Question

Classify each process as endothermic or exothermic. What is the sign of \(\Delta H\) for each process? Explain your answers. a. gasoline burning in an engine b. steam condensing on a mirror c. water boiling in a pot Provide at least two additional examples of exothermic processes and two additional examples of endothermic processes. Have each member of your group provide an example.

Short answer

a. Exothermic, \(\Delta H < 0\). b. Exothermic, \(\Delta H < 0\). c. Endothermic, \(\Delta H > 0\). Additional exothermic examples: slaking of lime and the Haber process. Additional endothermic examples: photosynthesis and melting of ice.

Step-by-step solution

Classify the Processes

Identify each process as endothermic or exothermic based on whether the process absorbs or releases energy. Endothermic processes require heat intake, while exothermic processes release heat.

Determine the Sign of \(\Delta H\)

For each process classified, determine the sign of the enthalpy change. Endothermic processes have a positive \(\Delta H\) because they absorb heat. Exothermic processes have a negative \(\Delta H\) because they release heat.

Analyze Gasoline Burning in an Engine

Gasoline burning in an engine is an exothermic process since energy is released in the form of heat and light. Therefore, the sign of \(\Delta H\) is negative for this process.

Analyze Steam Condensing on a Mirror

Steam condensing on a mirror is an exothermic process as heat is released when steam changes from a gas to a liquid. The sign of \(\Delta H\) is negative.

Analyze Water Boiling in a Pot

Water boiling in a pot is an endothermic process because it requires heat to change from liquid to gas. The sign of \(\Delta H\) is positive for this process.

Provide Additional Examples of Exothermic Processes

Request each group member to suggest an example of an exothermic process. Two possible examples are the reaction of calcium oxide with water ('slaking of lime') and the synthesis of ammonia (Haber process). Both of these processes release heat, which means \(\Delta H\) would be negative.

Provide Additional Examples of Endothermic Processes

Request each group member to suggest an example of an endothermic process. Two possible examples are the photosynthesis in plants and the melting of ice. Both of these processes absorb heat, so \(\Delta H\) would be positive.

Key concepts

Enthalpy Change

In the fascinating world of thermochemistry, one key player is enthalpy change, represented by the symbol \( \Delta H \). Think of it as a way to measure the heat change during a chemical reaction under constant pressure. This value can tell us a lot about the reaction's nature. When a reaction is endothermic, which literally means 'absorbing heat', \( \Delta H \) is positive because the system gains heat from the surroundings. A familiar everyday example is when you see water boiling in a pot to become steam; that's the system (water) needing heat to transform.

Now, flip the script, and you have exothermic processes—these are like generous friends giving away warmth. In these reactions, \( \Delta H \) is negative as the system releases heat. Imagine gasoline burning in an engine; it's literally producing heat and powering the vehicle. That's an exothermic process in action—energy going out, moving things forward, and \( \Delta H \) taking a dive into the negatives.

Heat Transfer in Chemical Reactions

Diving deeper into the essence of chemical reactions, it's paramount to understand the unveiling drama of heat transfer. During every chemical reaction, energy in the form of heat either enters or exits the stage. This process is what makes our steam-filled mirrors post-shower (steam condensing) and our world buzzing with life (the heat from the sun enabling photosynthesis).

How do we see this heat transfer?

It's all about the direction: a reaction that feels the chill and absorbs heat is like a snowman under the sun, trying to stay cool—it's an endothermic reaction needing extra energy. Conversely, exothermic reactions are the campfires of chemistry, radiating warmth outwards as in combustion or the glow of a firefly. Understanding this concept allows us to categorize virtually every chemical reaction into one of these two thermal tales.

Thermochemistry

Stepping back to view the broader picture, thermochemistry is the study of the energy and heat associated with chemical reactions and physical transformations. It's the arena where heat transfer and enthalpy change play their roles.

By grasping the essentials of thermochemistry, we begin to see the profound connection between the energy that flows within and between molecules and the macroscopic properties like temperature and heat that we can measure. Whether it's the melting of ice on a sunny day or the energetic dance of molecules in a hand-warmer during winter, thermochemistry provides the explanations we need to make sense of energy flow in our everyday lives and the wider universe.

• Endothermic Processes: Require heat; they're like a sponge soaking up water.
• Exothermic Processes: Give off heat; think of a baker taking hot bread out of the oven.

With a solid understanding of these principles, students can turn observations into scientific explanations, linking molecular motion to tangible experiences.