Problem 27 - Chemistry Matter and Change

Question

Determine Which of the following processes are exothermic? Endothermic? \begin{equation} \begin{array}{ll}{\text { a. } C_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{l}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{g})} & {\text { d. } \mathrm{NH}_{3}(\mathrm{g}) \rightarrow \mathrm{NH}_{3}(\mathrm{l})} \\\ {\text { b. } \mathrm{Br}_{2}(\mathrm{l}) \rightarrow \mathrm{Br}_{2}(\mathrm{s})} & {\text { e. } \mathrm{NaCl}(\mathrm{s}) \rightarrow \mathrm{NaCl}(\mathrm{l})} \\ {\text { c. } \mathrm{C}_{5} \mathrm{H}_{12}(\mathrm{g})+8 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 5 \mathrm{CO}_{2}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})}\end{array} \end{equation}

Step-by-Step Solution

Answer

Processes b, c, and d are exothermic; processes a and e are endothermic.

Step 1: Understanding Exothermic and Endothermic

Exothermic processes release energy, usually in the form of heat, to the surroundings. Endothermic processes absorb energy from the surroundings. In chemical reactions and phase changes, exothermic processes result in products at a lower energy state compared to the reactants, while endothermic processes result in products at a higher energy state.

Step 2: Analyzing Process a

Process a involves the phase change of ethanol (9:C_{2}H_{5}OH) from liquid to gas. Vaporization is an endothermic process because it requires energy (heat) to overcome the intermolecular forces present in the liquid.

Step 3: Analyzing Process b

Process b involves the phase change of bromine (9:Br_{2}) from liquid to solid (freezing). This process is exothermic because energy is released when intermolecular forces in the liquid strengthen into a solid structure.

Step 4: Analyzing Process c

Process c is the combustion of pentane (9:C_{5}H_{12}) with oxygen to form carbon dioxide and water. Combustion reactions are typically exothermic because they release a significant amount of energy in the form of heat.

Step 5: Analyzing Process d

Process d involves the condensation of ammonia (9:NH_{3}) from gas to liquid. Condensation is exothermic as it involves the release of energy when gaseous molecules come closer together to form a liquid.

Step 6: Analyzing Process e

Process e involves the melting of sodium chloride (9:NaCl) from solid to liquid. Melting is an endothermic process because energy is absorbed to break the lattice structure of the solid into a liquid.

Key Concepts

Chemical Reactions

Chemical reactions involve the transformation of reactants into products through the rearrangement of atoms. During these transformations, chemical bonds are either formed or broken, and energy changes occur. These energy changes are what determine whether a reaction is exothermic or endothermic.

• **Exothermic reactions** release energy, often in the form of heat or light. This means the products have less energy than the reactants because the difference in energy is given off to the surroundings.
• **Endothermic reactions** absorb energy from their surroundings. The products have more energy than the reactants because they take energy in, typically in the form of heat.

Understanding whether a chemical reaction is exothermic or endothermic is crucial in predicting how the reaction behaves and the conditions needed to initiate or sustain it.

Phase Changes

Phase changes are processes that involve the transformation of a substance from one physical state to another—such as solid, liquid, or gas. These transformations occur because of energy changes and often involve significant alterations in interactions between molecules.

• **Vaporization (e.g., liquid to gas)**: This is an endothermic process because it requires energy to break the intermolecular forces that hold the molecules in a liquid state.
• **Freezing and condensation (e.g., liquid to solid, gas to liquid)**: Both are exothermic as they release energy when molecules form stronger bonds during the transition to a more ordered phase.
• **Melting (e.g., solid to liquid)**: This is endothermic due to the need to absorb energy to disrupt the rigid structure of a solid into a more fluid liquid state.

Energy Absorption

Energy absorption is a hallmark of endothermic processes, where a substance takes in energy from its environment. This process is necessary for performing work such as increasing the kinetic energy of particles for phase changes or raising the energy of reactants in a chemical reaction.

• In **liquid to gas transitions** like vaporization, energy is absorbed to overcome the attractions holding molecules together.
• **Solid to liquid** transformations like melting also require energy to break the structured bonds within the solid.
• Chemical reactions that do not proceed spontaneously often absorb energy, needing an input to proceed and form higher-energy products.

The ability of a process to absorb energy has substantial implications in nature and industry, requiring careful control of conditions to maintain equilibrium.

Energy Release

Energy release characterizes exothermic processes where energy is emitted into the surrounding environment, often making these processes efficient and favorable. The release can appear as heat, light, or other forms of energy, helping to maintain a process or drive further chemical reactions.

• **Gas to liquid transitions like condensation** release energy as molecules come together to form a liquid.
• During **freezing**, energy is dissipated as the liquid molecules form a solid, overcoming the kinetic energy that kept them mobile.
• Common chemical processes like **combustion** release large amounts of heat, explaining why they are commonly used as energy sources.

These energy releases are integral to many natural cycles, including the maintenance of atmospheric conditions and supporting life through processes like cellular respiration.

Combustion Reactions

Combustion reactions are a specific type of chemical reaction typically involving a substance (often a hydrocarbon) reacting with oxygen to produce heat and light. These reactions are quintessential examples of exothermic processes and play a critical role in energy generation both biologically and industrially.

• During combustion, **hydrocarbons** break down, transforming into carbon dioxide and water, releasing substantial energy in the process.
• These reactions are responsible for the energy output in fuel-based systems like engines and industrial power plants.
• Combustion is vital to life, not only through mechanical applications but also in biological processes that help maintain cellular energy levels.

Due to their efficiency and energy output, combustion reactions are deeply studied and utilized to optimize energy use in technology and industry, even as their broader environmental impacts are carefully considered.