Problem 79 - Chemistry Matter and Change

Question

Write the sign of \(\Delta H_{ \text { sytem for each of the following }}\) changes in physical state. a. \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{s}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(1)\) b. \(\mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(1)\) \(\mathrm{c} . \mathrm{CH}_{3} \mathrm{OH}(1) \rightarrow \mathrm{CH}_{3} \mathrm{OH}(\mathrm{g})\) \(\mathrm{d} . \mathrm{NH}_{3}(1) \rightarrow \mathrm{NH}_{3}(\mathrm{s})\)

Step-by-Step Solution

Answer

a. \(\Delta H_{\text{system}} > 0\) b. \(\Delta H_{\text{system}} < 0\) c. \(\Delta H_{\text{system}} > 0\) d. \(\Delta H_{\text{system}} < 0\)

Step 1: Process a: C2H5OH(s) → C2H5OH(l)

This process involves the melting of a solid to form a liquid. Melting requires energy to be absorbed by the substance to overcome the intermolecular forces holding the solid together. Therefore, the process is endothermic, and since the system absorbs heat, the change in enthalpy will be positive: \(\Delta H_{\text{system}} > 0\).

Step 2: Process b: H2O(g) → H2O(l)

This process is the condensation of a gas to form a liquid. In condensation, gas particles come closer together and release energy while forming bonds. The process is exothermic, and since the system releases energy, the change in enthalpy will be negative: \(\Delta H_{\text{system}} < 0\).

Step 3: Process c: CH3OH(l) → CH3OH(g)

The process shown here is the vaporization of a liquid to form a gas. Vaporization involves breaking the intermolecular forces holding the liquid together, which requires energy to be added to the system. The process is endothermic, and since the system absorbs heat, the change in enthalpy will be positive: \(\Delta H_{\text{system}} > 0\).

Step 4: Process d: NH3(l) → NH3(s)

The given process is the freezing of a liquid to form a solid. In this process, the substance releases energy as its particles move closer together and form a more ordered structure. The process is exothermic, and since the system releases energy, the change in enthalpy will be negative: \(\Delta H_{\text{system}} < 0\).

Key Concepts

Phase Change

Understanding phase change is fundamental in the study of matter and energy interactions. A phase change refers to the transition of a substance from one state of matter to another, typically between solids, liquids, and gases. This transformation occurs when energy—in the form of heat—is either absorbed or released by the substance, thereby overcoming or reinforcing intermolecular forces.

Examples of phase changes include melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), as well as sublimation (solid to gas) and deposition (gas to solid). These processes are crucial in countless natural phenomena, like the water cycle, and in industrial applications, such as refrigeration. The sign of the enthalpy change, (Delta H), indicates whether the process is absorbing or releasing heat.

Endothermic Process

An endothermic process is a type of phase change where a system absorbs heat from its surroundings, resulting in a positive change in enthalpy ((Delta H > 0)). These processes are essential to understand as they provide insight into the energy requirements for certain reactions or changes in state.

Melting and Vaporization as Endothermic Processes

When a solid melts or a liquid vaporizes, energy is absorbed to break the intermolecular forces holding the particles together. For instance, when ice ((mathrm{H}_2(mathrm{O}((s))) melts into liquid water, it's an endothermic change. Similarly, vaporizing ethanol ((mathrm{C}_2(mathrm{H}_5(mathrm{OH}((mathrm{l}))) to a gaseous state demands energy. Teaching aids, such as interactive simulations, can help students visualize how energy absorption affects particle motion.

Exothermic Process

Conversely, in an exothermic process, the system releases heat into the surroundings, indicative of a negative enthalpy change ((Delta H < 0)). Such processes are commonly observed in everyday life and are vital in both natural settings and technological applications.

Freezing and Condensation as Exothermic Processes

Freezing is a classic example where a liquid turns into a solid, releasing heat in the process. The condensation of water vapor into liquid, such as morning dew condensing on grass, also exemplifies exothermic transformation. These changes often occur spontaneously, highlighting the natural tendency of systems to move towards a state of lower energy. A clear understanding of exothermic reactions is particularly important in thermodynamics, which delves into energy exchanges and transformations.

Chemical Thermodynamics

Chemical thermodynamics is a branch of physical chemistry that deals with the relationship between heat energy changes and chemical reactions or physical changes. It establishes the principles governing the transfer of energy, such as the laws of thermodynamics, and quantifies the properties of systems, like enthalpy, entropy, and free energy.

At the heart of this field is the concept that energy can neither be created nor destroyed (the first law of thermodynamics). Thermodynamics helps predict whether a chemical reaction or physical transformation will occur spontaneously, based on the changes in enthalpy ((Delta H)), entropy ((Delta S)), and temperature. By understanding these principles, students can foresee the direction and extent of chemical reactions, which is invaluable in fields like environmental science, engineering, and material science.