Enthalpies of Formation
Caleb Thomas
8 min read
Study Guide Overview
This study guide covers standard enthalpy of formation (ΔHf), including its definition, standard conditions, and the significance of zero ΔHf for elements in their standard states. It explains how to calculate ΔHrxn using the formula ΔHrxn = ΣnΔHf(products) - ΣmΔHf(reactants), with examples and common mistakes to avoid. It includes an AP-style practice question on enthalpy of combustion and concludes with final exam focus, key takeaways, common question types, and last-minute tips.
#Thermochemistry: Enthalpy of Formation Study Guide
Hey there, future AP Chem master! Let's dive into enthalpy of formation – a key concept that pops up all over the exam. This guide is designed to make sure you're not just memorizing, but truly understanding this material. Let's get started!
#Standard Enthalpy of Formation (ΔHf)
#What is it?
The standard enthalpy of formation (ΔHf) is the change in enthalpy when one mole of a compound is formed from its elements in their most stable state at standard conditions (25°C and 1 atm). Think of it as the energy 'cost' of building a compound from scratch 🏗️.
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Key Idea: It's always for one mole of product.
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Standard Conditions: 25°C (298 K) and 1 atm.
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Most Stable State: The form of an element that is most stable at standard conditions (e.g., O2(g), C(s, graphite)).
For elements in their standard states, ΔHf is always zero. This is a crucial point to remember!
#Visualizing ΔHf
Let's break down CO2 formation:
C(s, graphite) + O2(g) → CO2(g)
The ΔH for this reaction is the ΔHf of CO2. Notice how we're starting with the elements in their standard states? That's key!
Image Courtesy of Google Sites
Analogy: Think of ΔHf as the energy needed to assemble a Lego set (the compound) from individual Lego bricks (the elements in their standard states). Elements are the basic building blocks, and their ΔHf is zero because they are already in their simplest form.
#Calculating ΔHrxn Using ΔHf
#The Formula
Here's the magic formula to calculate the enthalpy change of a reaction (ΔHrxn) using standard enthalpies of formation:
ΔHrxn = ΣnΔHf(products) - ΣmΔHf(reactants)
- Σ means 'sum of'.
- n and m are the stoichiometric coefficients from the bala...
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