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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
⏱️ 4 min read
August 28, 2020
While it may not seem so, bonds contain energy😲. When you think about why some reactions are exothermic and others are endothermic, it is because of the breaking and forming of bonds! There is a general rule here, and that is: BREAKING BONDS REQUIRES ENERGY & MAKING BONDS RELEASES ENERGY.
Imagine a single molecule of H2. If you wanted to break that bond, you have to input energy. Think of it almost like having to snap a branch in half or stretching a rubber band until it snaps💥. In order to do this, we have to put energy into the system!
GIF Courtesy of MakeAGIF
Conversely, when you form a bond, energy is released. This is because of lower potential energy at these states. This leads to excess energy being released from the system. This can be seen in the following chart:
When atoms are bonded together in a correct formation, potential energy is released. When they are too close, repulsive forces make potential energy high and when they are too far apart, potential energy approaches zero since there are no attractive forces.
📝Read: AP Chemistry - Intramolecular Force and Potential Energy
Now that we've figured out that breaking bonds takes energy and that forming bonds releases energy, let's start examining how we quantize this energy. To begin, the energy that it takes to break a specific bond is called the bond dissociation energy, or BDE. This varies from bond to bond, but there are some basic trends:
Typically, a triple bond has a higher BDE than a double bond than does a single bond. This is because simply put, there's a more robust bond to break through. This should make logical sense, though it is worth noting🧐.
Similarly, a shorter bond will be stronger and a longer bond will be weaker. This is because of potential energy differences based on bond length.
Bond Dissociation Energy can be used to calculate the amount of energy released or absorbed during a chemical reaction. This is because of one simple principle: all a reaction is is the breaking of reactant bonds and the forming of product bonds. Therefore, we can use a simple formula:
ΔH = ΣH(broken) - ΣH(formed)
💡KNOW it's broken - formed, or reactants - products. In the next key topic, you are introduced to another formula that is products-reactants, so be careful.
Breaking Down This Formula
This formula essentially is the sum (that's what Σ, or sigma, means) of the bond dissociation energies of the reactant bonds broken minus the sum of the bond dissociation energies of the product bonds formed. An easy way to do this is to break all the bonds and then, from such, rebuild the products.
We are given the following thermodynamic data:
We know that: ΔH = ΣH(broken) - ΣH(formed)
Let's apply this formula: ΔH = (H-H + O=O) - (O-H + O-O + O-H) ΔH = (432 + 498) - (463 + 139 + 463) = -135 kJ/mol
Let's try another problem! Find the heat of reaction for the following:
CH4(g) + 2O2 (g) --> CO2(g) + 2H2O(g)
ΔH = [4(C-H) + 2(O=O) ] - [2(C=O) + 4(O-H)]
ΔH = [4(413) + 2(498)] - [2(799) + 4(463)] = -802 kJ/mol
This reaction is an exothermic reaction since the forming the bonds took more energy than breaking the bonds.
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