exchange between the substance and its surroundings.
For instance, when water freezes from liquid to solid, it releases energy into its environment, making the process exothermic. On the contrary, when water vaporizes from liquid to gas, it absorbs energy to overcome intermolecular forces, making the process endothermic.

• Freezing and condensation are exothermic processes as they release heat.
• Melting and vaporization are endothermic processes as they require heat absorption.

Knowing this helps predict whether a particular phase change will involve heat being absorbed or released.

Exploring Chemical Reactions

Chemical reactions often involve breaking and forming of bonds, leading to either the absorption or release of energy. In an exothermic reaction, more energy is released during the formation of bonds in the products than is consumed to break the bonds in the reactants.
Take the example of the reaction between hydrogen and oxygen gases to form water vapor. This process is exothermic since it releases substantial heat energy, which is why it is commonly used in fuel cells.
Understanding the energy changes during chemical reactions is crucial for designing efficient chemical processes and for energy-related applications.

Explaining Temperature Changes

Temperature changes in a substance can also indicate whether a process is exothermic or endothermic. A decrease in temperature suggests that the substance is releasing heat, which correlates with an exothermic process.
Take liquid water cooling from 25°C to 15°C; as it loses heat, it is exothermic. Conversely, a temperature increase would mean that the substance is absorbing heat, thus endothermic.
Temperature changes give us useful insights into the thermal behavior of a system during physical or chemical processes, allowing predictions about the direction of heat flow.

Comprehending Heat Absorption and Release

Heat absorption and release are central to understanding exothermic and endothermic processes. During exothermic reactions and phase changes, heat is released, often noticeable by a temperature increase in the surroundings.
Conversely, endothermic processes absorb heat, drawing energy from their surroundings, which can lead to a temperature drop around the system.

• Heat absorption corresponds with increases in potential energy of the molecules involved.
• Heat release results in a decrease in potential energy, typically transforming it into kinetic energy, raising temperatures.

Understanding these principles helps visualize heat flow during various physical and chemical transformations.