Question

Determine whether each process is exothermic or endothermic and indicate the sign of \(\Delta H\). a. natural gas burning on a stove b. isopropyl alcohol evaporating from skin c. water condensing from steam

Short answer

a. Natural gas burning is exothermic with (H) < 0, b. isopropyl alcohol evaporating is endothermic with (H) > 0, c. water condensing from steam is exothermic with (H) < 0.

Step-by-step solution

Identifying the Process for Natural Gas Burning

Determine if the process is exothermic or endothermic. When natural gas burns on a stove, it releases heat to the surroundings. This kind of process, which gives off heat, is called exothermic.

Determining the Sign of (H) for Natural Gas Burning

For an exothermic process, the change in enthalpy ((H)) is negative because the system (in this case, the gas) loses heat.

Identifying the Process for Isopropyl Alcohol Evaporating

Determine if the process is exothermic or endothermic. Isopropyl alcohol requires heat to evaporate from skin; it absorbs heat from the skin, which produces a cooling effect. Absorbing heat is characteristic of an endothermic process.

Determining the Sign of (H) for Isopropyl Alcohol Evaporation

For an endothermic process, the change in enthalpy ((H)) is positive because energy is absorbed from the surroundings into the system.

Identifying the Process for Water Condensing

Identify if the process of water condensing from steam is exothermic or endothermic. When water vapor condenses into liquid form, it releases heat to its surroundings. This indicates that it is an exothermic process.

Determining the Sign of (H) for Water Condensation

Since condensation is exothermic, the change in enthalpy ((H)) for water condensing from steam is negative due to the release of heat to the environment.

Key concepts

Exothermic Process

An exothermic process is characterized by the release of heat. This type of reaction occurs when the energy needed to break the bonds in the reactants is less than the energy released when new bonds form in the products. Imagine lighting a match; as it burns, it emits heat that you can feel if you hold your hand above it.

This release of heat is indicative of an exothermic reaction. The enthalpy change ((H)) for an exothermic process is negative, pointing out that the system is losing heat to its surroundings. For example, when natural gas burns, not only does it provide heat to cook food on a stove, but it also lessens the overall energy of the system, represented by a negative sign in the enthalpy change.

Endothermic Process

In contrast, an endothermic process absorbs heat from its surroundings, making the environment feel cooler. This occurs when the energy required to break the bonds in the reactants is greater than the energy released from forming new bonds in the products.

Think of the endothermic process like ice melting on a warm day; it takes in heat from the environment to change from a solid to a liquid. Here, the enthalpy change ((H)) for an endothermic reaction is positive because the system gains heat. A common example would be isopropyl alcohol evaporating from your skin, making the skin's surface feel cooler as it draws in heat.

Enthalpy Change

Enthalpy change ((H)) is a measure of the total heat content in a thermodynamic system and represents the amount of energy paired with a chemical reaction at constant pressure. It's a central concept when examining exothermic and endothermic processes because it indicates whether a system has lost or gained energy through heat transfer.

In formulas, enthalpy change is often denoted as (H), which can be calculated from the heat absorbed or released under constant pressure. If (H) is negative, the reaction is exothermic, showing heat release. Conversely, a positive (H) indicates an endothermic reaction where the system absorbs heat.

Heat Transfer

Heat transfer is the movement of thermal energy from one entity to another. It's an essential part of understanding thermochemistry as it explains how heat flows due to a temperature difference.

There are three primary modes of heat transfer: conduction, convection, and radiation. In the context of chemical processes, heat transfer often occurs through conduction or radiation, such as when heat moves from hot stovetop burners to a pot through direct contact, or from the sun to the Earth through space.

Understanding heat transfer allows us to determine whether a process is likely to be exothermic or endothermic by considering whether the system is giving off heat or absorbing it.