Chem 162 - Ch 9 - Thermochemistry

Thermochemistry Overview

• Definition: Thermochemistry is the study of changes in energy that accompany chemical reactions. It focuses on the energy changes involved in reactions.

• Notation: The Greek letter Delta (Δ) signifies a change in a quantity. For example, ΔE indicates a change in energy.

Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed or changed in form. This is foundational for understanding thermochemical processes.

Key Concepts

System and Surroundings

• System: The part of interest where the reaction occurs.

• Surroundings: Everything else surrounding the system that can exchange energy with it.

Work

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  Definition: Work is the force exerted through a distance. The formula is:[ W = \text{Force} \times \text{Distance} ]

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  Pressure-Volume Work:[ \Delta E = Q - P \Delta V ]Where Q is heat, P is pressure, and ΔV is the change in volume.

• Application: Often used in gas compression scenarios.

Types of Energy

• Potential Energy: The energy stored in chemical bonds, often referred to as chemical energy (ΔE).

• Kinetic Energy (K): The energy of motion.

Heat (Q)

• Definition: Heat is the energy that transfers into or out of a system due to temperature differences.

• Types of Reactions:

  • Endothermic Reactions:

    • Definition: Heat is absorbed by the system.

    • Characteristics: The reaction vessel feels cool; Q is positive. Example: Adding heat to the system increases energy.

  • Exothermic Reactions:

    • Definition: Heat is evolved (released) from the system.

    • Characteristics: The reaction vessel feels warm; Q is negative. Example: Reaction gives off heat.

Identifying Energy Sign Conventions

• Keywords indicate the sign of Q:

  • "Absorbed" or "added" = Positive Q.

  • "Evolved" or "removed" = Negative Q.

• Example Interpretations:

  • "100 kJ of heat is absorbed" = Q = +100 kJ.

  • "100 kJ of heat is evolved" = Q = -100 kJ.

Enthalpy (H)

• Definition: Enthalpy is an extensive property related to heat in chemical reactions.

• Key Relationship: ΔH = Q at constant pressure.

  • Change in enthalpy is synonymous with heat change for a reaction.

  • For endothermic reactions: ΔH is positive.

  • For exothermic reactions: ΔH is negative.

• State Function: Enthalpy is independent of the path taken between initial and final states, only the conditions matter.

Conclusion

• This content serves as an introduction to thermochemistry concepts, which will be explored in greater detail in subsequent examples and applications.