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Introduction to Enthalpy of Reaction

Emily Wilson

Emily Wilson

7 min read

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Study Guide Overview

This study guide covers enthalpy and energy in chemical reactions. Key concepts include: defining enthalpy (H) and enthalpy change (ΔH), distinguishing between exothermic and endothermic reactions, understanding the relationship between ΔH, heat (q), and work (w), and internal energy (E or U). The guide also provides practice questions and exam tips for the AP Chemistry exam.

AP Chemistry: Enthalpy & Energy - The Night Before Review 🚀

Hey! Let's make sure you're feeling confident about enthalpy and energy for tomorrow's AP Chem exam. This guide is designed to be quick, clear, and super helpful for your last-minute review. Let's get started! 💪

Thermodynamics: Enthalpy (H) and Energy (E)

What is Enthalpy?

Enthalpy (H) is like the total energy package of a system. It includes all the internal energy plus the energy related to pressure and volume. We usually focus on the change in enthalpy (ΔH), which tells us about heat flow in reactions or phase changes.

Key Concept

ΔH = Hfinal - Hinitial

This change in enthalpy (ΔH) is what we measure to understand if a reaction is releasing or absorbing heat.

Enthalpy Diagram

Image Courtesy of Chemistry LibreTexts

Enthalpy of Reaction (ΔHrxn)

ΔHrxn is the heat absorbed or released during a chemical reaction. It's our key to figuring out if a reaction is exothermic or endothermic.

  • Exothermic Reactions: ΔH is negative (-). Heat is released by the system. Think of a campfire 🔥 – it gives off heat.
  • Endothermic Reactions: ΔH is positive (+). Heat is absorbed by the system. Think of an ice pack getting cold as it melts 🧊 – it absorbs heat.
Exam Tip

Remember: Exothermic = Exit (heat exits), Endothermic = Enters (heat enters).

Examples of Exothermic and Endothermic Reactions:

  • Exothermic:
    • Combustion (burning) of fuels like propane (C3H8) or methane (CH4)
    • Neutralization reactions (acid + base)
  • Endothermic:
    • Dissolving ammonium nit...
Previous Topic - Energy of Phase ChangesNext Topic - Bond Enthalpies

Question 1 of 12

What is the correct formula for calculating the change in enthalpy (ΔHΔH) of a reaction? 🤔

ΔH=Hinitial−HfinalΔH = H_{initial} - H_{final}

ΔH=Hfinal+HinitialΔH = H_{final} + H_{initial}

ΔH=Hfinal−HinitialΔH = H_{final} - H_{initial}

ΔH=Hproducts∗HreactantsΔH = H_{products} * H_{reactants}