Under what circumstances are ΔE and ΔH essentially equal?

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Understand that ΔE represents the change in internal energy of a system, while ΔH represents the change in enthalpy.

Recall the relationship between ΔE and ΔH: ΔH = ΔE + PΔV, where P is pressure and ΔV is the change in volume.

Recognize that ΔE and ΔH are essentially equal when the term PΔV is negligible.

Identify the conditions under which PΔV is negligible: this typically occurs when the reaction involves no gases or when the volume change is very small, such as in reactions occurring in the liquid or solid phase.

Conclude that ΔE and ΔH are essentially equal under conditions of constant volume or when the work done by the system is minimal, such as in reactions involving only solids and liquids.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Internal Energy (ΔE)

Internal energy (ΔE) is the total energy contained within a system, encompassing kinetic and potential energy at the molecular level. It reflects the energy changes that occur during chemical reactions or physical processes. Understanding ΔE is crucial for analyzing energy transfers and transformations in thermodynamics.

Enthalpy (ΔH)

Enthalpy (ΔH) is a thermodynamic quantity that represents the total heat content of a system at constant pressure. It accounts for internal energy and the work done by the system due to volume changes. ΔH is particularly important in chemical reactions, as it indicates whether a reaction is exothermic or endothermic.

Constant Pressure Conditions

Under constant pressure conditions, the change in enthalpy (ΔH) is equal to the heat exchanged (q) during a process. When a reaction occurs in an open system at atmospheric pressure, the internal energy change (ΔE) and enthalpy change (ΔH) can be considered equal, especially when the volume change is negligible, allowing for simplified calculations in thermodynamics.

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