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⚛️ Unit 1 - Atomic Structure and Properties
1.1Moles and Molar Mass
1.2Mass Spectroscopy of Elements
1.3Elemental Composition of Pure Substances
1.4Composition of Mixtures
1.5Atomic Structure and Electron Configurations
1.6Photoelectron Spectroscopy & Graph Interp.
🤓 Unit 2 - Molecular and Ionic Compound Structures and Properties
2.0Unit 2 Overview: Molecular and Ionic Bonding
2.1Types of Chemical Bonds
2.2Intramolecular Force and Potential Energy
2.3Ionic Bonding and Ionic Solids
2.4Metallic Bonding and Alloys
2.5Lewis Dot Diagrams
2.6Resonance and Formal Charge
🌀 Unit 3 - Intermolecular Forces and Properties
3.0Unit 3 Overview: Intermolecular Forces and Properties
3.2Properties of Solids
3.3Solids, Liquids, and Gases
3.4The Ideal Gas Law
3.5The Kinetic Molecular Theory of Gases
3.6Deviations from the Ideal Gas Law
3.7Mixtures and Solutions
3.8Representations of Solutions
3.9Separation of Solids/Mixtures
3.10Solubility and Solubility Rules
3.11Spectroscopy and the Electromagnetic Spectrum
3.12Quantum Mechanics and the Photoelectric Effect
🧪 Unit 4 - Chemical Reactions
4.0Unit 4 Overview: Chemical Reactions
4.1Recognizing Chemical Reactions
4.2Net Ionic Equations
4.4Physical vs. Chemical Changes
4.5Stoichiometry & Calculations
4.6Titrations - Intro and Calculations
4.8Intro to Acid-Base Neutralization Reactions
👟 Unit 5 - Kinetics
5.0Unit 5 Overview: Kinetics
5.1Defining Rate of Reaction
5.2Introduction to Rate Laws
5.3Rate and Concentration Change
5.4Writing Rate Laws
5.5Collision Model of Kinetics
5.6Reaction Energy and Graphs w/ Energy
5.7Reaction Mechanisms and Elementary Steps
5.8Writing Rate Laws Using Mechanisms
🔥 Unit 6 - Thermodynamics
6.0 Unit 6 Overview: Thermochemistry and Reaction Thermodynamics
6.1Endothermic Processes vs. Exothermic Processes
6.2Energy Diagrams of Reactions
6.3Kinetic Energy, Heat Transfer, and Thermal Equilibrium
6.4Heat Capacity and Coffee-Cup Calorimetry
6.5Phase Changes and Energy
6.6Introduction to Enthalpy of Reaction
6.7Bond Enthalpy and Bond Dissociation Energy
6.8Enthalpies of Formation
⚖️ Unit 7 - Equilibrium
🍊 Unit 8 - Acids and Bases
8.0Unit 8 Overview: Acids and Bases
8.1Introduction to Acids and Bases
Unit 9 - Applications of Thermodynamics
🤺 AP Chemistry Essentials
🧐 Multiple Choice Questions
AP Chemistry Self-Study and Homeschool
⏱️ 3 min read
August 27, 2020
Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. Basically, enthalpy helps describe heat energy within a system and how this heat energy moves through, in, and out of a system🔄. However, as we've previously discussed, you cannot measure absolute enthalpy, but rather you can only measure changes in enthalpy, or ΔH.
Image Courtesy of Chemistry LibreTexts
In Unit 6.1, we identified two types of reactions: exothermic and endothermic reactions. You may think of an exothermic reaction as "hot" or "heat producing" and an endothermic reaction as "cold" or "heat absorbing". While you can think of these reactions in terms of temperature🌡️ or in terms of the change in enthalpy.
The change in enthalpy due to a reaction is called the enthalpy of reaction and describes whether a system is losing or gaining energy (at constant pressure). If ΔH is positive, the system is gaining energy, and if ΔH is negative, the system is losing energy🔥.
Now that we've learned about what enthalpy of reaction means, let's look at the difference between the signs of ΔH (+ or -). If ΔH for a reaction is positive, the system is gaining energy, meaning the reaction is endothermic. This might seem counterintuitive, but since the surroundings is losing energy, we will see the reaction cooling down the environment, which is our observational definition of endothermic.
Similarly, if a reaction's ΔH is negative, the reaction is exothermic, since energy is being released into the surroundings (the system is losing heat).
Image Courtesy of Mr Lowe
Energy, enthalpy, and heat, what is the difference between these things? And what is work?
As we went over before, energy is generally the capacity to do work or produce heat.
Internal energy⚡, which is referred to as E sometimes, is the sum of the kinetic energy in a system and the potential energy in a system. You could calculate the change in E, or ΔE, using the following formula: ΔE = q + w.
Heat🕯️, or q, refers to the transfer of energy between two objects that occurs due to a temperature change. Remember, heat flows from the source, which is the hotter substance, to the sink, which is the cooler substance.
Work🏃, or w, refers to force over a distance. Work is really a physics term, but know that if you are gaining work, or have positive work, a gas is compressed, volume decreases, and E increases. Vice versa applies as well: if you are losing work, or have negative work, a gas is expanded, volume increases, and E decreases. You don't really have to know this but w=-PΔV. Work is usually expressed in joules. Basically:
Positive work = compression = volume decreases = energy of the system increases
Negative work = expansion = volume increases = energy of the system decreases
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