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Energy of Phase Changes

Caleb Thomas

Caleb Thomas

8 min read

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Study Guide Overview

This study guide covers phase changes and energy in chemistry, focusing on heating/cooling curves, phase diagrams, and related calculations. It explains heat of fusion and heat of vaporization, provides example problems and solutions, including an AP-style FRQ, and offers exam tips. Key concepts include the triple point and critical point on phase diagrams, and the application of formulas like q=mcΔT. Practice multiple-choice and free-response questions are included with solutions.

Phase Changes & Energy: Your Ultimate AP Chem Review 🚀

Hey there, future AP Chem master! Let's break down phase changes and energy, making sure you're totally prepped for anything the exam throws your way. We'll go through heating/cooling curves, phase diagrams, and tackle some practice problems. Let's get started!

Heating Curves: The Endothermic Journey 📈

Heating curves show how temperature changes as you add energy to a substance. It's all about that endothermic life!

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###### Image Courtesy of Schoenherr & Diamantopoulos Chemistry Videos
  • X-axis: Time or energy added
  • Y-axis: Temperature 🌡️
  • As you add energy, you move from solid → liquid → gas.

Plateaus: The Phase Change Zones 🧊🔥

Key Concept

Those flat lines aren't mistakes! They represent phase changes where energy goes into breaking intermolecular forces (IMFs), not raising the temperature.

  • Melting: Solid → Liquid. Energy is the heat of fusion (Hf).
  • Vaporizing: Liquid → Gas. Energy is the heat of vaporization (Hv).
Quick Fact

Hv is almost always greater than Hf because all IMFs must be broken to vaporize, while only some are broken during melting.

Memory Aid

Memory Aid: Heating Curve

Think of a mountain climb:

  1. Slopes: You're gaining altitude (temperature) – use q = mcΔT.
  2. Plateaus: You're crossing a flat area (phase change) – use Hf(m) or Hv(m).

Example Calculation: Ice to Steam 🧊→ 😤

Let's say we want to know how much energy is needed to change 30.0g of ice at -20°C to steam at 140°C. Here's the breakdown:

  • Given values:

    • Specific heat of ice (cice) = 2.108 J/g°C
    • Specific heat of water (cwater) = 4.18 J/g°C
    • Specific heat of steam (csteam) = 2.010 J/g°C
    • Hf of H2O = 334 J/g
    • Hv of H2O = 2260 J/g

  • Steps:

    1. 🧊Heating ice: q = mcΔT = (30.0 g)(2.108 J/g°C)(20°C) = 1264.8 J
    2. Melting ice: q = Hf(m) = (334 J/g)(30.0 g) = 10020 J
    3. 💧Heating water: q = mcΔT = (30.0 g...

Question 1 of 9

On a heating curve, what does the x-axis represent? 🤔

Temperature

Pressure

Time or energy added

Phase of substance